Keywords

13.1 Introduction

For sustainable agriculture, the use of microbial based biofertilizers has prestigious role in enhancing level of crop productivity and in food safety. Microorganisms as invisible soil engineers maintain soil health, construct a hub for different biogeochemical cycles (Gadd 2010) and many soil microorganisms such as bacteria, actinomycetes, cyanobacteria and mycorrhiza present an eco-friendly approach for improved uptake of nutrients and enhanced plant growth. Microorganisms particularly plant growth promoting rhizobacteria (PGPR) are dwelled in rhizospheric region and efficiently colonize the roots of plants and confer tolerance in plants against several abiotic and biotic stresses (Prasad et al. 2015). Plant growth promoting microorganisms make nutrients available to plants by numerous mechanisms such as atmospheric nitrogen fixation, solubilizing the nutrients fixed in the soil matrix, and production of phytohormones. Such microorganisms make sure for further enhancement of micronutrients in plants, as they play a key role in organic material mineralization and as well as transforming inorganic nutrients. Microorganisms can also influence nutrient availability through presenting different characteristics such as chelation, solubilization and oxidation or reduction (Khan 2005) and also conferred resistance from pathogens causing diseases to the host plant by the secretions of antibiotics (Bonfante and Genre 2015). The aim of modern agriculture system, besides augmented crop yield, is also to produce nutritious safe food crops with improved level of micronutrients in the edible portion of crop plants. Human population is mainly dependent on crop based foods for the basic diet, and having foods with poor level of essential micronutrients creates serious health issues in humans. Deficiency of micronutrients (zinc, iron, selenium, copper, manganese and vitamins) in both humans and plants is narrated as ‘hidden hunger’ (Sharma et al. 2016), and bestows threat of malnutrition among world population. Therefore, implementation of biofortification strategy is an important mode for providing the preeminent solution for producing food crops with elevated level of necessary micronutrients. ‘Biofortification’ and ‘standard fortification’ are two different terms, where biofortification is related with consigning the nutrients aggregation inside plant cells whereas latter involves use of additives with the foods. Biofortification process deals with several approaches for enhancing bioavailability of key nutrients in crops.

13.2 Micronutrient Associated Malnutrition

Micronutrient deficiency occurs in humans, where populations of developing countries intake diet in the form of staple foods characterized by reduced bioavailability of essential micronutrients. Prevalence of micronutrient deficiency increase the risk of extensive disease burden in low and middle income countries (Black 2014), where populations of impecunious people cannot afford costlier nutrient rich foods and other nutrient supplements and suffers from wide varieties of micronutrients malnutrition associated ailments. According to the United Nations System Standing Committee on Nutrition (UNSSCN) (2004) micronutrients starvation are associated with more than 50% of all child mortality and also present the foremost risk factor for maternal mortality. Zinc (Zn), iron (Fe), and selenium (Se) are considered as important micronutrients and these are required in appropriate amount through routine diet for maintaining several life processes. Deficiency of one or more micronutrients creates negative impact on human health express in wide arrays of diseases (Fig. 13.1). The micronutrient zinc is most essential for all organisms including humans, and also has important structural roles in several proteins. Zinc deficiency is most prevalent micronutrient dearth and is associated with numerous human health related issues such as impairments of physical growth, greater risk of various infections, retarded growth, deferred wound healing, diarrhea, skeletal abnormalities and increased risk of abortion (Salgueiro et al. 2000).

