The Importance of Phytohormones and Microbes in Biofertilizers

Abstract

Plant growth is dependent on meristems where cell proliferations (cell division and growth) give rise to new plant structures and allow the plant to increase in size. We provided scientific linkages and evidence to show that the growth promoting factors in biofertilizers regulating cell proliferation and ultimately modulating plant growth and development are phytohormones. The known biological functions of phytohormones (cytokinins, auxins, gibberellins, etc.) are in tandem with the observed physiological characteristics and crop yield of plants. When light, water and mineral nutrients are not limiting, phytohormones especially cytokinins, in biofertilizers help to drive plant growth by progressing faster through the various plant cell cycle checkpoints leading to the production of more cells. In the soil matrices, PGPRs (Plant Growth Promoting Rhizobacteria) have the ability to promote plant growth via various mechanisms such as nitrogen fixation, phosphorus and zinc solubilization. Some PGPRs secrete phytohormones, especially cytokinins, and can be cultured and developed into a biofertilizer. In the near future, a hybrid approach of combining organic and conventional fertilization regimes will be the likely scenario as we have achieved a better understanding of plant growth and development through the regulatory controls on the cell proliferation processes by phytohormones and mineral nutrients delivered by fertilizers. The futuristic green biofertilizer should come in the form of granules in which the active plant growth promoting and soil improving substances and/or suitable microbes, with carefully selected mineral nutrients, are embedded in the packing materials giving slow and sustained release over a desired period.

1 Introduction

1.1 General Plant Growth and Mineral Nutrition

General plant growth and development require 16 chemical elements (Marschner 1995). These essential elements can be classified into groups of non-mineral and mineral elements. Non-mineral elements, hydrogen (H), oxygen (O) and carbon (C), are found in air and water. Plants require these elements as raw ingredients, in the form of carbon dioxide (CO₂) and water (H₂O), to produce their own food via photosynthesis. During photosynthesis, CO₂ and H₂O are converted into glucose and ultimately to complex sugars, starch and cellulose using the energy from the sun light. Starch and complex sugar in turn provide source of carbon, energy and polymeric substrates of the plant for their growth and biosynthesis processes (Rolland et al. 2002).

Unlike non-mineral nutrients, mineral nutrients are obtained from the soil (Bronick and Lal 2005). These nutrients can be classified into macronutrients and micronutrients according to their relative concentration in plant tissues (Shaviv and Mikkelsen 1993; Taiz and Zeiger 2010). Macronutrients can be further divided into primary and secondary nutrients. Amongst all the mineral nutrients, primary nutrients, nitrogen (N), phosphorus (P) and potassium (K), are utilized by plants in the largest amounts for growth and survival and thus these three components are most lacking in nutrient-depleted soil. The secondary macronutrients are calcium (Ca), magnesium (Mg) and sulfur (S). Under natural conditions, the soil contains sufficient amount of secondary macronutrients and hence supplementation through fertilization is not always necessary. Furthermore, large amounts of Ca and Mg are added when lime is applied to reduce soil acidity; while S content in the soil is maintained by those released from the organic matter that underwent slow decomposition.

Micronutrients, boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn), on the other hand are required by plants in much smaller (micro) amount (Hänsch and Mendel 2009). They are also commonly termed as trace elements. Like S, these elements are recycled from decomposed organic matter. The above-mentioned elements in the form of micronutrient and macronutrients are mandatory for plant growth and development. However, their availability for plant uptake is determined not only by the amount present in the soil; soil composition and physicochemical properties (e.g. soil texture, soil structure and soil pH) also determine the extent to which the nutrients are bioavailable to the plants.

1.2 Soil Properties and Nutrient Availability

The soil is a complex physical, chemical and biological substrate (Bronick and Lal 2005; Berendsen et al. 2012). It is the most common medium in which many plants grow, and thus good soil condition is the prerequisite for promoting favorable plant growth which subsequently affects crop yield.

The properties contributing to the soil’s function and ability to support plant life are the soil structure, texture and pH. Many ecological and plant physiological processes are heavily influenced by these factors: nutrient cycling, erosion, root penetration and gas exchange are some of the processes more directly related to plant growth.

Soil texture refers to the proportion of sand, silt, clay and organic matter in the soil, which is influenced by the geographical location and seasons. Soil texture influences nutrient and water retention in the soil which in turn benefits the plants (Bronick and Lal 2005). Clay and organic soil have much better nutrients and water retention capacity than sandy soil. In soils with poor nutrient and water retention capacity, leaching and loss of soil nutrients into groundwater occur as nutrients drain away, along with the water that is not being retained. This results in less nutrients being available for plant uptake. When the soil has more clay and organic matter, then water might be retained for too long causing the soil to become waterlogged. Under waterlogged conditions, the oxygen content in the soil depletes, plant roots might also rot due to prolonged soaking in water and aerobic respiration at the roots ceases. Production of nitrates, N source for plants, is also inhibited by the anaerobic condition of waterlogged soil (Crawford and Glass 1998). Hence, soil that contains optimum portions of sand, silt, clay and organic matter is ideal for farming and agricultural use.

Apart from the soil composition, soil structure, i.e. aggregation of soil particles, is also an important property of productive soil. It is the key factor that determines the functioning of the soil and enables the soil to support plant life on top of moderating environmental quality with respect to soil carbon sequestration and water quality (Bronick and Lal 2005). The pore spaces created between the particles in the aggregates affect water and air movement within the rhizospheres, nutrient availability for plant root growth and microbial activity. Thus, favorable soil structure helps to improve soil fertility, agronomic productivity and enhance soil porosity while lowering erodibility of the soil (Bronick and Lal 2005).

Soil pH, a measure of the acidity or alkalinity of the soil, is another important property that directly affects the availability of nutrients for plant uptake. At low pH, macronutrients tend to be less available; while at high pH, micronutrients tend to be less available. Under most circumstances, most soil has low pH level due to the release of hydrogen ion from the reaction between soil water and carbon dioxide produced during organic matter decomposition. Lime application can raise the soil pH level to the ideal range of 6.0–6.5. This slightly acidic pH range promotes root growth, weathering of rocks that releases minerals (Ca, K, Mg and Mn), and increases the solubility of carbonates, sulfates and phosphates (Taiz and Zeiger 2010). Beneficial plant bacterial activities, such as microbial nitrogen fixation and conversion of sulfur to forms suitable for plant uptake, also become more prevalent. Furthermore, the added lime also contributes to the pool of Ca and Mg for plant use and enhances the soil structure which subsequently promotes water and air movement.

Although nutrients occur naturally in the soil, some nutrients should be added to the soil such as lime or fertilizer to sustain plant growth and especially under situations where there are significant biomass removal periodically (Shaviv and Mikkelsen 1993; Chen 2006).

1.3 Fertilizers

Sustainable agriculture ideally should produce good crop yields with minimal impact on important ecological factors such as soil fertility (Tilman 1998; Mäder et al. 2002; Chen 2006). Mäder et al. (2002) defined fertile soil as a soil that provides essential nutrients for crop plant growth, supports a diverse and active biotic community, exhibits a typical soil structure, and allows for an undisturbed decomposition. Such “ideal” fertile soil is, however, yet to be achieved widely in the current green revolution that practices high-intensity agriculture. Intensive agriculture, often referred to as conventional agriculture, has successfully increased crop yields to meet the demands of the growing global population, but it leads to serious environmental costs (Tilman 1998). These costs include contamination of groundwater, release of greenhouse gases, change in the natural soil structure, loss of crop genetic diversity, eutrophication of water bodies and aquatic ecosystems and alteration of aquatic food webs.

Over the years, an alternative agriculture practice known as organic farming , is steadily gaining wide acceptance and practice. Agriculture products from organic farming are also marketed globally. According to the survey conducted by Foundation Ecology and Agriculture (SÖL), 4.0 % (as of 2004) of the agricultural land in Asia is managed organically; a rapid rise from the 0.33 % in 2001 (Hsieh 2005). The survey also revealed that a significant percentage of agricultural land in other continents were also managed organically, e.g. Oceania: 42 %; Latin American: 24 %; North America: 6 %; Europe: 23 %; Africa: 1 % (as of 2004). The 21-year-long organic farming study conducted by Mäder et al. (2002) showed that this method is also environment friendly. Nutrient inputs (N, P, K) and energy required to produce a crop dry matter unit was significantly reduced to 34, 51, 20 and 56 %, respectively, as compared to the conventional practice. There was, however, 20 % less crop yield. But how sustainable is organic farming? Until today, the answer still remains elusive.

The success of both farming methods is highly dependent on soil fertility, i.e. mineral nutrients. Mineral nutrients from the soil are dissolved in water and absorbed through plant roots. These nutrients may occur naturally in the soil but the bioavailable amount may not be sufficient to support healthy and robust plant growth. Furthermore, farming (especially the conventional method) may also deplete the soil of nutrients, especially primary macronutrients—N, P and K. To overcome this nutritional limitation, farmers and gardeners add nutrients externally through the application of fertilizers to compensate for the shortage of mineral nutrients in the soil. Globally, fertilizer usage has been increasing steadily over the years; typically, 40–70 % of a food production company’s operating cost is spent on fertilizer usage.

In general, fertilizers refer to the substances added to the soil to increase its fertility. While most fertilizers are applied to the soil, some are formulated to be sprayed on leaves and the other aerial plant parts. Fertilizers can be derived from either organic sources or can be chemically synthesized. Regardless of the route of application and how they are derived, fertilizers supply the plants with nutrients that are generally absorbed in the form of inorganic ions (Taiz and Zeiger 2010). Nevertheless, there is new evidence to demonstrate that plants are able to absorb proteins directly and without involving other organisms (Paungfoo-Lonhienne et al. 2008).

Fertilizers play a pivotal role in regulating the growth of crop plants, and thus the reliability of food supply. The application of fertilizers is important for plant growth via cell proliferation (cell division and enlargement; detailed mechanisms will be discussed in Sects. 2 and 3), and the periodic replenishment of essential nutrients, especially the primary macronutrients, N, P, K, which are most likely to be depleted in heavily utilized soil, and other trace micronutrient, is essential to maintain soil fertility.

