Keywords

8.1 Introduction

In modern cultivation process, indiscriminate use of fertilizers, particularly the nitrogenous and phosphorous, has led to substantial pollution of soil, air, and water. Excessive use of these chemicals exerts deleterious effects on soil microorganism, affects the fertility status of soil, and also pollutes the environment. The application of these fertilizers on a long-term basis often leads to reduction in pH and exchangeable bases, thus making them unavailable to crops, and the productivity of crop declines (Meena et al. 2013a; Bahadur et al. 2014; Maurya et al. 2014; Jat et al. 2015; Kumar et al. 2015, 2016b; Ahmad et al. 2016). To obviate this problem and obtain higher plant yields, farmers have become increasingly dependent on chemical sources of nitrogen and phosphorus. Besides being costly, the production of chemical fertilizers depletes nonrenewable resources, the oil and natural gas used to produce these fertilizers, and poses human and environmental hazards. Sustainable agriculture is vitally important in today’s world because it offers the potential to meet our future agricultural needs, something that conventional agriculture will not be able to do. Recently there has been a great interest in eco-friendly and sustainable agriculture (Meena et al. 2016a, b; Parewa et al. 2014; Prakash and Verma 2016; Priyadharsini and Muthukumar 2016; Kumar et al. 2016a, 2017).

Toward a sustainable agricultural vision, crops produced need to be equipped with disease resistance, salt tolerance, drought tolerance, heavy metal stress tolerance, and better nutritional value (Vejan et al. 2016). To fulfill the above desired crop properties, one possibility is to use soil microorganisms (bacteria, fungi, algae, etc.) that increase the nutrient uptake capacity and water use efficiency. Among these potential soil microorganisms, bacteria known as PGPR are the most promising. In this sense, PGPR may be used to enhance plant health and promote plant growth rate without environmental contaminations. PGPR are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion (Saharan and Nehra 2011).

PGPR shows an important role in the sustainable agriculture industry. The increasing demand for crop production with a significant reduction of synthetic chemical fertilizer and pesticide use is a big challenge nowadays. The use of PGPR has been proven to be an environmentally sound way of increasing crop yields by facilitating plant growth through either a direct or indirect mechanism (Meena et al. 2015a, b, f; Raghavendra et al. 2016; Zahedi 2016; Rawat et al. 2016; Jaiswal et al. 2016; Jha and Subramanian 2016). The mechanisms of PGPR include regulating hormonal and nutritional balance, inducing resistance against plant pathogens, and solubilizing nutrients for easy uptake by plants. In addition, PGPR show synergistic and antagonistic interactions with microorganisms within the rhizosphere and beyond in bulk soil, which indirectly boosts plant growth rate. For decades, varieties of PGPR have been studied and some of them have been commercialized, including the species Pseudomonas, Bacillus, Enterobacter, Klebsiella, Azotobacter, Variovorax, Azospirillum, and Serratia (Vejan et al. 2016). The successful utilization of PGPR is dependent on its survival in soil, the compatibility with the crop on which it is inoculated, the interaction ability with indigenous microflora in soil, and environmental factors. According to Nakkeeran et al. (2005), an ideal PGPR should possess high rhizosphere competence, enhance plant growth capabilities, have a broad spectrum of action, be safe for the environment, be compatible with other rhizobacteria, and be tolerant to heat, UV radiation, and oxidizing agent (Yasin et al. 2016; Meena et al. 2016c, d; Saha et al. 2016a, b; Yadav and Sidhu 2016; Dotaniya et al. 2016).

8.2 Plant Growth-Promoting Rhizobacterial (PGPR) Forms

Plant growth-promoting rhizobacteria can be classified into extracellular plant growth-promoting rhizobacteria (ePGPR) and intracellular plant growth-promoting rhizobacteria (iPGPR). ePGPR may exist in the rhizosphere, on the rhizoplane, or in the spaces between the cells of root cortex, while iPGPR locate generally inside the specialized nodular structures of root cells (Gupta et al. 2015; Verma et al. 2014, 2015a, b; Sharma et al. 2016; Meena et al. 2013b, 2014a; Das and Pradhan 2016; Dominguez-Nunez et al. 2016) (Table 8.1).