Fig. 13.1
figure 1

Schematic representation of health effects of micronutrient (Zn, Fe and Se) malnutrition in humans

Full size image

Deficiency of iron causes chlorosis in plants and results in reduced crop yield, and eventually affects human health through food-chain, specifically to people whose diets generally rely on plant resources. Iron deficiency engenders nutritional anemia and also associated with impaired immune functions in children and as well impaired neurocognitive development (Murray-Kolb 2013). Selenium is another instance of essential micronutrient possessing role in wide range of metabolic pathways such as antioxidant defense and thyroid hormone metabolism. Selenium (Se) deficiency may linked with numerous ailments including heart diseases, reduced male fertility, hypothyroidism, weakened immune system and high risk of infections, cancer, oxidative stress-related conditions and epilepsy (Hatfield et al. 2014). Deficiency of vitamins (Vitamin B6, Vitamin B12, Vitamin C, Vitamin E and folic acid), zinc and iron also results in DNA damage through using similar strategies as radiation and various chemicals, and therefore considered a major factor to cause cancer and other disabilities. To circumvent problems associated with micronutrients malnutrition, investigation of those strategies are required which can improve the nutrient assimilation in plants.

13.3 Approaches for Biofortification

13.3.1 Biofortification Through Genetic Modification

Biofortification of vital food crops through genetic amendment and various biotechnological techniques is a sustainable solution for alleviating the micronutrient malnutrition. The techniques of genetic modification are being optimized for the development and production of healthy foods in addition to step up in the levels and activity of biologically active components in food crop system. Techniques of genetic modification have typically been targeted at escalating yields of staple food crops in developing countries. Though, the food crops with improved nutritional quality have gathered less consideration. An excellent example of a genetically modified biofortified crop is golden rice. Ordinary rice is not able to synthesise beta-carotene; however, due to genetic modification, golden rice can produce rice with beta-carotene in it. Moreover, stearidonic acid assimilation in soybean crop is also reported through genetic transformation (Singh et al. 2017).

13.3.2 Transgenic Approaches

Biotechnological techniques accredit the screening and selection of flourishing genotypes, isolation as well as cloning of favorable traits and formation of transgenic crops for sustainable agriculture system. Transgenic approaches can be used to increase the micronutrient content of staple food crops such as legumes and cereals, which can be achieved by insertion of specific genetic trait with the ability to produce the desired nutrients that are typically deficient in recipients. It involves the characterization, insertion or deletion of specific gene to improve the desired trait like nutritional quality from donor organisms. This may be achieved by the introduction of genes that code for trace element binding proteins, over expression of storage proteins already present or the expression of other proteins that are responsible for micronutrient uptake in plants. Furthermore, metabolic pathways from any microorganism and other organisms can also be applied into crops to utilize alternative pathways for metabolic engineering. Thus, these technologies provide a powerful tool that is unconstrained by the gene pool of the host. In addition, the transgenic approaches can be targeted to the edible portions of commercial crops (Hirschi 2009). As shown in Table 13.1, several crops have been genetically modified with traits of macronutrient and micronutrient that may provide reimbursement to consumers (Newell 2008).

Table 13.1 Genetically modified crops with description of macronutrient and micronutrient assimilation
Full size table

13.3.3 Agronomic Biofortification

Biofortification through agronomical approach can be achieved through the implication of nutrient-rich fertilizers to foliage or soil to increase the micronutrient concentration in edible crop parts and thus increase the intake of essential micronutrients by consumers. Interaction between micronutrient and macronutrient can influence the efficiency of agronomic biofortification. Good quantity of macro elements (N, P and K) in crop has a positive effect on development of root architecture and transportation of nutrients from vegetative tissues to the seeds. Consequently, there is increased concentration of micronutrients in edible parts of the food crop (Prasad et al. 2014). However, when food crops are grown where mineral elements become straight away unavailable in the soil system, targeted application of soluble chemical fertilizers to foliar parts and roots are practised.

