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ABSTRACT

Microbes of the rhizomicrobiome play key roles in nutrient acquisition and assimilation, improved soil texture, secreting, and modulating extracellular molecules such as hormones, secondary metabolites, antibiotics, and various signal compounds, all leading to enhancement of plant growth. The microbes and compounds they secrete constitute valuable bio-stimulants and play pivotal roles in modulating plant stress responses. Research has demonstrated that inoculating plants with plant-growth promoting rhizobacteria (PGPR) or treating plants with microbeto-plant signal compounds can be an effective strategy to stimulate crop growth. Furthermore, these strategies can improve crop tolerance for the biotic stresses.

The health of crop plants is influenced by a myriad of biotic and abiotic interactions in soil and in the rhizosphere. While the growers routinely emphasize the physical and chemical approaches to manage the soil environment and plant growth, the use of microbes for this purpose is less common. An exception to this is the use of rhizobacterial inoculants for efficient nitrogen fixation in legumes.  As early as in 1687, Hooke first suggested the existence of a relationship between root exudates and the soil environment. The soil volume around the roots of a plant is denoted as the rhizosphere. This volume of soil is rich in nutrients because as much as 40% of the photosynthates are secreted from the roots in this region. As a result, the rhizosphere supports large and dynamic communities of microorganisms which are capable of exerting beneficial, neutral or deleterious effects on the plant growth. Beneficial plant associated microorganisms in the rhizosphere are part of a vast and largely untapped natural biological resources that interact with crop plants and with deleterious microorganisms. These beneficial microorganisms can be a significant component of management practices to increase the yield. This approach fits well with sustainable management practices because of its potential environmental benefits due to less application of agricultural chemicals.

There are bacteria that can be isolated from the rhizosphere which when back inoculated through seed or seed piece bacterization, stimulate the plant growth. Specific strains of Pseudomonas fluorescens-Pseudomonas putida rapidly colonize the roots of the growing plants and cause significant yield increases. Enhanced plant growth caused by these bacteria is often associated with reduction in the root zone populations of deleterious microorganisms. These beneficial strains are also potent biological control agents of certain soil borne plant pathogens. The term plant growth promoting rhizobacteria or PGPR was first defined by Kloepper and Schroth (1978) to describe the soil bacteria that colonize the roots of plants following inoculation onto seeds and that enhance the plant growth. Some of the rhizobacteria can also enter the roots and establish endophytic population. In addition to Pseudomonas, other bacteria viz., Azospirilum, Azotobacter, Bacillus, Klebsiella, Hafnia, Enterobacteria, Alcaligens, Arthrobacter and Serratia have also been shown to enhance the plant growth in many instances. The ultimate success of plant growth promotion by PGPR is largely dependent on the establishment of a significant population along with an elongated root system and their persistence throughout the growing season at the sites where deleterious rhizosphere microorganisms (DRMO) may become active. Specific strains of PGPR, when introduced through seed bacterization, rapidly colonize the germinating seeds (spermosphere) which profusely exudates a wide range of amino acids, carbohydrates and organic acids during germination. Effective seed colonization results in subsequent colonization of the developing roots.

 Mechanisms of plant growth promotion

Beneficial Rhizo-bacteria have been utilized to improve water and nutrient uptake, abiotic and biotic stress tolerance. The mechanisms by which PGPR enhance the plant growth may be direct or indirect. PGPR can promote the plant growth indirectly by suppressing the plant diseases caused by plant pathogens or by suppressing the DRMO. These DRMO are not direct pathogens of the plants and they do not parasitize the plants but they play a major role in reducing the plant growth and yield. It has been proved conclusively that PGPR exert their plant growth promoting activity by depriving the native micro flora of iron. PGPR produce extracellular low molecular weight microbial iron transport agents called ‘siderophores’ which effectively capture available iron in the root zone environment making it unavailable to the native micro flora and thereby suppressing them. Additionally, the PGPR synthesize antifungal antibiotics, cell wall degrading enzymes and hydrogen cyanide which suppress the growth of fungal pathogens. Another indirect means of plant growth promotion by PGPR is by suppression of plant diseases by induction of systemic resistance which has been termed as induced systemic resistance or ISR.