Table 8.1 PGPR and their effect on growth parameters/yields of crop/fruit plants
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The bacterial/rhizobacterial genera such as Agrobacterium, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Burkholderia, Caulobacter, Chromobacterium, Erwinia, Flavobacterium, Micrococcus, Pseudomonas, and Serratia belong to ePGPR. iPGPR belongs to the family of Rhizobiaceae which includes Allorhizobium, Bradyrhizobium, Mesorhizobium and Rhizobium, endophytes, and Frankia species, both of which can symbiotically fix atmospheric nitrogen with the higher plants (Bhattacharyya and Jha 2012; Shrivastava et al. 2016; Velazquez et al. 2016; Meena et al. 2015c, e; Teotia et al. 2016; Bahadur et al. 2016b).

PGPR plays an important role in enhancing plant growth through a wide variety of mechanisms. The mode of action of PGPR that promotes plant growth includes (i) abiotic stress tolerance in plants, (ii) nutrient fixation for easy uptake by plant, (iii) plant growth regulators, (iv) production of siderophores, (v) production of volatile organic compounds, and (vi) prevention of plant diseases (by production of protection enzyme such as chitinase, glucanase, and ACC deaminase) (Fig. 8.1).

Fig. 8.1
figure 1

Direct and indirect role of PGPR in enhancing plant growth

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However, the mode of action of different PGPR varies depending on the type of host plants. Plant growth is influenced by a variety of stresses due to the soil environment, which is a major constraint for sustainable agricultural production (Sindhu et al. 2016; Meena et al. 2014b, 2015d; Singh et al. 2016).

8.3 Abiotic Stress Tolerance in Plants

Abiotic stresses affect the productivity of agricultural crops as well as the microbial activity in soil. PGPR mitigate most effectively the impact of abiotic stresses (drought, low temperature, salinity, metal toxicity, and high temperature) on plants through the production of exopolysaccharides (EPS) and biofilm formation (Nada et al. 2012). Symbiotic fungi (arbuscular mycorrhizal fungi) and dual symbiotic systems (endophytic rhizospheric bacteria and symbiotic fungi) also tend to mitigate the abiotic stress in plants. Plant growth-promoting rhizobacteria (PGPR) colonize the rhizosphere of many plant species and confer beneficial effects, such as increased plant growth and reduced susceptibility to diseases caused by plant pathogenic fungi, bacteria, viruses, and nematodes (Kloepper 2004; Singh et al. 2015; Meena et al. 2013c, 2016e; Bahadur et al. 2016a; Masood and Bano 2016).

8.3.1 Drought

Drought stress limits crop growth and productivity, especially in arid and semiarid regions. Some microbial species and/or strains that inhabit plant rhizosphere use different mechanisms to mitigate negative effects of drought on plants (Table 8.2).

Table 8.2 Effect of microorganisms on drought mitigation in crops
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PGPR mitigate the impact of drought on plants through a process called induced systemic tolerance (IST), which includes (a) bacterial production of cytokinins, which causes the accumulation of abscisic acid (ABA) in leaves, which in its turn results in the closing of stomata; (b) the production of antioxidants (e.g., the enzyme catalase) which causes the degradation of reactive forms of oxygen; and (c) the bacterial-produced ACC deaminase which degrades the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). PGPR also produce osmolytes and bacterial exopolysaccharides (EPS) to ensure survival of plants under drought-stressed conditions (Kaushal and Wani 2016). Some PGPR also elicit physical or chemical changes related to plant defense, a process referred as “induced systemic resistance” (ISR) (Van Loon 2004; Meena et al. 2017). Timmusk and Wagner (1999) reported that inoculation with the PGPR Paenibacillus polymyxa enhanced the drought tolerance of Arabidopsis thaliana.