13.3.4 Chemical Fertilizer

Effectiveness of chemical fertilizer on crop performance is influenced by type of fertilizer. The interaction between chemical fertilizer and different forms of nutrients can have positive, negative or neutral effect on food crop in yield and nutrient bioavailability. The implication of fertilizer with soil is often the most efficient manner. However, soils often contain huge amounts of iron, but only little amount of iron is phytoavailable. The implication of inorganic Fe fertilizers to such soils is more often futile as it rapidly becomes unavailable to plant root system through adsorption, oxidation reactions and precipitation. For this reason, Fe chelators are often used as soil Fe fertilizers (Rengel et al. 1999). Zinc is commonly applied to crops as ZnSO4 or as synthetic chelators (Shuman 1998). The application of Zn fertilizers to the soil system is effective for increasing Zn concentrations in cereal grains, growing mostly in soils and foliar applications of either ZnSO4 or Zn chelators can increase Zn concentrations in plant via ample Zn mobility in the phloem. Similarly, applications of Zn fertilizers in soil and foliar can increase Zn concentrations in leaf, tuber and fruit (Shuman 1998; Rengel et al. 1999).

13.3.5 Biofortification Through Plant Growth Promoting Microorganisms (PGPM)

Biofortification of crops through implications of PGPMs can be considered as a promising accompanying measure, which along with transgenic varieties, can lead to augmented micronutrient concentrations in food crop system, besides improving yield and soil fertility. Plant growth promoting microorganism’s have been reported to biofortify the micronutrient contents in food crops besides improving the soil fertility and crop yield (Rana et al. 2012). In addition, plant growth promoting microorganisms also facilitate the plant growth through N2 fixation, insoluble phosphorus solubilization, production of phytohormones, lowering of ethylene concentration, antibiotics and antifungal metabolites synthesis and induced systemic resistance. In this way, PGPM are also known to boost the soil fertility in return, the plant acquiesce by supplying essential nutrients, growth regulators and enhancing the ethylene mediated stress by 1-aminocyclopropane-1-carboxylate (ACC) deaminase production along with improved plant stress tolerance to drought, salinity, metal and toxicity of pesticide (Singh and Prasad 2014; Singh and Singh 2017). Moreover, the potentiality of PGPM in agriculture is progressively increased as it provides an attractive approach to replace the exploitation of chemical fertilizers, pesticides and other supplements. Subsequently, biofortification of crops through application of PGPMs can be therefore considered as a potential supplementary approach, which along with breeding varieties, can escort to augment the concentrations of micronutrient in wheat crop, besides improving yield and soil fertility (Singh et al. 2017).

13.4 Mechanisms of Plant Growth Promoting Microorganisms

A variety of plant growth promoting microorganisms (PGPM) has been reported to enhance plant growth and productivity by means of various mechanisms. Illustration of some of the mechanisms is as follows.

13.4.1 Iron Chelation

Iron is a vital component for all forms of life including prokaryotes as well as eukaryotes. It is the component of electron transport carrier, cofactor of various enzymes and important part of various constituent such as hemoglobin. Due to the aerobic environment conditions, iron is present in its oxidized form (Fe3+, insoluble at neutral pH) instead of reduced form (Fe2+, soluble at neutral pH) which are taken up by plants. To sequester the iron, many fungi, bacteria and some plants have an unusual adaptation to produce low molecular weight compounds called as siderophores, a group of low molecular weight compounds (<10 KD) those have immense affinity towards Fe3+ ions. Siderophores are PGPM secreted compounds that are ultimately taken up by plants therefore transporting molecule of iron to the plants cells. Plant roots might be able of take up siderophore and use them as sources of iron. Therefore, microbial siderophore can enhance plant growth by improving iron uptake as well as by inhibiting the plant pathogen by means of competition ultimately leads to the iron biofortification in plants and their grains (Srivastava et al. 2013). Many researchers has been reported the siderophore production in wide range of bacterial species viz. Bacillus, Pseudomonas, Azotobacter, Arthrobacter, Burkholderia, Enterobacter, Rhodospirrilum, Serratia, Azospirillum and Rhizobium and fungal species viz. Aspergillus Penicillium Rhizopus, Syncephalastrum (Leong and Neilands 1982; Das et al. 2007; Duran et al. 2016; Srivastava et al. 2013). There are many types of siderophore such as hydroxamate, catacholate and carboxalate that are secreted by microbes varies from species to species. Furthermore, mixed type of siderophore has been secreted by many baceterial species (Wandersman and Delepelaire 2004). Hence we can say that use of siderophore producing PGPM is better approach over other conventional methods such as chemical fertilizers to enhance iron content in plants and grains.