N, P and K are the three major mineral nutrients essential for plant growth and development (Orhan et al. 2006). Among these three nutrients, N is the mineral nutrient needed in greatest abundance by plants (Crawford 1995). N is the key constituent of molecules such as amino acids, amides, proteins, nucleic acids, nucleotides, coenzymes that are essential for various biological functions and comprises about 1.5 % of a plant’s dry weight (Taiz and Zeiger 2010). In addition to serving as a nutrient and as an osmolyte, N also functions as a signal that reprograms N and C metabolism and influences root and shoot growth (Crawford 1995; Wang et al. 2007). N level in the soil is often lowered due to plant uptake, leaching and microbial denitrification, resulting in the dependence on N fertilizers to sustain the productivity of any modern intensive agriculture (Crawford and Glass 1998).

After N, P is the second most frequently limiting macronutrient for plant growth (Schachtman et al. 1998). Making up about 0.2 % of a plant dry weight, P is a component of key molecules such as nucleic acids, phospholipids, sugar phosphates, coenzymes and ATP (Schachtman et al. 1998; Taiz and Zeiger 2010). Phosphorus in the form of orthophosphate (Pi), is also required in the regulation of metabolic pathways and enzymatic reactions (Theodorou and Plaxton 1993).

Potassium, K, generally constitutes 1 % of a plant dry weight and thus it is the nutrient to be absorbed in second largest amount, after N. It is essential as a cofactor for enzyme activities involved in nutrient absorption, respiration, transpiration and photosynthesis. Unlike N and P, K does not become a part of endogenous organic compounds but remain as a cation in the plant tissues. It is also the crucial for the establishment of cell turgor and maintenance of cell electroneutrality (Taiz and Zeiger 2010).

A wide variety of fertilizers are available commercially and they can be classified into chemical fertilizers and organic/green fertilizers. Another class of fertilizers, biofertilizers , is also gaining worldwide attention, due to the awareness of the detrimental effects of chemical fertilizers imposed on the environment globally and of the improved knowledge on the relationships between plants and microorganisms occurring in the soil (Malusá and Vassilev 2014). Thus, agricultural fertilizers currently available in the market can be classified traditionally into three broad categories, namely chemical fertilizers, organic fertilizers and biofertilizers.

Chemical fertilizers are chemically synthesized compounds that contain specific nutrients, macro and/or micronutrients. This group of fertilizers provides the plants with nutrients in inorganic forms. In this review, we refer chemical fertilizers specifically to synthetic fertilizers containing N, P and K in various ratios in terms of weight percentage, otherwise also known as NPK fertilizers or inorganic fertilizers (Shaviv and Mikkelsen 1993).

Organic fertilizers are fertilizers generally derived from natural sources such as plant and animal matter. The commonly known examples are composts (decomposed plant materials) and manure (animal excrement). Meat and bone meal is another form of organic material being used as organic fertilizers (Jeng et al. 2006). In this review, we consider composts and manures as organic fertilizers.

Biofertilizers are the new and emerging entities in the realm of agricultural fertilizers or “Biostimulants”. Biofertilizers are defined as fertilizers that enhance plant growth via the activities of microorganisms, i.e. conversion of nutritionally important elements/compounds from the “unavailable state” to (bio)available form(s) and production of active ingredients, particularly phytohormones (e.g. cytokinins, auxins, gibberellins etc.), amino acids and proteins. To date, the definition of biofertilizers is still unclear and remains highly debatable. No consensus has been agreed upon on the inclusion of indirect microbial activities, such as biocontrol properties targeting pathogens and conferring resistance against pathogens, as plant growth enhancing properties (Malusá and Vassilev 2014). Interestingly, there are also some people who do not consider biofertilizers as organic fertilizers. For the purpose of this review, we would, however, like to define biofertilizers as organic products containing biomass-based structural matrix, e.g. composts, humic acid and fulvic acid, with different types of useful (natural) microorganisms that enhance plant growth through their biological activities. Thus, in this review, our holistic classification of organic fertilizers is to include biofertilizers , unless otherwise stated.

In the following sections, we will discuss briefly on the advantages and disadvantages of chemical and organic fertilizers, while biofertilizers will be discussed in greater depth in Sect. 5.

1.3.1 Chemical Fertilizers

Chemical fertilizers are NPK-based formulation that has been widely used for over 100 years. Chemical fertilizers, e.g. urea, are the preferred choice as they are deemed to be highly effective and can be transported more economically. They supply the plants with mineral nutrients in the form of organic/inorganic salts that are easily and readily taken up by the plants via the roots, giving rise to immediate or quick plant growth improvement. Most importantly, chemical fertilizers supplement the soil with nutrients in desired ratios of N:P:K at a low cost.

Despite the advantages, a century-long usage of chemical fertilizers has seemed to reveal that these fertilizers are “losing” its growth promoting efficacy and increasingly, creating environmental problems. It has been reported that large fractions of N fertilizers applied to agriculture systems are lost as N₂, trace gases and nitrate leachate (Adesemoye et al. 2010), as the soil structure degrades and loses its mineral and water retention capacity. Larger amount of chemical fertilizers is thus required to achieve the same effect. The rampant use of chemical fertilizers in turn leads to eutrophication of water resources, pollution and contamination of soil, further degradation of soil structure, reduced soil fertility and reduced fertilizer efficiency (Vitousek et al. 1997; Tilman 1998; Mahdi et al. 2010; Xiang et al. 2012). Prolonged supply of high amount of N to plants also causes plant tissues softening, causing the plant to be more sensitive to pests and diseases (Chen 2006).

Chemical fertilizers also pose adverse effects on the biological properties of the soil. Its continuous application usually causes the soil to become acidic and repels earthworms, beneficial entity in fertile soil. Acidic soil also alters microbial species composition and diversity in the rhizosphere and results in the destruction and hindrance of beneficial microbial activities such as organic matter decomposition and symbiotic interaction with plants. Also, biologically inactive soil contains less organic matter and do not release as much nutrients as biologically active soil (Chandramohan et al. 2013). Over time, soil treated only with chemical fertilizers will lose its organic matter and disrupt the interactions with the living organisms, namely earthworms and microorganisms, which contribute immensely to soil “health” (Tilman 1998).

1.3.2 Organic Fertilizers

It is increasingly evident that the intensive agricultural methods employed as part of the conventional agriculture have been proven to be unsustainable (Tilman 1998; Mäder et al. 2002). Small but growing cohorts of farmers/growers have recognized this issue and have turned to utilize an alternate farming method, i.e. organic agriculture. Organic agriculture is defined by the FAO/WHO as “a holistic production management system which promotes and enhances agro-ecosystem health, including biodiversity , biological cycles and soil biological activity. It emphasizes the use of sustainable management practices in all aspects of the farm management and taking into account of the regional conditions and locally adapted systems. This is accomplished by using, wherever possible, agronomic, biological and mechanical methods, as opposed to using synthetic materials, to fulfill any specific function within the system” (FAO/WHO 2015). In comparison to conventional agriculture, organic agriculture is deemed to be an effective farming system with self-sustainability features.

Theoretically, organic agriculture is self-sustainable but it still requires the input of external fertilizers to replenish the nutrients that are quickly sequestered by the growing plants. To adhere to its organic practices, organic agriculture supply their soil with N by more natural means such as growing cover crops, mainly leguminous species that have the ability to fix atmospheric N. It makes use of organic fertilizers instead of chemical fertilizers. Organic fertilizers, also termed as green fertilizers, however, work on a different basis as compared to chemical fertilizers.

Organic fertilizers, including biofertilizers , supplement the soil with nutrients, but usually at a much lower concentration and slower release rate as compared to chemical fertilizers. N is released slowly from organic fertilizers (compost) due to its slow mineralization rate (Hernández et al. 2010) and its availability is dependent on soil properties (Fricke and Vogtmann 1993). P is contributed at a percentage of 20–40 % (Fricke and Vogtmann 1993) of its total low P content (vermicompost: 0.014 ± 0.0009 %; compost: 0.015 ± 0.0009 % (Hernández et al. 2010)). K is, however, contributed at an exceptionally higher percentage of 85 % (Fricke and Vogtmann 1993).

Generally, organic fertilizers have lower mineral nutrient contents which are often not well characterized and quantified, and may vary between production batches and methods when compared with chemical fertilizers (Shaviv and Mikkelsen 1993; Mäder et al. 2002; Chen 2006; Mahdi et al. 2010). Thus, within the plant industry, there is certainly a need to develop/produce organic fertilizers in a reproducible way in order to gain wider acceptance by farmers utilizing the conventional farming approach, i.e. quality control and assurance for organic fertilizers as a reliable commercial product. Furthermore, the rate of nutrient release in some poorly prepared organic fertilizers may not meet the needs of the vigorously growing plants, accustomed to conventional chemical fertilizers. Hence, larger amount of organic fertilizers has to be applied under certain situations. This in turn incurs more cost to the growers as compared with using the traditional chemical fertilizers.

Organic fertilizers are, however, desirable over chemical fertilizers due to its ability to improve the soil structure via the enhancement of soil biological diversity. As discussed earlier (Sect. 1.2), soil properties play a very vital role in determining plant growth and nutrient availability for plant uptake. Thus, organic fertilizers are more valued for their soil-improving qualities and to a lesser extent for their mineral nutrient, mainly N and P, contribution.

In comparison to chemical fertilizers, the environmentally friendly organic fertilizers (composts and manure ) do appear to be the solution to attain sustainability in modern agriculture. However, due to our current limited understanding about the growth stimulating mechanism(s) of organic fertilizers on plant growth, we will not be able to achieve optimal and sustainable agriculture yield by relying solely on organic fertilizers at present. We believe that the combined use of chemical and/or organic fertilizer with the active ingredients obtained from biofertilizers , would help the present day farmers to achieve sustainable farming with maximal yield (Shafi et al. 2012; Qin et al. 2015; Song et al. 2015).