PGPR mitigate the impact of drought on plants through a process so-called induced systemic tolerance (IST), which includes: (a) Bacterial production of cytokinins, which causes the accumulation of abscisic acid (ABA) in leaves, which in its turn results in the closing of stomata, (b) The production of antioxidants (e.g., the enzyme catalase) causes the degradation of reactive forms of oxygen, (c) The bacterial-produced ACC deaminase degrades the ethylene precursor 1-aminocyclopropane- 1-carboxylate (ACC). PGPR also produce osmolytes and bacterial exopolysaccharides (EPS) to ensure survival of plants under drought stressed conditions (Kaushal and Wani 2016). Some PGPR also elicit physical or chemical changes related to plant defence, a process refers as ‘induced systemic resistance’ (ISR) (Van Loon 2004). Timmusk and Wagner (1999) reported that inoculation with the PGPR Paenibacillus polymyxa enhanced the drought tolerance of Arabidopsis thaliana.

8.3.2 Excess Moisture

With the changing climatic scenario these stressful conditions of excessive moisture, microorganisms take up the available oxygen while toxic substances accumulate in the soil. In such conditions, plants reduce the permeability of roots, water absorption, and nutrient uptake, which reduce the growth of aboveground plant parts and roots (Nada et al. 2012). Provoked by excessive moisture, roots release large quantities of aminocyclopropane carboxylate-1 (ACC) into the soil. Some groups of bacteria degrade ACC and reduce its concentration in the soil by secreting the enzyme ACC deaminase. In excessively moist soil, bacteria such as Enterobacter cloacae and Pseudomonas putida predominate over fungi and actinomycetes (Grichko and Glick 2001). Mycorrhizal fungi mitigate the stress caused in plants by excessive moisture.

8.3.3 Temperatures

All organisms respond to a sudden increase in temperature by inducing the synthesis of specific group of polypeptides known as heat shock proteins (HSPs). HSPs consist of chaperons (such as GroEL, DnaK, DnaJ, GroES, ClpB, ClpA, ClpX, small heat shock proteins (sHSP), and proteases). Chaperons are involved in the proper folding of denatured proteins, and proteases are required for the degradation of irreversibly damaged proteins (Grover et al. 2011). A thermotolerant P. aeruginosa strain AMK-P6 isolated from semiarid location showed induction of HSPs when exposed to high temperature (Ali et al. 2009). Some bacterial species and strains affect plant tolerance to high temperature. So, Pseudomonas sp. strain NBRI0987 causes thermotolerance in sorghum seedlings, which consequently synthesize high molecular weight proteins in leaves, thus increasing the plant biomass. Meena et al. (2015) also reported that Pseudomonas aeruginosa (strain 2CpS1) reduced cell membrane injury (%) in wheat under high temperature stress.

The bacterium Burkholderia phytofirmans PSJN colonizes grapevine residues and protects the plant against heat and frost through increases in the levels of starch, proline, and phenols (Ait Barka et al. 2006). Inoculation of wheat seeds with Serratia marcescens strain SRM and Pantoea dispersa strain 1A increases the seedling’s biomass and nutrient uptake at low temperatures. In an experiment, Turan et al. (2012) reported that application of Bacillus megaterium M3 and Bacillus subtilis OSU142 alleviates the low-temperature deleterious effect in wheat and barley.

8.3.4 Salinity

Microorganisms use different mechanisms to alleviate the salinity stress in agricultural crops (Table 8.3). Inoculation of wheat seedlings with bacteria that produce exopolysaccharides (EPS) affects the restriction of sodium uptake and stimulation of plant growth under conditions of stress caused by high salinity. Corn, beans, and clover inoculated with AM fungi improved their osmoregulation and increased proline accumulation which resulted in salinity resistance.

Table 8.3 Effect of microorganisms on mitigation of salinity stress in agricultural crops
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8.3.5 Heavy Metals

Heavy metals affect the soil microbial population, their effects depending on the element in question and its concentration on one side and the bacterial species/strain on the other. Some heavy metals are essential micronutrients that are required in small quantities for the growth of microorganisms and plants. Microorganisms bind soluble heavy metals in three ways (biosorption, bioaccumulation, and the binding by metabolic products), which indirectly reduce the negative impact of heavy metals on plants. Methylobacterium oryzae and Burkholderia sp. reduce nickel and cadmium stress in tomato by reducing their uptake and translocation (Marquez et al. 2007). Heavy metals such as Cd, Ni, and Pb disrupt the water regime in plants. Proline accumulation in plant cells is a biomarker for stress induced by heavy metals (Ciupa et al. 2013).