13.4.2 Zinc Solubilizer

Zinc is one of the essential nutrients required for growth and metabolic activities. Zinc ions takes part in many physiological activities, it act as cofactor in various enzymes; take part in defense; play role in cell division and growth in prokaryotes as well as in eukaryotes. Zinc ions are highly reactive in nature and present in close interaction with soil constituents therefore soluble zinc is very low in soil. Generally, it is found in the form of oxides, phosphates and carbonates. Plant associated microorganisms adopt several mechanisms to solubilize zinc such as chelation (Whiting et al. 2001), reduction in soil pH (Subramanian et al. 2009), or through improving root growth and root absorptive area (Burkert and Robson 1994). Zinc chelation by microbes and making them available for plants roots is a well known phenomenon. Microbes produce chealating compounds, which forms complex upon binding with zinc. In addition, they releases chealated zinc at the root surface and enhance the zinc availability ultimately lead to the zinc biofotification in plants. Whiting et al. (2001) reported production of metallophores as the possible strategies used by bacteria to chelate Zn. Reduction in soil pH also enhance availability of zinc. Decline in pH has been reported, when Pseudomonas and Bacillus spp. solubilized zinc complex compounds (ZnS, ZnO and ZnCO3) into zinc ions in a broth culture (Saravanan et al. 2004). Zinc solubilization methods differ from one microorganism to another to improve Zn availability in soil system and plant tissues. Many microbes including many bacterial and fungal species (Pseudomonas, Microbacterium, Enterobacter, Bacillus, Arbuscular mycorrhizae) have the incredible capability to solubilize Zn from complex compounds (Whiting et al. 2001; Fasim et al. 2002; Subramanian et al. 2009) and consequently take part in improvement of food quality and nutrient status of plants and grains.

13.4.3 Biofertilizers

In the present era, use of chemical fertilizers to enhance plant growth, productivity and to replenish soil nutrient status is very common. But several problems coincide with the use of chemical fertilizers such as high cost, unavailability of large portion of nutrients, toxic and non-degradable nature, leading to enhancement of environmental pollution and making land unsuitable for cultivation. Therefore as an alternative strategy, application of biofertilizers can be used to enhance crop productivity and biofortification of nutrients in grains. Biofertilizers which are the fertilizers based on source of biological origin such as microbes (including bacteria and fungi) are used instead of synthetic compounds. Use of biofertilizers is an ecofriendly approach as they are of biological origin and keeps environment healthy due to its low persistence. The objective of biofertilizers is to increase the soil organic contents improve soil structure and reduce loss of nutrients such as nitrogen, iron, zinc, calcium and phosphorus (Lal and Greenland 1979). Biofertilizers serves as source of all nutrients due to their ability to solubilize complex form of nutrients into soluble form (Singh et al. 2010, 2013, 2018). Plants utilize phosphorus in the form of orthophosphate (Pi). Plant growth promoting microorganisms possess various mechanisms to enhance phosphate solubilization (Fig. 13.2). Jones et al. (1998) reported 3.1–4.7 times more efficacy in mycorrhizal associated plants for phosphorus uptake than nonmycorrhizal plants. Commercially biofertilizers are developed by coating of various bacteria (Azotobacter, Azospirillum, Rhizobium, Pseudomonas and Bacillus) on seeds, a process called bacterization. Azotobacter chrococcum secretes azotobacterin, Bacillus megaterium secretes phosphobacterin, which is used for preparation of biofertilizers (Kumar and Bohra 2006). These bacteria may or may not form symbiotic association but enhances lateral root hairs of plants to increase mineral and water absorption, also increases nitrogen availability, secretes plant growth stimulating substances such as vitamin, auxins, gibberellic acid, cytokinins which leads to increase in photosynthesis capacity ultimately enhancing nutrient status in plants. Rhizobial biofertilizers are able to fix 50–150 N/ha/annum. It has been well known, that application of biofertilizers with plants, significantly increases plant growth, high nutrient status, low level of pathogen attack (Gupta et al. 2003; Yadav et al. 2016). Therefore, such free living and symbiotic microorganisms are promoted to reduce the dependence on chemical fertilizers. Some biofertilizers are listed in Table 13.2.