Besides functioning as bio-control agents, plant growth promoting rhizo-bacteria PGPR protect plants against pathogens by eliciting bio-chemical and molecular defense responses within the plant. PGPR can trigger ISR in plants, which activates pathogenesis related genes, mediated by phyto-hormone signaling pathways and defense regulatory proteins to prime plants against future pathogen attack. Bacterial signal compounds and microbe-associated molecular triggers, such as chitin oligomers, have been shown to modulate ISR induction in plants. Pathogen cell-surface factors such as flagellins and O-antigen of lipo-polysaccharides elicit ISR, whereas analogs of salicylic acid and jasmonic acid trigger ethylene to elicit NPR1 mediated systemic acquired resistance (SAR) in plants.

 Alternatively, PGPR may increase plant growth by other, more direct means also e.g. by associative nitrogen fixation that is transported to the plants, production of IAA and other phytohormones, enhancement of phosphate uptake, solubilization of nutrition such as phosphorus, promoting mycorrizhal functions, regulation of ethylene production in roots and by decreasing heavy metal toxicity. PGPR usually display more than one of these mechanisms in the rhizoshere.

Some PGPR (Pseudomonas putida MTCC5279) ameliorated drought stress in plants by modulating membrane integrity, osmolyte accumulation (proline, glycine betaine) and ROS scavenging ability. Stress responses were positively modulated by the bacteria resulting in differential expression of genes involved in ethylene biosynthesis, salicylic acid (PR1), jasmonate transcription activation, SOD, CAT, APX, and NAC1 (transcription factors expressed under abiotic stress), LEA and DHN (dehydrins). Application of thuricin 17 produced by Bacillus thuringiensis NEB17 to soybean (Glycine max) under water-deficit conditions resulted in modification of root structures and increased root and nodule biomass, root length, root ABA, and total nitrogen content. Helpful microbes also help plants cope with flooding stress. Rice (Oryza sativa) seedlings inoculated with an ACC deaminase producing strain of Pseudomonas fluorescens REN1 increased root elongation under constantly flooded conditions. Plant growth promotion and yield increase by seed or seed piece bacterization with PGPR has been well demonstrated in many crops. Potato yields increased from 5 to 33% in the field plots of California and Idaho in America. The treatment of sugar beet seeds with various strains of Pseudomonas spp. resulted in yield increase of 4-8 t/ha. Similarly, treatment with various strains of Pseudomonas spp. resulted in increase in root weight of radish ranging from 60-144% in most of the field trials. The most carefully investigated case of PGPR is that of potatoes in Holland. When the crop was planted repeatedly on the same fields or in short rotations, the tuber yield decreased by at least 10%. This had nothing to do with nutrient status but probably due to accumulation of DRMO. Seed bacterization with PGPR strains suppressed the DRMO and stimulated the plant growth and increased the yield. Similarly, the growth promotion has been demonstrated in many other crops like wheat, rice, soybean, mustard, sugarcane, lettuce etc. in many countries including India.

Conclusion

There are innumerable research reports that several rhizosphere bacteria have the potential to suppress the plant pathogens and promote the plant growth. In spite of this, the use of PGPR is less in commercial agriculture. This is mainly due to inconsistent performance of these bacteria in the fields where heterogeneity in biotic and abiotic factors is high. Focus should be given for the development of PGPR strains to ensure better survival and activity in the fields. Emphasis should also be given on compatibility of PGPR strains with different chemicals used in commercial agriculture so that PGPR can fit in the integrated crop management (ICM) practices.

References

 Kloepper, J. W., Leong, J., Teintze, M., and Schroth, M. N. (1978). Enhanced plant growth by siderophores produced by plant growth-promoting rhizobacteria. Nature 286, 885–886.

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Potential of plant growth promoting rhizobacteria in increasing crop yield.pdf