In order to understand how organic fertilizers and phytohormones work in tandem to govern plant growth and development, and the other associated useful plant performance characteristics such as conferring resistance against pathogens, the fundamentals of plant growth mechanisms and cell proliferation regulations by various substrates (sucrose, phytohormones, availability of nutrients, etc.) will be revisited and discussed in depth and in relation to fertilizers.

2 Plant Growth Mechanisms

Plant growth is dependent on meristems, groups of dividing cells that give rise to new plant structures (Steeves and Sussex 1989; Coen and Meyerowitz 1991; Wolters and Jürgens 2009) and enable the plant to increase in size continuously throughout its lifetime (Huala and Sussex 1993). In order to be competent and ready to divide, proliferative cells in the plant meristems have to undergo four distinctive phases of the cell cycle: postmitotic interphase (G1), DNA synthetic phase (S), postsynthetic interphase (G2) and mitosis (M phase); and subdivision phases of mitosis, cytokinesis and G1-phase (G0); all of which are governed by a series of checkpoints (Francis and Sorrell 2001) regulated by cyclin-dependent kinases (CDKs) (Den Boer and Murray 2000; Inzé 2003; Dewitte and Murray 2003) (Fig. 1). Cyclins, CDK inhibitors, retinoblastoma proteins, E2F/DP transcription factors, histones, sucrose and phytohormones (specifically cytokinins, auxins, gibberellin and abscisic acid) are some of the other factors involved in the regulation of the cell cycles (Inzé 2003; Souza et al. 2010).

Fig. 1

Schematic diagram of the plant cell cycle and regulatory actions of phytohormones. Cell cycle has four distinct phases, G1 (postmitotic interphase), S (DNA synthetic phase), G2 (postsynthetic interphase) and M (mitosis); and subdivision phases of mitosis, cytokinesis and G1-phase (G0); all of which are governed by a series of checkpoints, mainly between G1 and S (G1/S checkpoint) and between G2 and M (G2/M checkpoint). The plant cell cycle is predominantly regulated by phytohormones while other factors exerting varying degrees of controls under different environmental (abiotic) and biotic circumstances in which the whole plant is exposed to. Generally, auxins, cytokinins and gibberellin play stimulatory roles; while abscisic acid (ABA), ethylene and jasmonates inhibit the progression of cell cycle. Systemin, a plant peptide hormone, down regulates cell cycle by promoting the biosynthesis of jasmonates, while salicylic acid up-regulates cell cycle progression by inhibiting the biosynthesis of ethylene

Meristems have the ability to divide and differentiate but can only give rise to certain structures; for example, the root meristems give rise to roots while the shoot meristems may result in leaves, flowers, axillary buds and internodes (Huala and Sussex 1993). Primary shoot apical meristem (SAM) that arises during embryogenesis together with additional meristems formed after seed germination plays a crucial role in giving the plants its various forms (Steeves and Sussex 1989). Primary SAM provides the plant with the main axis while the other meristems determine the development of the shoot branches, and the brunches temporal and spatial development determines the complexity of the branching pattern (Shimizu-Sato and Mori 2001). Plant forms although plastic in nature, influenced by environmental cues, are still genetically governed and thus retain their species-specific forms (Shimizu-Sato and Mori 2001).

Axillary meristems are typically located on the adaxial (upper) region where the primary organ, such as leaf axil and stems join (Grbić and Bleecker 2000; Long and Barton 2000; Shimizu-Sato and Mori 2001). Being secondary meristems, axillary meristems are crucial for the continuous development of the plant morphology, such as lateral plant growth (Bennett and Leyser 2006). Lateral plant growth is, however, governed by a mechanism known as apical dominance. Apical dominance, broadly defined as the inhibitory control of the shoot apex over the outgrowth of lateral buds, is one of the mechanisms that ensure the plants survival with a reservoir of meristems to replace damaged primary shoot (Shimizu-Sato and Mori 2001). Apical dominance, however, can be released by development programs, hormonal and environmental cues (Turnbull et al. 1997; Bangerth el al. 2000; Shimizu-Sato and Mori 2001; Schmülling 2002). Increased cytokinin concentrations and changes in phytohormone ratio(s) (e.g. auxins:cytokinins) are some of the hormonal cues that enable the plant to overcome apical dominance.

3 Plant Growth Regulation

Plant growth is regulated by various abiotic and biotic factors such as temperature, light intensity, water availability, soil compositions and characteristics, nutrient availability, phytohormones (plant hormones) availability, interactions with the immediate organisms (microorganisms, fungi, other plants) in its surroundings, and other factors (for reviews, see Steeves and Sussex 1989; Mok 1994; Rolland et al. 2002; Van Loon 2007; Wolters and Jürgens 2009).

In this review, we will focus our discussion on sucrose and phytohormones availability in relation to fertilizer types and usage.

3.1 Regulation by Sucrose Availability

Photosynthesis is the fundamental process in plants and sugars are produced in the process. Sugars are essential for the plant growth as they are the source of carbon, energy and polymer substrates for biosynthesis (Rolland et al. 2002). They are transported via the plants phloem from the sites of photosynthesis (usually the leaves) to the various sink organs such as roots, flowers, developing fruits and seeds, mainly in the form of sucrose. Sucrose concentration plays an important role in regulating various plant growth processes. Low sucrose concentration stimulates leaf photosynthetic activities, nutrient mobilization and export from the sink organs, while high sucrose concentration inhibits photosynthetic activities but stimulate growth and storage in the sink organs (Wang and Ruan 2013). Before sucrose can be utilized for metabolism and biosynthesis, they are normally converted to simpler forms like glucose and fructose by invertase or UDP-glucose and fructose by sucrose synthase (Wang and Ruan 2013). In addition to their essential roles as source of carbon, energy and building blocks for plant growth, sucrose and its cleavage compound hexose have important hormone-like functions as signaling molecules that regulate specific gene expression (Rolland et al. 2002; Wang and Ruan 2013).

A close correlation was observed between the supply of sucrose and the expressions of cyclins, specifically d-type cyclins (CycD2 and CycD3), that induce the cell to progress beyond the G1 phase and become committed to complete the full plant cell cycle (Riou-Khamlichi et al. 2000) (Fig. 1). Expression of CycD2 only requires sucrose while CycD3 expression requires the presence of sucrose and phytohormones, specifically cytokinins and auxins (Koning 1994; Riou-Khamlichi et al. 2000). In other words, sucrose is essential in the upstream regulation prior to hormonal regulation of CycD3 expression. Sucrose is also required for the activation of mitotic entry by activating the transcription of key components that drives the G2 to M transition (Skylar et al. 2011) (Fig. 1). Thus, it is evident that sucrose could be involved in the reactivation of cell from the state of growth arrest (Souza et al. 2010). Apart from driving plant cell cycle transition indirectly, sucrose is also required for general plant growth such as tuberization in potato (Šimko 1994), to induce formation of adventitious roots in Arabidopsis seedlings (Takahashi et al. 2003), to induce flowering (Roldán et al. 1999) and various other growth processes (for a review, see Gibson 2005).

3.2 Regulation by Phytohormones Availability

Phytohormones are naturally occurring substances known to be crucial for regulating various aspects of physiology and development throughout the lifecycle (for reviews, see Bleecker and Kende 2000; Mok and Mok 2001; Pimenta Lange and Lange 2006; Matsubayashi and Sakagami 2006; Wolters and Jürgens 2009; Zhao 2010; Pacifici et al. 2015). Some of the growth regulatory functions include cell division and expansion, cell elongation, stem elongation, inhibition, root growth, activation of bud growth, branch development, promoting or delay in leaf senescence and chlorophyll production. Regulatory functions of cytokinins and auxins will be discussed in detail in the following sections. Other classes of phytohormones, such as gibberellins, ethylene, abscisic acid and strigolactones, are listed in Table 1 and will not be discussed in detail.

Table 1 List of hormonal and non-hormonal plant growth promoters and their known biological functions

3.2.1 Cytokinins

The vast majority of naturally occurring cytokinins are a group of adenine compounds with an isoprene, modified isoprene, or aromatic side chain attached to the N⁶ amino group. Refer to Fig. 2a for the basic structure of cytokinins and Fig. 2b for the representative cytokinins. Cytokinins occur in both free and tRNA-bound forms, and are essential in regulating various physiological processes in plants (Letham and Palni 1983; Haberer and Kieber 2002; Stirk and Van Staden 2010). The biosynthetic gene ipt, which encodes the enzyme isopentenyltransferase, is responsible for the synthesis and expression of cytokinins (Kamínek et al. 1997). This enzyme is produced in the roots and shoots (Chen et al. 1985), with the root apical meristems being the major site of synthesis. Isopentenyltransferase is essential in the first step of cytokinin synthesis. It transfers the isopentenyl moiety from dimethylallyl diphosphate (DMAPP) to ATP/ADP, which is more efficiently utilized by the plant isopentenyltransferase compared to AMP (Kakimoto 2001). In contrast, bacterial cytokinin synthesis, which shares a similar pathway, are able to start the first step by transferring isopentenyl moiety from 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (HMBDP) to AMP (Ueda et al. 2012) (Fig. 3).