Fig. 13.2
figure 2

Phosphate solubilization mechanisms of plant growth promoting microorganisms

Full size image
Table 13.2 Biofertilizers, their mode of action and crops benefitted
Full size table

13.4.4 Biocontrol Agents

Under natural environmental conditions, plants are continuously exposed to various pathogenic bacteria and fungi, which causes disease in plants leading to the reduction in crop production or death of the plants. Therefore, plant diseases need to be controlled to maintain the quality and nutritional status of plants. Different approaches may be used to prevent or control plant diseases. In present circumstances, many pesticides are used to prevent disease but the disadvantage is that the pest may adapted towards pesticide and it is also cost intensive as well as act as environmental pollutant due to its persistence. Therefore, there is a novel approach i.e. application of biocontrol agent to suppress or demise pathogen growth. Biocontrol agents are microorganisms, which controls the fungi, insect, pest and any other pathogen (Beattie 2006). Generally bacteria, fungi, virus and protozoans are used as biocontrol agents. Generally plant growth promoting microorganisms produces various substances that protect plants against pathogens by direct interactions i.e. antagonistic activity or indirectly by inducing host resistance (Induce systemic resistance or systemic acquired resistance). Plant growth promoting microorganisms that indirectly act on pathogens may have some mechanisms to control plant pathogen such as the following (Table 13.3).

Table 13.3 Major biocontrol agents, their target pathogen and mechanism of action
Full size table
  1. 1.

    The PGPR may have ability to produce siderophore that chelates iron, which makes iron unavailable for plant pathogens (Singh et al. 2017)

  2. 2.

    The PGPR may possess capacity to secrete some anti-pathogenic metabolites such as antibiotics, cell wall degrading hydrolytic enzyme (Glucanases, chitinases, proteases, lipase, pectinases) or hydrogen cyanide (HCN), which suppresses pathogen growth (Maksimov et al. 2011).

  3. 3.

    The PGPR may compete for nutrients and niche with pathogen (Kamilov et al. 2005).

  4. 4.

    The PGPR may stimulate Induced Systemic Resistance (ISR) or Systemic Acquired Resistance (SAR) (Van loon et al. 1998).

A wide range of microorganisms are reported to act as biocontrol agents, as they possess one or more than one mechanisms to suppress pathogen attack or growth. Due to less persistence in environment, specificity for target pest, cost effectiveness and ecofriendly nature, biocontrol agents are good alternative to pesticides.

13.5 Conclusion

Development of crops with elevated concentration of micronutrients is immensely and urgently needed to combat the problem of micronutrient deficiency. Plant growth promoting microorganisms (PGPMs) make interaction with plants and exert plant growth promoting activities and enhance the capability of the plant for uptake of micronutrients from surrounding soils. Zinc solubilization and siderophore secreting microorganisms enhance the level of zinc and iron in the various edible portions of crop plants and provide an alternative strategy to fortify micronutrients and produce micronutrients rich foods. Application of such microorganisms inoculants reduces the dependency on costly biofortification approaches i.e. agronomic and genetic approaches. In future, formulation of microorganisms possessing multiple beneficial traits can be applied for biofortification strategies to tackle the problem of hidden hunger. Using plant growth promoting microorganisms for biofortification as part of green technology approach can be a better strategy to achieve environmental sustainability.