8.4 Plant Growth Regulators

A wide range of microorganisms found in the rhizosphere are able to produce substances that regulate plant growth and development. Plant growth-promoting rhizobacteria produce phytohormones such as auxins, cytokinins, gibberellins, and ethylene which can affect cell proliferation in the root architecture by overproduction of lateral roots and root hairs with a subsequent increase of nutrient and water uptake (Arora et al. 2013).

Indoleacetic acid (IAA): Among plant growth regulators, IAA is the most common natural auxin found in plants and its positive effect on root growth. Rhizobacterial strains synthesize ~80% of IAA colonized the seed or root surfaces is proposed to act in conjunction with endogenous IAA in plant to stimulate cell proliferation and enhance the host’s uptake of minerals and nutrients from the soil. The IAA affects plant cell division, extension, and differentiation; stimulates seed and tuber germination; increases the rate of xylem and root development; controls processes of vegetative growth; initiates lateral and adventitious root formation; mediates responses to light, gravity, and florescence; and affects photosynthesis, pigment formation, biosynthesis of various metabolites, and resistance to stressful conditions (Spaepen and Vanderleyden 2011). Tryptophan is an amino acid commonly found in root exudates and has been identified as main precursor molecule for biosynthesis of IAA in bacteria. The biosynthesis of indoleacetic acid by plant growth-promoting rhizobacteria involves formation via indole-3-pyruvic acid and indole-3-acetic aldehyde, which is the most common mechanism in bacteria like Pseudomonas, Rhizobium, Bradyrhizobium, Agrobacterium, Enterobacter, and Klebsiella (Shilev 2013).

Cytokinins, gibberellins and ethylene: Several plant growth-promoting rhizobacteria, e.g., Azotobacter sp., Rhizobium sp., Pantoea agglomerans, Rhodospirillum rubrum, Pseudomonas fluorescens, Bacillus subtilis, and Paenibacillus polymyxa, can produce either cytokinins or gibberellins or both for plant growth promotion. Ethylene is a key phytohormone that has a wide range of biological activities that can affect plant growth and development in a large number of different ways including promoting root initiation, inhibiting root elongation, promoting fruit ripening, promoting lower wilting, stimulating seed germination, promoting leaf abscission, and activating the synthesis of other plant hormones (Glick 2007). The high concentration of ethylene induces defoliation and other cellular processes that may lead to reduced crop performance. The enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) is a prerequisite for ethylene production, catalyzed by ACC oxidase. Iqbal et al. (2012) reported improved nodule number, nodule dry weight, fresh biomass, grain yield, straw yield, and nitrogen content in grains of lentil as a result of lowering of the ethylene production via inoculation with plant growth-promoting strains of Pseudomonas sp. containing ACC deaminase along with R. leguminosarum. Currently, bacterial strains exhibiting ACC deaminase activity have been identified in a wide range of genera such as Acinetobacter, Achromobacter, Agrobacterium, Alcaligenes, Azospirillum, Bacillus, Burkholderia, Enterobacter, Pseudomonas, Ralstonia, Serratia, Rhizobium, etc. (Kang et al. 2010).

8.5 Role of PGPR as a Biofertilizer

PGPR have the activity to fix atmospheric nitrogen and also enhance nutrient uptake from soils, thus reducing the need for fertilizers and preventing the accumulation of nitrates and phosphates in agricultural soils. A reduction in fertilizer use would lessen the effects of water contamination from fertilizer runoff and lead to savings for farmers. As per findings of Mishra et al. (2013),

biofertilizer is a mixture of live or latent cells encouraging N-fixing, P-solubilization, K- solubilization and Zn-solublization or cellulolytic microorganisms used for biopriming of soil, seed, roots, or composting areas with the purpose of increasing the quantity of those mutualistic beneficial microorganisms and accelerating those microbial processes, which augment the availability of nutrients that can then be easily assimilated and absorbed by the plants.