Fig. 2

Chemical structures of cytokinins and its basic structure. a Adenine structure and the numbering system for the nomenclature of cytokinins. b Representative cytokinins: trans-zeatin (tz), kinetin (K), N⁶-[2-isopentyl]adenine (iP), N⁶-benzyladenine (BA) and N⁶-isopentyladenosine (iPR). Note that the isoprene, modified isoprene, or aromatic side chains are attached to the N⁶ amino group of adenine compounds

Fig. 3

A model for cytokinins biosynthesis and metabolic pathway in plants and bacteria. In plants, the isopentenyl moiety from dimethylallyl diphosphate (DMAPP) is transferred to ATP/ADP while the bacterial pathways start off with AMP. Bacteria cytokinin biosynthesis may also start by transferring isopentenyl moiety from 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (HMBDP) to AMP. Active cytokinins are shaded (adapted from: Haberer and Kieber (2002), Kakimoto (2003), Sakakibara (2006), Tarkowski et al. (2009) and Frébort et al. (2011))

Cytokinins are classified according to their side-chain configurations as either isoprenoid or aromatic cytokinins (Stirk and Van Staden 2010) with the latter being the rarer form (Kakimoto 2003). Naturally occurring isoprenoid cytokinins are either isopentenyladenine (iP)-type, which carries an isopentenyl N⁶ side chain, or zeatin-type, which carries hydroxylated isopentenyl N⁶ side chain (Kakimoto 2003). Zeatin-type cytokinins can occur in cis or trans configuration, depending on the hydroxylation of the methyl group on the side chain; and trans-zeatin and its derivatives have higher biological activity than the cis forms. Zeatin-type cytokinins are also the main constituents in plants (Mok et al. 2000). Cytokinins also occur in different forms, namely in the form of free base, riboside, or ribotide (or nucleotide); with the free base form being biologically active and the riboside form being the form of transportation via the xylem system. The riboside type of cytokinins are later converted to their active form by another enzyme at the shoot (Sakakibara 2006).

The biological concentrations of these phytohormones are closely regulated by the rates of biosynthesis, metabolism, inactivation and degradation with homeostasis under the influence of both internal and external factors (Sakakibara 2006). Any exchange or interconversion of cytokinins between plants and its various external components can potentially influence and even disrupt cytokinin homeostasis in the plant and alter its growth pattern as low concentrations of cytokinins are needed to elicit a physiological response (Letham and Palni 1983; Stirk and Van Staden 2010).

3.2.1.1 Role of Cytokinins in Plant Growth

Cytokinins are crucial for activating cell division and to regulate plant growth from the cellular level through to the tissue, organ and whole plant level (Letham and Palni 1983; Francis and Sorrell 2001; Schmülling 2002; Sakakibara 2006). On the cellular level, cytokinins upregulate plant d-type cyclin (CycD3) at the G1 checkpoint and the phosphoregulation of the CDK at the G2/M checkpoint, thereby inducing a continuum of cell cycle activation that leads to plant growth (Francis and Sorrell 2001). The main cytokinin driver of the cell cycle is zeatin, which peaks its concentrations at the end of S phase, during the G2/M phase transition and in the late G1 phase.

Developmental processes such as formation of embryo vasculature , nutritional signaling, leaf expansion, branching, chlorophyll production, root growth, promotion of seed germination and delay of senescence are also heavily influenced by cytokinins (Letham and Palni 1983; Mok 1994; Schmülling 2002; Howell et al. 2003). The initiation and outgrowth of axillary buds, released from shoot apical dominance, were reported to be well correlated with the cytokinins levels (Turnbull et al. 1997; Bangerth et al. 2000; Shimizu-Sato and Mori 2001; Yong et al. 2014). It has been known that plants with reduced endogenous cytokinins have distinct morphological and developmental alterations such as shorter shoot internodes, delayed flowering, fewer flowers and reduced leaf surface area with smaller vasculature, smaller shoot apical meristems with reduced cell division, enhanced root growth and a larger root meristem (Schmülling 2002). Thus, any change in the levels of endogenous cytokinins could alter the regulation of the above-mentioned physiological processes and result in the disruption of normal plant growth (Letham and Palni 1983; Schmülling 2002). However, it is also important to note that different classes of phytohormones interact in a synergistic way for regulation of physiological processes and optimum plant growth. The roles of these phytohormones will be discussed in their respective sections.

Cytokinins have been reported to function as local and long-range chemical signals in plants. They are transported via the xylem and phloem (Hwang and Sakakibara 2006) and the transpiration stream from the root tips to aerial plant parts (Yong et al. 2000; Schmülling 2002; Stirk and Van Staden 2010; Yong et al. 2014). Studies conducted by Ma et al. (2002) showed that cytokinins synthesized in the embryo function as local signal for increased meristematic activity. Reallocation of nutrients, minerals and nonmetabolizable substances are also initiated with an increase in cytokinins concentrations in leaves (Leopold and Kawase 1964; Gersani and Kende 1982; Mauk and Noodén 1992), a phenomenon termed as cytokinin-induced nutrient mobilization . It has also been suggested by Frugier et al. (2008) that cytokinins may function as the central signal for controlling lateral organ differentiation. Their study revealed that a local increase in cytokinin concentrations within the roots induces nodule organogenesis while repressing lateral root formation (Fig. 4). Cytokinins functioning as long-range biochemical signals help to coordinate root–shoot development (Schmülling 2002; Stirk and Van Staden 2010), communicate root biotic interactions (e.g. with Rhizobium, Yong et al. 2014) and environmental stresses such as nutritional status, low temperatures, salinity and drought to the shoots (Goicoechea et al. 1996; Yong et al. 2000; Emery and Atkins 2002; Schmülling 2002), a phenomenon termed as root-to-shoot signaling.

Fig. 4

Proposed role of cytokinin in nodulation and infection events. a Nod factor perception by lysin motif (LysM)-containing receptors elicits calcium signaling through a calcium- and calmodulin-dependent kinase (CCaMK). This in turn leads to localized biosynthesis and/or activation of cytokinin signaling by an as yet unknown mechanism. Part of this signaling cascade can be bypassed by bacterially produced cytokinins (Rhizobium pTZS) or cytokinin-like molecules, enabling Nod factor-independent nodulation. Epidermally produced cytokinin might be translocated to the cortex by diffusion and/or by selective transport from cell to cell. Alternatively, an intermediate messenger (*) might travel to the cortex to elicit de novo localized cytokinin signaling. b Cytokinin perception by LHK1 or ortholog MtCRE1, and signaling through cytokinin response regulators (RRs) leads to initiation of nodule organogenesis (cell divisions). This requires transcription factors such as NSP1, NSP2 and ERN, as well as downstream functions, such as NIN and ENOD40. In the epidermis, NIN is required for infection thread formation but also negatively regulates root susceptibility to rhizobial signaling. Hypothetically, cytokinin might participate in this process by signaling through an unknown histidine kinase receptor(s) (HK). In this scenario, cytokinin signaling contributes to, but is not fully responsible for, reprogramming of gene expression, possibly by regulating the activity and/or localization of transcriptional factors, such as NSP2, which is known to relocate from the nuclear envelope to the nucleus upon Nod factor signaling. Cytokinin might also be involved in both local and systemic feedback regulation of infection. In Lotus, LHK1 is not required for initiation and progression of infection events, but it participates in negative regulation of root susceptibility to infection. c In M. truncatula, both nodule inception and infection thread progression, but not initiation, are tightly linked to MtCRE1 function. d Cytokinin might also participate in systemic autoregulatory feedback mechanisms, possibly involving HAR1, to restrict nodule number (reprinted from Trends in Plant Science, 13, F. Frugier, S. Kosuta, J.D. Murray, M. Crespi, K. Szczyglowski, Cytokinin: secret agent of symbiosis, 115–120, Copyright (2008), with permission from Elsevier)

3.2.1.2 Flow of Cytokinins Through the Environment

Cytokinins are widely distributed throughout the plant kingdom (Stirk and Van Staden 2010), and are widely available and highly fluid within the environment (Fig. 5). In the following section, we will discuss the various plant-related sources of cytokinins and their distribution in the environment.

Fig. 5

Diagrammatic scheme showing the movement of cytokinins through the environment. Dotted circles indicate sites of cytokinins biosynthesis. 1 sites of cytokinin biosynthesis in vascular plants including roots, flowers and fruits; 2 cytokinin translocation via the xylem from the roots; 3 cytokinin translocation via the phloem from aerial plant organs; 4 cytokinins derived from degradation of leaf litter; 5 movement of cytokinins through the soil due to rainwater and irrigation; 6 free-living microorganisms (bacteria, fungi, Cyanophyta and Chlorophyta) in the rhizosphere; 7 microorganisms, including nematodes in a symbiotic relationship (either beneficial or parasitic) with the host plant’s roots; 8 cytokinins released into the soil by root exudates and cytokinin uptake by the roots; 9 air-borne microorganisms (bacteria and fungi) in a parasitic relationship with the host plant; 10 insects infecting the host plant to form galls; 11 agricultural input including irrigation with water that contains cytokinins and application of natural (e.g. seaweed concentrates) and synthetic cytokinins for crop improvement (reprinted from Plant Growth Regulation, 62, 2010, 101–116, Flow of cytokinins through the environment, W.A. Stirk and J. Van Staden, Fig. 1, with kind permission from Springer Science and Business Media)

3.2.1.3 Sources of Cytokinins

Cytokinins can be derived from various sources and the most prominent source would be from plants as they are called phytohormones. Cytokinins have been known to be released directly into the soil from plant roots (Van Staden 1976). Studies conducted by Arthur et al. (2001) showed that tomato seedlings metabolize the cytokinins taken up from their external environment before releasing them back to the external environment. Plant parts, such as cotyledons, flowers and leaf litter, are also sources of cytokinins (Letham and Palni 1983; Stirk and Van Staden 2010). Hence, root exudates and any plant parts that contain cytokinins are potential sources that may contribute to the pool of cytokinins available in the environment for the uptake by other plants.

Microalgae of both prokaryotic and eukaryotic nature, namely Cyanophyta and Chlorophyta, respectively, are also natural sources of cytokinins. Ördög et al. (2004), Stirk et al. (2003, 2009) and Burkiewicz (1987) have detected cytokinins and cytokinins-like activity in isolated microalgae samples. Stirk et al. (1999) had successfully proven the presence of cytokinins, specifically iP, in microalgae with the use of GC-MS. Also, studies on Chlorella had expressed highest biosynthetic rates for iPR, iP and cZ in samples harvested 8 h into the light period compared to samples harvested 8 h into the dark period (Stirk et al. 2011). These studies indicated that microalgae are capable of synthesizing cytokinins which might be released into the soil during cell decomposition, thereby further contributing to the pool of cytokinins available for plant uptake (Stirk and Van Staden 2010).

