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2.1.5 Modulation of Phytohormones:

Phytohormones are flagging atoms that are liable for controlling all aspects of vegetation.
PGPR instigated direct development advancement components to incorporate the creation of

phytohormones, for example, auxins, gibberellins (GAs), cytokinins (CK), and nitric oxide
(NO). In different words, phytohormones are substance couriers that arrange cell exercises of

plants (Williams, 2011). At the point when the ecological condition comes to the problematic
level the degrees of endogenous phytohormones are regularly deficient for ideal

development (de Garcia Salamone et al., 2005).

Among several auxins, indole-3-acetic acid (IAA) is the most common natural auxin found in

plants and its firm effect on root growth (Miransari, 2014). Auxin production is widespread in
soil bacteria. This specificity is discovered in plenty of soil bacteria as well as in

streptomycetes, methylobacteria, cyanobacteria, and archaea. Some of these are free-living or
symbiotic PGPB. Bacterial IAA is capable for enhancement of lateral and fortuitous rooting

which results in improved mineral and nutrient uptake and root exudation that further

stimulates bacterial propagation on the roots (Lambrecht et al., 2000). Most Rhizobium
species produce IAA (Badenoch-Jones et al., 1983) and several studies have suggested that

changes in auxin levels in the host plant are necessary for nodule organogenesis (Mathesius
et al., 1998). Effect of environmental stresses can be prevented by PGPB-synthesizing

IAA (Frankenberger Jr et al., 2020).

2.1.6 Cytokines and Gibberellins
Several plant growth promoting rhizobacteria Azotobacter sp., Rhizobium sp., Pantoea

agglomerans, Rhodospirillum rubrum, Pseudomonas fluorescens, Bacillus subtilis and

Paenibacillus polymyxa can produce cytokinins or gibberellins or both can produce either
cytokinins or gibberellins or both for plant growth promotion (Kang et al., 2010). As

compared with cytokines produced by certain species of phytopathogens, level of cytokines
produced by PGPR is low hence it gives the stimulatory effect on the plants while the effect

of the cytokine of pathogens is destructive. Inoculation of seeds with cytokinin-producing

bacteria results in higher cytokinin content in the plants which cause positive impacts on
plant development (Arkhipova et al., 2005).

2.1.7 Ethylene Synthesis is based on three enzymes (a) S-adenosyl-L-methionine (SAM)

synthetase, which catalyzes the conversion of methionine to SAM (Giovanelli et al., 1980)
(b) 1- aminocyclopropane-1-carboxylic acid (ACC) synthase, which mediates the hydrolysis

of SAM to ACC and 50-methylthioadenosine (Hyodo et al., 1993) and (c) ACC oxidase

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