Meanwhile, biofertilizer products are usually based on the plant growth-promoting microorganisms (PGPMs). These PGPMs can be classified into three dominant groups of microorganisms, arbuscular mycorrhizal fungi (AMF), PGPR, and N-fixing rhizobia, which are deemed to be beneficial to plant growth and nutrition. However, it has been reported that PGPR have been used worldwide as biofertilizers, contributing to increased crop yields and soil fertility (Gupta et al. 2015). Hence, with the potential contribution of the PGPR, this leads to sustained agriculture and forestry. Previous studies show that a biofertilizer prepared by combining PGPR with composts could enhance PGP effects and biocontrol of plants. Bacillus sp. and Pseudomonas sp. are two PGPR that have been reported to be effective biocontrol agents. Among these bacterial species, Bacillus subtilis, Bacillus amyloliquefaciens, and Bacillus cereus are the most effective species at controlling plant diseases through various mechanisms. The ability to form endospores allows PGPR, especially Bacillus sp. and Pseudomonas sp., to survive in a wide range of environmental conditions, thus facilitating the effective formulation of biofertilizer. Sufficient densities of PGPR in biofertilizer provide a beneficial role in creating a proper rhizosphere for PGPR and converting nutritionally important elements through biological process, for example, increasing the availability of N, P, and K, as well as inhibiting pathogen growth. The high availability of N, P, and K could enhance soil fertility, improve antagonistic isolate’s biocontrol effects, and extend microorganisms survival rates in soil (Vejan et al. 2016).

8.6 Biological Nitrogen Fixation (BNF)

The use of chemical nitrogen fertilizers in agriculture not only depletes nonrenewable energy resources but also poses human and environmental hazards, besides being very expensive. For sustainable crop production, PGPR may be used to enhance plant health and promote plant growth rate without environmental contamination (Armada et al. 2014). The efficient BNF microorganisms are considered one of the major mechanisms by which plants benefit from the association of micropartners. One of the benefits that diazotrophic microorganisms provide to plants is fixed nitrogen in exchange for fixed carbon released as root exudates (Zahir et al. 2004). The BNF contributes 180 × 106 Mt/year globally, out of which symbiotic associations produce ~80% and the rest comes from free-living or associative systems (Saharan and Nehra 2011). The use of biofertilizer and bio-enhancer such as N-fixing bacteria and beneficial microorganism can reduce chemical fertilizer applications and consequently lower production cost (Meena 2013). PGPR can fix atmospheric nitrogen either symbiotically or nonsymbiotically.

8.7 Symbiotic Nitrogen Fixation

Two groups of nitrogen-fixing rhizobacteria (NFR) have been studied extensively, which includes Rhizobia and Frankia. The Frankia forms root nodules in symbiosis with more than 200 species of nonleguminous woody plants in 24 genera of angiosperms (Welsh et al. 2009). When rhizobia colonize the roots from nonlegume plant in a nonspecific relationship, the strains from this genus may behave as PGPR (Saharan and Nehra 2011). The beneficial effects of the symbiotic association between rhizobia and legumes are well known, and these have been intensively investigated. Moreover, previous studies have shown that free-living bacteria as well as rhizobial strains can promote the growth of cereal plants by contributing to N-economy through their ability to fix N2 (Biswas et al. 2000).