To date, various studies have proven that certain bacteria are important sources of cytokinins (Philip and Torrey 1972; Upadhyaya et al. 1991; Arkhipova et al. 2005; Kudoyarova et al. 2014) and harbor the potential for the discovery and extractions of cytokinins from nature for plant industry applications. Interestingly, there are novel cytokinins that are yet to be discovered and characterized from these microbial sources. Genetic studies have successfully identified biosynthetic gene responsible for the expression of cytokinins in various bacteria (Powell and Morris 1986; Crespi et al. 1992; Binns 1994; for a review, see Taylor et al. 2003). It has also been reported that bacteria enhance or promote plant cytokinin production. Agrobacterium tumefaciens, which induces crown galls in plants, was reported to be capable of transferring and integrating part of their Ti-plasmid DNA into the host plants’ genome (Sakakibara et al. 2005). The integrated bacterial genome which encodes an enzyme, adenosine phosphate-isopentenyltransferase, confers the host plant with the ability to synthesize cytokinin via an alternative biosynthesis pathway leading to increased plant cytokinin production. Unlike plants, bacteria produce iP-type cytokinins. These iP-type cytokinins could be taken up by plants, gets converted to Z (Mok and Mok 2001) and subsequently to ZR. ZR is then transported within the xylem to target sites where it gets cleaved into the bioactive form (Z) to drive growth (active cell cycle) using NPK (raw materials) to achieve optimal growth. Studies conducted by Ueda et al. (2012), however, proved that A. tumefaciens was capable of efficient biosynthesis of tZ during tumor formation in infected galls. Thus, there is a possibility that other bacteria capable of synthesizing zeatin-type cytokinins remains to be discovered.

Apart from bacteria, other studies have also detected cytokinins production in various mycorrhizal fungi . Crafts and Miller (1974) had successfully obtained crystalized Z and ZR from the media in which Rhizopogon roseolus (Corda) Hollos, a fungus, had been cultured; presenting definite evidence for the production of cytokinins by the various mycorrhizal fungi screened. Studies conducted by Barea and Azcón-Aguilar (1982), Ng et al. (1982) and Kraigher et al. (1991) had also successfully detected substances with cytokinin-like activity and cytokinins in growth cultures of the various mycorrhizal fungi screened. Thus, it is possible that many cytokinin-producing fungi are contributing to the pool of cytokinins in the environment and these fungal species have yet to be identified.

Parasitic nematodes, root-knot nematodes ( Meloidogyne species) and cyst nematodes (Heterodera species), the common cause of gall and cyst formation in plant roots (Stirk and Van Staden 2010) are known to be capable of exuding cytokinin-like substances (Bird and Loveys 1980). De Meutter et al. (2003) had detected the presence of cytokinins, specifically BA, iP and Z, from in vitro grown nematode exudates or lysates in concentrations high enough to be biologically active. The secretion of cytokinins together with host plant cytokinins had been reported to trigger a change in the nematodes’ surface lipophilicity (Akhkha et al. 2002), which might be an infection strategy against the host plant’s defense responses. It is also likely that the cytokinins help to establish and/or maintain the feeding cells as a nutrient sink (De Meutter et al. 2003) as roots with overexpressed cytokinins oxidase have reduced gall formation (Lohar et al. 2004).

Similar to nematodes, insect larvae had been reported to synthesize and secrete cytokinins into the plant tissues (Mapes and Davies 2001). Results from the experiments conducted by Van Staden and Bennett (1991) and McDermott et al. (1996) revealed an elevation in cytokinin concentrations in plant tissues that were infected with insect larvae compared to healthy tissue. Elevated concentration of up to 53 times was recorded in the cytokinin profile comparison between the insect larvae with the gall structure and the plant stem tissue. This observation suggested that the insect larvae are capable of synthesizing and secreting their own cytokinins (Mapes and Davies 2001).

Although the roles of cytokinins exuded by nematode and insect larvae are unclear, it is likely that high concentrations of cytokinins are secreted in the gall so that the feeding site remains as an active nutrient sink (Stirk and Van Staden 2010) as elevated concentration of cytokinins may function as local signal for the reallocation of nutrients and photoassimilates; similar to the cytokinin-induced nutrient mobilization phenomenon observed in plants. Thus, both nematodes and insect larvae are also potential sources of cytokinins which could contribute to the environmental cytokinins pool as exudates or lysates.

3.2.2 Auxins

Apart from cytokinins, auxins are another important class of phytohormones that play crucial roles in regulating various plant growth processes (for a review, see Zhao 2010). Auxins are synthesized at the root (Pacifici et al. 2015), in the shoot apex and young expanding leaves (Al-Babili and Bouwmeester 2015). From the site of biosynthesis, auxins are transported basipetally via auxin transporters, which in turn create an auxin gradient across the plant (Al-Babili and Bouwmeester 2015). Peaks of auxin concentration and gradient then act as positional information for auxin activity and maintenance of correct cell division, polarity and fates at the root apex (Sabatini et al. 1999; Al-Babili and Bouwmeester 2015). Auxins also regulate the synthesis of other hormones such as strigolactones (Al-Babili and Bouwmeester 2015), trigger organ primordia initiation and play a role in the initiation and maintenance of the organ founder cell populations (Wolters and Jürgens 2009).

At the cellular level, auxins induce the expression of CycD3 and CDKs, both play crucial roles governing the various checkpoints of the cell cycle, especially in the transition of G1/S phase (Wang and Ruan 2013). Auxin signaling is also required in the later phase of G2/M transition to complete the mitosis process (Wang and Ruan 2013). The mechanisms on how auxins are involved in the initiation of the various cell cycle stages have been reviewed by Wang and Ruan (2013).

Auxins are known to inhibit axillary bud growth, however, the mechanisms of axillary bud outgrowth are dependent on the ratio of cytokinins to auxin rather than the absolute concentration levels of either hormone (Shimizu-Sato and Mori 2001). Direct application of auxin to axillary buds, however, cannot prevent bud growth (Shimizu-Sato and Mori 2001). Apart from regulating axillary bud growth, cytokinin to auxin ratios also determine the development of roots and shoots. A balanced ratio of the two hormones keep the cells in undifferentiated state, while low cytokinin to auxin ratios promote root development and high ratios promote shoot development (Haberer and Kieber 2002). Apart from interacting with cytokinins, interactions between auxins and other phytohormones have been reported as well. Examples include interaction with ABA to regulate root growth (Zhao et al. 2015), with sugar for cell division and expansion regulation (Wang and Ruan 2013) and with brassinosteroids to regulate differential growth (Zhou et al. 2013).

Like cytokinins, auxins can be found in various fertilizer sources and these included vermicomposts (Zhang et al. 2015) and humic acids (Canellas et al. 2002). Hayat et al. (2010) provided a useful listing of bacteria (Azospirillum, Azobacter, Bacillus, Kluyvera, Paenibacillus, Pseudomonas, Rhizobacteria, Rhizobium) that produce auxins although not all the research described in the review paper provided unequivocal evidence for the occurrence of auxins using mass spectrometery. Interestingly, Patten and Glick (2002) provided scientific evidence for a direct role of auxins produced by Pseudomonas putida in regulating mung bean root development.

4 Non-hormonal Plant Growth Promoters (Humic Substances)

Humic substances are the major components of natural organic matter found in soil, water and organic deposits such as sediments, peats, coals, leaf litters and composts (for reviews, see Piccolo et al. 1992; Piccolo 2002; International Humic Substances Society 2007). The bioactive components of humic substances are humic acid and fulvic acid. Fulvic acid is essentially a polymerized humic acid and thus this section will focus on humic substances in general as both humic acid and fulvic acid share similar if not the same chemical characteristics. Both humic and fulvic acids are available commercially in the forms of pellets.

Studies have often shown positive effects of humic substances on seed germination, root initiation and total plant biomass (Rauthan and Schnitzer 1981; Chen et al. 2004). This is achieved via their ability to improve soil structure and their hormone-like activity on plants (Piccolo et al. 1992; Atiyeh et al. 2002). In plants, humic acids are involved in cell respiration, oxidative phosphorylation, protein synthesis, photosynthesis and various enzymatic reactions (Atiyeh et al. 2002) by improving iron, and possibly zinc nutrition (Chen et al. 2004). At the roots, humic acids enhance root initiation, root hair proliferation and mineral nutrient uptake by increasing the permeability of membranes of the root cells (Atiyeh et al. 2002).

Piccolo (2002) stated that the beneficial effects which humic substances have on the physical properties of soil and their role in the soil environment are significantly greater than that attributed to their contributions to sustaining plant growth, and have provided a comprehensive review on how humic substances contribute to the soil properties and environment. Piccolo’s review (2002) provided insights that the hydrophobic nature of humic components protects compounds that are easily degradable and enhance their persistence in soil. This contributes to the accumulation of organic matter which harbors beneficial effects on the rhizosphere and plant growth. Review by Bronick and Lal (2005) also corroborated this view that humic acids help improve the soil condition by increasing the aggregate stability of the soil structure which results in better plant growth and higher yield. Furthermore, humic acid, being a weak acid, could function as a buffer that keeps the soil at the optimal pH for both plant and microbial growth.

5 Biofertilizers

With the ever growing concerns in environmental-related issues and increasing efforts to promote more environmentally friendly farming practices in conventional farms and plantations, the usage of biofertilizers is gaining global acceptance. Unlike conventional chemically synthesized fertilizers that contain N, P and K, biofertilizers are biomass-based structural matrix (e.g. compost , humic acid , etc.) that contain live or latent cells of microorganisms that have the ability to augment nutrients for plant assimilation through microbial processes such as atmospheric nitrogen fixation, phosphate solubilization, cellulolytic degradation and production of phytohormones (Vessey 2003; Van Loon 2007; Lugtenberg and Kamilova 2009; Mishra et al. 2013; Ahemad and Kibret 2014; Owen et al. 2014). It has been reported that the application of biofertilizers to the seeds and the soil, has helped to increase nutrient availability for plant uptake, increased and/or improved plant growth parameters and increased crop yield up to 10–20 % without any adverse effect on the environment (Bhattacharjee and Dey 2014). Thus, biofertilizers are plausible means to tap onto the natural nutrient cycle without posing any threat on the environment.

The use of biofertilizers in our modern farming practice should be encouraged so as to reduce the adverse effects of long-term chemical fertilizers usage. There is currently a wide range of biofertilizers available commercially (please refer to Table 2 for the mode of action of the various types of biofertilizers and their known microorganisms) and we will focus our discussion on PGPR and vermicomposts .

Table 2 Different types of biofertilizers and the known microorganisms

5.1 Plant Growth Promoting Rhizobacteria (PGPR)

PGPR are bacteria found within the rhizosphere and have the ability to promote plant growth (for reviews, see Kloepper et al. 1989; Vessey 2003; Hayat et al. 2010; Ahemad and Kibret 2014) via various mechanisms such as nitrogen fixation, phosphorus and zinc solubilization, which help to enhance the availability of plant nutrients for absorption (Çakmakçi et al. 2006; Mahdi et al. 2010). The use of PGPR has been reported to increase plant uptake of nitrogen from fertilizer (Adesemoye et al. 2010), and aid to sustain soil productivity and environmental health by reducing dependence on chemical fertilizers (Shaviv and Mikkelsen 1993). PGPR are also referred to as biocontrol agents due to their ability to reduce the incidence or severity of plant diseases (Beattie 2006). Applications of PGPR have been investigated in various plants and crops such as maize, wheat, oat, barley, peas, canola, soy, potatoes, tomatoes, lentils, radicchio, cucumber and chickpea (Gray and Smith 2005; Gopalakrishnan et al. 2015).

While some would consider bacteria localized on the epidermis of plant leaves to be PGPR (Maksimov et al. 2011), we consider PGPR to be the bacteria found within the rhizosphere, free-living or in association with plant roots. However, most PGPR are bacteria that form close association with the plants on the root surface (rhizoplane) or penetrate into the radicular tissues of the root. Most bacterial growth usually occurs at the junctions between epidermal cells and areas where side roots appear (Lugtenberg and Kamilova 2009). Some researchers speculated that PGPR must colonize the root surface efficiently, compete well against other microbes present within the same rhizosphere for nutrients secreted by the root and for sites that can be occupied on the root before being able to exert beneficial effects on the plants (Lugtenberg and Kamilova 2009).

In general, PGPR can affect plant growth in two different ways, directly or indirectly. Direct effects include the various positive influences that PGPR have on plant growth which occur in the absence of pathogens (Lugtenberg and Kamilova 2009). Minimizing or preventing deleterious effects of plant pathogenic organisms via production of antagonistic substances or induction of plant resistance against pathogens is referred to as indirect effects (Glick 1995). It is difficult to classify the effects of PGPR on plant growth into the two distinct groups as a direct effect might lead to an indirect influence. For example, production of phytohormones helps to enhance growth (direct effect) but may also induce disease resistance (indirect effect). Thus, our review will not classify the resultant effects into direct or indirect groups.

PGPR share similar functions with the other groups of biofertilizers . The common functions include converting atmospheric nitrogen (Döbereiner 1992) and facilitating the uptake of nutrients such as phosphorus and zinc via solubilizing inaccessible forms trapped in insoluble compounds. Unlike the other groups of biofertilizers that mainly exert one type of positive effect, PGPR enhance plant growth in many more ways. One of the most prominent enhancements is the secretion of bacterial phytohormones (Glick 1995), specifically cytokinins (Philip and Torrey 1972; Upadhyaya et al. 1991), which promotes plant growth. The phytohormones, specifically auxins which the bacteria synthesize using the tryptophan present in root exudates (Kravchenko et al. 2004), also promote better root system formation, thereby enhancing water and nutrient absorption (Patten and Glick 2002). These in turn help the plants to pass through the pathogen-sensitive early development stage more rapidly (Maksimov et al. 2011). This characteristic is especially important as studies with added inorganic nitrogen (to increase nitrogen fixation) suggested that plant growth promotion is caused by the production of plant growth factors such as phytohormones rather than nitrogen fixation (Okon et al. 1998).

Apart from providing the plants with phytohormones, PGPR are also known to stimulate plant growth by releasing volatiles and cofactor pyrrolquinoline quinone (PQQ) (Ryu et al. 2003; Lugtenberg and Kamilova 2009). Volatiles are reported to increase photosynthetic efficiency and chlorophyll content in Arabidopsis thaliana through the modulation of endogenous signaling of glucose and abscisic acid sensing (Zhang et al. 2008). PQQ on the other hand functions as antioxidants and cofactor of enzymes involved in antifungal activity and induction of systemic resistance (Lugtenberg and Kamilova 2009; Choi et al. 2008).

Other major substances known to be synthesized by PGPR and are beneficial to the plants include antibiotics, siderophores and hydrolytic enzymes. PGPR antibiotics are oligopeptides that inhibit cell wall synthesis in pathogens at the cell wall synthesis initiation stage. The antibiotics disrupt the functions of ribosomes and inhibit the formation of initiation complex on small subunit of ribosomes (Maksimov et al. 2011). PGPR antibiotics are said to be effective against Gram-positive and Gram-negative bacteria and pathogenic fungi (Maksimov et al. 2011).

Microbial siderophores synthesis by PGPR is induced by low ferric ion level in the environment. Siderophores have high affinity to ferric ions and have the ability to solubilize and extract ferric ions from mineral or organic complexes (Wandersman and Delepelaire 2004). Thus, this increases the pool of iron available for plant assimilation. It was reported that microbial siderophores help to enhance iron uptake by plants that are able to recognize the bacterial ferric-siderophore complex (Masalha et al. 2000; Katiyar and Goel 2004; Dimkpa et al. 2009). Iron uptake by plants in the presence of other metals such as nickel and cadmium is also enhanced by siderophores (Burd et al. 1998; Dimkpa et al. 2008). By enhancing plant iron uptake, pathogens are deprived of the iron that is much needed for their growth and development and thus reducing the occurrence of plant diseases (Maksimov et al. 2011). Calcium assimilation by plants is also enhanced by siderophores.

The production of bacterial hydrolytic enzymes, e.g. chitinases, glucanases, proteases, lipases, that lyse fungal cells, volatile compounds and their toxins are also ways which PGPR help reduce and/or prevent pathogenic diseases (Neeraja et al. 2010; Maksimov et al. 2011) and suppress nematode populations within the rhizosphere (Youssef and Eissa 2014).

PGPR also help facilitate plant growth and development by reducing the stress response within plants via decreased ethylene levels. Ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) is converted into 2-oxobutanoate and ammonia by bacterial ACC deaminase (Glick et al. 2007), relieving the inhibition of root growth (Van Loon 2007) while rendering the plants to be more resistant against stress due to heavy metals (Ca²⁺ and Ni²⁺), salt, draught and phytopathogenic bacteria (Glick and Bashan 1997; Lugtenberg and Kamilova 2009; Van Loon 2007). Studies by Talaat et al. (2015) also provide concrete evidence on the application of suitable microorganisms to help the plants gain tolerance against salinity stress via alteration of nutrient acquisition and accumulation of compatible solutes.

It is also noteworthy that PGPR play the role of rhizoremediator by degrading soil pollutants (Lugtenberg and Kamilova 2009). Siderophores produced by PGPR are able to aid in the bioremediation of soil pollutants (Crowley 2006) by isolating and degrading heavy metals and toxic organic matters such as metabolites of pathogenic origins (Maksimov et al. 2011) reducing the occurrence of contaminated crops that may pose adverse effects when consumed. Cleaner soil also allows more microorganisms and organisms (such as earthworms) to flourish, conferring beneficial effects on the plants.

Another way that plants benefit from the association with PGPR is the activation of defense mechanism—induced systemic resistance (ISR) against pathogens (Van Loon et al. 1998). Exudates produced by PGPR are able to stimulate ISR by activating components such as lipoxygenases, lipid peroxidases and reactive oxygen species (Maksimov et al. 2011) conferring protection against diseases caused by different organisms (Lugtenberg and Kamilova 2009) by reducing the rate of disease development in terms of severity or number of diseased plants (Van Loon 2007). ISR activation is dependent on jasmonic acid and ethylene signaling (Van Loon 2007). Systemic acquired resistance (SAR) which also enhances resistance against diseases is, however, induced by pathogens and dependent on salicylic acid (SA) signaling (Van Loon 2007). It is important to note that PGPR that elicit ISR in one plant species may not do so in another due to interaction specificity between rhizobacteria and plants (Van Loon 2007).

5.2 Vermicomposts

Another form of biofertilizer that is gaining widespread acceptance globally is vermicompost and vermicompost tea , a leachate of the vermicompost. Vermicompost is the highly valued compost produced by earthworms (for a review, see Edwards et al. 2010). Vermicomposting not only helps to reduce organic wastes in volume, but also turning them into humus-like substance that is finer than compost and generally contains high concentration of mineral matter. This makes vermicompost a very good fertilizer that is up to 70 times more efficient than conventional manure (Červená et al. 2013). The earthworm activity also stimulates and increases the diversity of microbial activity. Typically vermicompost is applied at low concentration to the plant growth medium or as soil drench or foliar spray.

Vermicomposts have been reported to have beneficial effects on plant growth such as improved seed germination, enhanced seedling growth and development, and increased plant productivity (Atiyeh et al. 2002; for a review, see Edwards et al. 2010). It enhances plant growth by improving the physical structure and moisture retention capacity of the soils (Arancon et al. 2004) while supplying the plants with N in stable form (Chaoui et al. 2003) and phytohormones or phytohormone-like compounds produced by the microorganisms present within.