8.8 Nonsymbiotic Nitrogen Fixers

The nonsymbiotic biological N fixation is basically carried out by free-living diazotrophs that belong to the genera like Azospirillum (Bashan and de-Bashan 2010), Burkholderia, Gluconacetobacter, Pseudomonas (Mirza et al. 2006), Azotobacter, Arthrobacter, Acinetobacter, Bacillus, Enterobacter, Erwinia, Flavobacterium, Klebsiella, Acetobacter, etc. associated with the plant rhizosphere and fix atmospheric N2 into form which is taken up by the plants. These are free-living rhizobacteria and live outside the plant cells and do not produce nodules. Tan et al. (2015) observed that multi-strain biofertilizer with a locally isolated PGPR (UPMB19, Lysinibacillus xylanilyticus) and an indigenous rhizobia (UPMR30, Bradyrhizobium japonicum) promoted rice shoot (6–20%) and root growth (19–76%), tiller numbers (4–32%), plant dry weight (13–22%), and nutrient accumulations (0.2–30%) and substantially increased the BNF of the plant. The combined PGPR and rhizobia inoculation reduced the plant dependence on chemical N-fertilizer through their synergistic BNF activities and contributed up to 22% of N2 fixed from the atmosphere. Biswas et al. (2000) also reported that biological nitrogen fixation by diazotrophic PGPR strains may be a contributing factor of rice growth promotion in addition to other mechanisms.

8.9 Phosphate Solubilization

Phosphorus (P) is second most important plant nutrient, but most of P remains fixed in soil which is not available to plants. Microorganisms offer a biological rescue system capable of solubilizing the insoluble inorganic P of soil and make it available to the plants. The ability of some microorganisms to convert insoluble P to an accessible form, like orthophosphate, is an important trait for increasing plant yields (Rodriguez et al. 2006). Many scientists have reported the ability of different bacterial species to solubilize insoluble inorganic phosphate compounds such as dicalcium phosphate, tricalcium phosphate, rock phosphate, and hydroxyapatite. These bacteria solubilize phosphate through the production of acids and by some other mechanism and are termed as phosphate-solubilizing bacteria or rhizobacteria (PSB or PSR). A number of metabolites are released by these strains which strongly affect the environment and increase nutrient availability for the plants, viz., Bacillus subtilis, B. licheniformis, B. megaterium var. phosphaticum, and Pseudomonas lutea.

Bacterial genera like Azospirillum, Azotobacter, Bacillus, Beijerinckia, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Microbacterium, Pseudomonas, Rhizobium, and Serratia are reported as the most significant PSB (Mehnaz and Lazarovits 2006). Lavakush et al. (2014) revealed that treatment combination of CPC [PGPR strains, e.g., Pseudomonas aeruginosa BHUJY16, P. aeruginosa BHUJY20, Pseudomonas putida BHUJY13, P. putida BHUJY23, and Pseudomonas fluorescens BHUJY29 were known as combined Pseudomonas culture (CPC)] with Azotobacter chroococcum, Azospirillum brasilense, and 30 kg ha−1 P2O5 is saving 50% chemical fertilizer than treatment combination of CPC with A. chroococcum, A. brasilense, and 60 kg ha−1 P2O5 and also enhances significant plant growth attributes, yields, and nutrient content in rice crops. These combinations of microbial consortia may be used as efficient bi-inoculants for integrated nutrient management for rice production under sustainable agriculture. It is environment-friendly, economically cheaper alternate to chemical fertilizer and efficient combination for enhancing rice production as well as enhancing the soil fertility and health.

8.10 Production of Siderophores

The efficient microorganisms have evolved specialized mechanisms for the assimilation of iron, including the production of low molecular weight iron-chelating compounds known as siderophores, which transport this element into their cells. Siderophores are divided into three main families depending on the characteristic functional group, i.e., hydroxamates, catecholates, and carboxylates. At present more than 500 different types of siderophores are known, of which ~270 have been structurally characterized (Cornelis 2010).