Our group has been actively characterizing the phytohormones in vermicomposts and their leachate (vermicompost tea) . Recently, a new method has been successfully established by our group for the analysis of phytohormones present within vermicompost (Zhang et al. 2015) and quantitative evidence of the various growth regulating factors, such as phytohormones, i.e. cytokinins, auxins, gibberellins and brassinosteroids, present in vermicompost tea and leachate have been provided by Zhang et al. (2014) and Aremu et al. (2015). Hopefully, the phytohormone screening approach developed for vermicomposts can be extended to all types of organic fertilizers. Aremu et al. (2015) have also provided insightful discussion on the importance of different phytohormones on their roles in regulating plant growth and development. Results from these studies indicated that vermicomposts harbor a rich diversity of plant growth promoting factors, specifically phytohormones. The origins of these “subterranean” phytohormones are likely to be linked to the symbiotic microbes living in the gut of the earthworms. There is also a possibility that vermicomposts may contain other factors that are beneficial for the plants that have yet to be detected. The beneficial effects of vermicomposts can also be attributed to the presence of humic acids or growth regulators associated with humic acids as demonstrated by Arancon et al. (2004) and Canellas et al. (2002).

Synergistic relationship between vermicompost and PGPR had been reported to improve plant growth, reduce plant mortality and increase microbial biomass (Sahni et al. 2008; Song et al. 2015). This could be due to the reason that vermicompost contains humus which allows PGPR to thrive well and multiply in population. Thus, farming practice can turn to a new biofertilization regime which utilizes both the vermicomposts and PGPR to reap the full synergistic benefits of these natural resources that are beneficial for the plants and to maintain good soil and environmental health.

6 How Biofertilizers Work in Tandem with Microorganisms and Phytohormones to Influence Plant Growth?

An extensive review of published literature (Gharib et al. 2008; Datta et al. 2009; Edwards et al. 2010; Hayat et al. 2010; Mahdi et al. 2010; Shafi et al. 2012; Ahemad and Kibret 2014; Bhattacharjee and Dey 2014; Qin et al. 2015; Sarma et al. 2015; Song et al. 2015) and our extensive field observations have shown that organic fertilizers, despite having low NPK value, can sometimes produce the same growth promoting effect and/or achieving comparable yields, when compared to plants grown using conventional chemical fertilizers with high NPK ratios (e.g. 10–21) (for reviews, see Shaviv and Mikkelsen 1993; Chen 2006). Hence, there must be some growth-promoting factors present in organic fertilizers and these positive factors are certainly not the NPK mineral nutrients per se, that are driving plant growth and development. These salient and positive growth enhancement observations had been noted by many farmers/growers in the plant industry and a plausible scientific explanation remains elusive. In this review, we provided scientific evidence that the growth promoting factors in biofertilizers modulating plant growth and development are phytohormones, and that the known biological functions of phytohormones are in tandem with the observed physiological characteristics and crop yield (Fig. 6).

Fig. 6

The contribution of chemical, organic and biofertilizers to plant growth and development through cell proliferation. Chemical and organic fertilizers provide nutrients, e.g. N, P, K, that are essential for the basic cellular structural machinery and the biosynthesis of DNA, enzymes, cells walls, etc. These processes are active when the meristematic cells are dividing, i.e. cell undergoes the entire cell cycle process, and new cells are produced. Under normal conditions, plants grow at fairly predictable pace due in part to cell cycle regulation governed predominantly by the availability of resources (mineral nutrients, water), suitable environmental conditions (adequate sunlight, optimal temperature) and phytohormones. Thus, the addition of mineral nutrients via chemical fertilization will not necessarily increase the rate of plant growth per se, when there are other limitations imposed on the plant. The application of biofertilizers supplies the plants with phytohormones (in addition to those synthesized endogenously by the plants) that help the plant meristems to overcome the various cell cycle checkpoints’ “restrictions” and to facilitate active cell proliferation. The calibrated and integrated usage of different fertilizers (both chemical and organic) to supply the plants with ample nutrients for their cellular structural needs and appropriate phytohormonal signals to proceed through cell cycle checkpoints will eventually lead to active plant growth

At the whole plant level and in relation to the plant–soil continuum, the interactions between the whole plant and the microorganisms present in the soil can be best illustrated by Fig. 7. The soil and the entire subterranean root system form a diverse and intimate association of “biological networks and entities” comprising of plant roots, microbes (bacteria, fungi) and many very small organisms (nematodes, earthworms, etc.). Amidst the complex array of biological networks and entities is the soil matrix and water medium where multitudes of biological activities (e.g. microbial biochemical activities like enzyme production, plant exudations [allelopathic] and uptake, ingestion by earthworms, etc.) and interactions are taking place. It is therefore conceivable that the soil matrix and water medium contain many naturally produced substances, biological metabolites and these include the phytohormones and their precursors (see Sect. 5). From a holistic perspective, one may view the entire plant subterranean root system as a “receiver” of the multitudes of biochemical signals and this information allows the plants to “sense” the prevailing soil conditions for water, nutrient and phytohormone availability. The selected biochemical signals are “received” at the root tips, “assimilated”, and sent to the various plant parts. These signals induce most responses at the actively growing areas within the plants. The actively growing areas within any plant are the plant meristems found mainly in the aboveground shoot system: shoot apices, axillary buds, flower buds and the root tips (belowground). The growth rates of these meristems are governed by the various phytohormonal chemical signals arriving there, from the roots. Most of these phytohormonal chemical signals have their origins in the subterranean soil and they normally travel with the transpirational water flow through the xylem and onto many plant parts. Closer to the meristems where there may not be any functional conduits leading from the main transport tubes to the meristems, these chemical signals may travel via the phloem and/or through cell-to-cell linkages via diffusion and onto the meristems. Nevertheless, the xylem represents the main conduit for the root-to-shoot transmission of the phytohormonal signals for plant growth and development, and there are ample scientific evidence to support this growth regulatory mechanism (Yong et al. 2000, 2014; for reviews, see Schmülling 2002; Stirk and Van Staden 2010).

Fig. 7

Schematic diagram to describe how biofertilizers work in tandem with microorganisms and phytohormones to influence plant growth. 1 Soil provides the plants with general nutrients like N, P and K, that serve as cellular structural materials and building blocks for components such as nucleic acids, ATP and enzymes; and cofactors for enzyme activities. Fertilizers, either conventional or organic in origin, are added to restore the “nutrient loss” from biomass removal and/or through leaching or weathering processes. 2 Application of biofertilizers introduces microorganisms (e.g. bacteria and fungi) into the rhizosphere. 3 Microorganisms may be free-living or in symbiosis with the plants (at the roots). 4 Certain microorganisms produce various useful enzymes for improving soil availability of certain nutrients, and other useful substances including phytohormones or possibly, their precursors (mainly cytokinins and auxins). 5 The phytohormones (and/or their precursors), present in the soil, are then taken up by the plant, via the roots, and 6 transported to/or near the sites of active growth, i.e. shoot apices and axillary meristems, through the xylem. 7 The phytohormones help the cells to bypass the G0 phase and 8 proceed beyond the checkpoints (G1/S and G2/M) that results in active cell divisions, producing more cells at the meristems, and leading eventually to plant growth. 9 Phytohormones transported to the other parts of the plants also regulate various biological processes such as the opening of stomata, chloroplast production, release from apical dominance, flower development and root development. 10 Earthworms produce vermicompost that contains phytohormones (through earthworms’ intestinal microbial activities) that are released into the soil and can be taken up by the plants. 11 Vermicompost also contains humus that allows beneficial bacteria (either of earthworm gut origin and/or soil origin) to thrive and multiply, and thereby increasing the bioactivity of the soil. Humus also help to improve the soil structure that allow better water and air movement within the rhizospheres and thereby increasing soil fertility

6.1 Improving Current Fertilizer Regime in Light of the Linkages Between Plant Growth and Microorganisms-Derived Phytohormones

For the general plant industry at present, the sole application of each type of fertilizer (either chemical or organic) appears to be the current trend adopted by many growers/farmers. However, there is a gradual and progressive shift by some of the conventional farmers/growers toward using organic additives (organic fertilizers and biofertilizers ) to supplement their chemically based NPK fertilizer applications in the farms/plantations. This is because many conventional farmers/growers have realized that the long-term and prolonged usage of chemical fertilizers on their lands had somewhat led to a drop in plant growth promoting efficacy and subsequently lower yield. Due to the complexity of these biotic and abiotic processes (some of which are still unclear) and interactions, it was often impossible to give farmers/growers adequate scientific explanations. Some research suggested a plausible link between lower yields (despite increasing chemical fertilization) and diminishing microbial activities (“poor soil health”). Such ambiguity paves the way for more intensive research toward understanding the role of soil microbes (and their associated phytohormonal content), in relation to plant growth and fertilizer formulations. Nevertheless, various scientific studies had been conducted on the use of a combination of fertilizers, chemical and organic (Shafi et al. 2012), organic and biofertilizers (Gharib et al. 2008; Sarma et al. 2015), vermicompost and PGPR (Song et al. 2015) and even a combination of all three types (chemical, organic and biofertilizers; Datta et al. 2009). The studies have proven that the integrated use of different fertilizers is highly beneficial in terms of crop yield and environmental friendliness.

Some conventional plant industries have started using humic acid (commercially available in pellet forms) and/or biocharcoal, in their farming practices. Humic acid pellets are known to improve the soil properties, which are likely to be degraded by prolonged chemical fertilizer application. Other conventional farmers/growers have used vermicomposts which not only improve the soil composition and structure but also introduce beneficial microorganisms. These microorganisms in turn provide the necessary phytohormones needed to support plant growth and development. While organic fertilizers and biofertilizers contain phytohormones (varying levels) and are environmentally friendly, they may fail to deliver stable and predictable growth stimulation comparable to chemical fertilizers as it is difficult to manage the batch–batch variation in microbial activities and missing mineral nutrients. Moving forward, plant industry that uses organic fertilizer or biofertilizers could consider fortifying their fertilizers with selected micro- and macromineral nutrients so as to supplement the low amount of nutrients, especially NPK and other trace elements, to improve or to maintain the yield.