Siderophore production confers competitive advantages to PGPR that can colonize roots and exclude other microorganisms from this ecological niche. Under highly competitive conditions, the ability to acquire iron via siderophores may determine the outcome of competition for different carbon sources that are available as a result of root exudation or rhizodeposition. Among most of the bacterial siderophores studied, those produced by pseudomonads are known for their high affinity to the ferric ion. The potent siderophore, pyoverdin, for example, can inhibit the growth of bacteria and fungi that present less potent siderophores in iron-depleted media in vitro (Kloepper et al. 1980a). A pseudobactin siderophore produced by P. putida B10 strain was also able to suppress Fusarium oxysporum in soil deficient in iron; this suppression was lost when the soil was replenished with iron, a condition that represses the production of iron chelators by microorganisms (Kloepper et al. 1980b). Recent studies have demonstrated the suppression of soilborne fungal pathogens through the release of iron-chelating siderophores by fluorescent pseudomonads (Dwivedi and Johri 2003). Two fluorescent Pseudomonads, Pseudomonas fluorescens NCIM 5164 and Pseudomonas aeruginosa NCIM 2036, produced siderophores under iron-limiting conditions. Both the Pseudomonas sp. were further tested as seed inoculants and found to be very effective in seed germination and plant growth promotion of Triticum aestivum and Apios americana plants under pot culture conditions (Bholay et al. 2012).

8.11 PGPR as Biocontrol Agent

PGPR are indigenous to soil and the plant rhizosphere and play a major role in the biocontrol of plant pathogens. They can suppress a broad spectrum of bacterial, fungal nematode, and viral diseases. Most of the PGPR produce antifungal metabolites (AFMs), i.e., phenazines, pyrrolnitrin, 2, 4-diacetylphloroglucinol (DAPG), pyoluteorin, viscosinamide, and tensin (Table 8.4).

Table 8.4 PGPR used as biocontrol agents against different diseases, pathogens, and insects affecting different crops
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Among PGPR, Pseudomonas is the best-characterized biocontrol agent at molecular level. Pseudomonads possess many traits that make them well suited as biocontrol and growth-promoting agents (Saharan and Nehra 2011). These include the ability to (i) grow rapidly in vitro and to be mass produced, (ii) rapidly utilize seed and root exudates, (iii) colonize and multiply in the rhizosphere and spermosphere environments and in the interior of the plant, (iv) produce a wide spectrum of bioactive metabolites (i.e., antibiotics, siderophores, volatiles, and growth-promoting substances), (v) compete aggressively with other microorganisms, and (vi) adapt to environmental stresses. In addition, Pseudomonas is responsible for the natural suppressiveness of some soilborne pathogens (Weller et al. 2002). The major weakness of Pseudomonas as biocontrol agents is their inability to produce resting spores (as do many Bacillus sp.), which complicates formulation of the bacteria for commercial use. Yuttavanichakul et al. (2012) study demonstrated that isolate PGPR A20 and A45 (108 cell per ml) significantly reduced disease incidence and disease severity when 105 or 106 spores per ml of Aspergillus niger were applied to plants. PGPR isolates A20 and A45 co-inoculated with commercial Bradyrhizobium sp. TAL 173 (108 cell per ml) provided further protection of the seed from the pathogenic fungus A. niger and promoted the growth of peanut plants.

8.12 Conclusions

As long as the human population continues to increase, the world will have to withstand the escalating demand for food. Seven decades ago, the Green Revolution increased agricultural production globally, saving about one billion people from starvation and undernourishment; it triggered the development of chemical fertilizers, along with other advances. This has to be put to rest; the conventional crop approach cannot be practiced anymore since anthropogenic activities such as intensive agriculture, crop monocultures, and the use of agrochemicals are grave concerns and disturb the ecosystem. The present review indicates the development and formulations of PGPR in biological promotion of different characteristics of plant growth. Use of PGPR can play an important role toward achieving the objectives of sustainable agriculture. Several benefits of PGPR in terms of biofertilization, biocontrol, and bioremediation exert a positive influence on crop productivity and ecosystem functioning. The development of stable formulations of antagonistic PGPR in sustainable agricultural systems thus established as another promising approach replacing the use of chemical fertilizers. Besides, PGPR are protecting natural environments as well as biological resources by playing a significant role in integrated pest management system (IPM). Thus, it is becoming increasingly apparent that most of the PGPR strains can promote plant growth by several mechanisms, though most studies currently focus on individual mechanisms and have not yet been able to sort out the relative contributions of different processes that are also responsible for successful plant growth promotion. However, carefully controlled field trials of crop plants inoculated along with rhizobacteria are necessary for maximum commercial exploitation of PGPR strains.