6.2 The Novel Futuristic Green Biofertilizer—With Microbial Phytohormones

In our current plant industry and fertilization practices, the shortage of mineral nutrients can be assessed rapidly and easily overcome by adding fertilizers. However, the apparent shortage of phytohormones in promoting plant growth and development has never been recognized as an important tenet of whole plant nutrition. Ironically, the importance of phytohormones in promoting in vitro plant growth and development in the plant tissue culture industry is widely recognized and supplementing phytohormones within a desired mineral nutrient formulation is the standard practice (for reviews, see George and Sherrington 1984; George 1993). Unknown to many farmers/growers from the scientific perspective of plant cell cycle regulation through phytohormones, the use of organic fertilizers and biofertilizers (with their microbes and naturally occurring phytohormones and especially cytokinins) by either an organic farm (routinely) or as a periodic supplement to their conventional farming methods, help to increase the levels of growth promoting phytohormones in the subterranean root environment. With the new understanding about the pivotal role of phytohormones in regulating plant cell proliferation when mineral nutrients are sufficient, we believe that plant industry productivity or yield can be further enhanced with the supplementation (or natural occurrence) of phytohormones like cytokinins, auxins, gibberellins, etc., within the current fertilizer formulation, and possibly through microbial avenues with varying levels of controlled release technologies (Bashan et al. 2014).

Moving forward, we propose a new practice of introducing phytohormones, specifically cytokinins, in current agricultural/horticultural plant nutrition methodologies. Cytokinins appear to be one of the limiting factors in regulating plant growth due to its scarcity and fluidity in the fragile subterranean environment, as discussed in earlier sections. Thus, we believe that agricultural/horticultural yield/productivity can be greatly enhanced with the supplementation of cytokinins and other phytohormones in fertilizers. The preferred sources of cytokinins in any futuristic green biofertilizer should preferably be “natural” and originating from microorganisms (e.g. bacteria like Azospirillum and Rhizobium) and natural sources (coconuts, macroalgae, or seaweeds) that are widely available to the growers/farmers (for reviews, see Letham and Palni 1983; Stirk et al. 2003; Ördög et al. 2004; Yong et al. 2009). Microbial production of cytokinins has been well documented (Phillips and Torrey 1972; Ng et al. 1982; Burkiewicz 1987; Kraigher et al. 1991; Upadhyaya et al. 1991; Arkhipova et al. 2005; Kudoyarova et al. 2014) and could be cultured economically in large quantity using bioreactors and formulated for agricultural/horticultural use (Fig. 8). Thus, cytokinin-producing bacteria harbors the greatest potential for large-scale cytokinin production and can be developed into the next generation of green fertilizer for agricultural/horticultural applications.

Fig. 8

Formulations of inoculants as biofertilizers for agricultural and environmental uses. (reprinted from Plant and Soil, 378, 2014, 1–33, Advances in plant growth promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013), Y. Bashan, L.E. de-Bashan, S.R. Prabhu, Juan-Pablo Hernandez, Fig. 2, with kind permission from Springer Science and Business Media)

The new generation of green biofertilizer is likely to come in the form of granules, and/or coated by natural/hydrophobic polymers or as matrices in which the active plant growth promoting (e.g. cytokinins, auxins, gibberellins, etc.) and soil improving substances (e.g. humic acid) , and/or suitable microbes, with carefully selected mineral nutrients, are embedded in packing materials (e.g. alginate) that restricts the rapid dissolution of the fertilizer, and consequently providing the plants with a sustained source of phytohormones, mineral nutrients, amino acids, proteins, etc. (Fig. 9). The current slow and controlled release or “stabilized” fertilizers is a useful design template to develop the futuristic fertilizer (for reviews, see Shaviv and Mikkelsen 1993; Bashan et al. 2014). The selection of specific mineral nutrients to be embedded in the novel futuristic green fertilizer should be formulated only after a proper chemical analysis of the targeted soil type or locality had been carried out. In situations where certain macro- and micronutrients may be biologically unavailable, suitable and effective microbial populations for solubilizing the chemically fixed phosphorus and to improve the availability of other macro and micronutrients, could be added onto these granules. When there are threats to plant health, the novel fertilizer may be formulated to have some levels of biocontrol properties against certain pathogens. Some other organic compounds such as humic acids and vermicomposts with favorable soil structure improving properties (such as water and mineral nutrient retention) and soil microbial activity-enhancing properties, could also be added in order to augment the overall performance of the novel futuristic green biofertilizer. Bashan et al. (2014) provided interesting and useful methodologies toward designing varying levels of controlled release centered on either a seed (Fig. 10a) or a microbial pellet (Fig. 10b). These useful microbial-inoculant design conceptual templates can be broaden further to include the futuristic green fertilizer granule design that may carry some useful substances (see Sects. 3.2 and 5) and perhaps, concomitantly embedding two different microbes with different biological functions in the soil. With above-mentioned benefits, together with a reasonable storage life (up to 3–6 months), the novel futuristic green biofertilizer in the form of granules is likely to be favored by the conventional farmers/growers.

Fig. 9

Encapsulating bacteria for use in industrial fermentation and as an inoculant for agricultural/environmental use (reprinted from Plant and Soil, 378, 2014, 1–33, Advances in plant growth promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013), Y. Bashan, L.E. de-Bashan, S.R. Prabhu, Juan-Pablo Hernandez, Fig. 3, with kind permission from Springer Science and Business Media)

Fig. 10

Schematic representation of the various strategies available for a multilayered seed inoculation and b microbial pellets for soil inoculation (reprinted from Plant and Soil, 378, 2014, 1–33, Advances in plant growth promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013), Y. Bashan, L.E. de-Bashan, S.R. Prabhu, Juan-Pablo Hernandez, Fig. 4, with kind permission from Springer Science and Business Media)

The ideal fertilizers should have the following three characteristics:

• A single application should supply enough mineral nutrients and phytohormones throughout the entire growing season to meet plant demand for optimum growth;

• Maximal plant growth stimulation thus allowing the largest financial return for the cost of input;

• Minimum detrimental ecological effects on the soil, water and atmospheric environment.

7 Conclusion

Plant growth is dependent on meristems where cell proliferations give rise to new plant structures and allow the plant to increase in size. We provided scientific linkages and evidence to show that the growth promoting factors in biofertilizers regulating cell proliferation and ultimately modulating plant growth and development are phytohormones. The known biological functions of phytohormones (cytokinins, auxins, gibberellins, etc.) are in tandem with the observed physiological characteristics and crop yield of plants. When light, water and mineral nutrients are not limiting, phytohormones, especially cytokinins, in biofertilizers help to drive plant growth by progressing faster through the various plant cell cycle checkpoints leading to the production of more cells.

There is an enormous diversity of microbes found within the soil matrices of the subterranean environment where the plant root system exists. Within the rhizosphere, there is a group of PGPR bacteria that has the ability to promote plant growth via various mechanisms such as nitrogen fixation, phosphorus and zinc solubilization. Another prominent ability of some PGPR is the secretion of bacterial phytohormones, like cytokinins and auxins and other useful substances. Phytohormone-producing PGPR harbors the greatest potential for large-scale phytohormone (especially cytokinins) production and can be developed into the next generation of green fertilizer with microbial phytohormones and/or microbial inoculant for agricultural/horticultural applications.

Evidently, the above literature reviews and discussions reveal that biofertilizers bring about great advantages and improvements to our modern and intensive conventional agriculture practice. In addition, although not discussed in depth in this review, certain biofertilizers may also confer natural biocontrol property that would be useful for disease management. The long-term and fundamental sustainable criterion for any futuristic farmland/plantation is essentially soil health. Good soil provides the foundation for healthy plant growth with minimal external mineral nutrient addition. However, despite its environmental friendliness, any fertilizer regime that solely relies upon biofertilizer may not be feasible in terms of crop productivity per unit area when compared to conventional agriculture practice. Large amount of biofertilizers would be required as they generally contain lower mineral nutrient content, variable elemental composition and/or releasing nutrients at a much slower rate that is unable to sustain maximum plant growth over a limited time period (Chen 2006). Thus, for those involved solely in organic farming practices, we suggest that their organic fertilizers be fortified with selected macro- and micronutrients when there is a drop in horticultural productivity/crop yield. Conversely, for those involved in conventional farming practices, adding organic and biofertilizers periodically is the remedy to reduce chemical fertilizer usage while maintaining their expected yields. Under certain circumstances, selective biofertilizer application to support conventional farming practices is considered the best way to restore the effective microbial populations in order to solubilize chemically fixed phosphorus and to improve the availability of other macro- and micronutrients for plant uptake.

In the near future, we envisage that a hybrid approach of combining organic and conventional fertilization regimes will be widely accepted throughout the global plant industry. This is evident from the 15-year-long study conducted by Qin et al. (2015) which demonstrated that the combined application of organic fertilizers (manure was investigated in that study), together with chemical fertilizer are of great importance to improving agricultural economy as well as sustaining soil heath and quality. Moving forward, the new generation of green biofertilizer should come in the form of granules in which the active plant growth promoting (e.g. cytokinins, auxins, gibberellins, etc.) and soil improving substances (e.g. humic acid) and/or suitable microbes (“inoculants”), with carefully selected mineral nutrients, are embedded in the packing materials giving slow and sustained release over a desired period. The futuristic green fertilizers should provide the plants with a sustained source of phytohormones and mineral nutrients. In situations where certain macro- and micronutrients may be unavailable, suitable and effective microbial populations for solubilizing the chemically fixed phosphorus and to improve the availability of other macro- and micronutrients could be added onto these green fertilizer granules.