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Role of Endophytes, Plant Growth Promoting Rhizobacteria

the precursor of ethylene into ammonia and α-ketobutyrate resulting low

level of ethylene in plant. IAA is the type of auxin class of phytohormone

which has a key role in plant growth. PGPR those produces IAA promotes

root and shoot growth and increased water uptake under saline condition

(Kumar et al., 2019). During stress condition ABA plays an important role

in stomata closure that protects plants from losing water. Production of cyto­

kinin is another common feature of PGPR. Cytokinin positively regulates

cell growth, root and shoot development and tissue differentiation. PGPR,

producing a high concentration of cytokinin, potentially improves tolerance

in plants against stress conditions. For instance, Bacillus subtilis enhanced

drought resistance and shoot biomass in lettuce plant by elevating the level

of cytokinin (Arkhipova et al., 2007). Several genera of PGPR are also used

in the agricultural sector as bio-fertilizer which facilitate enhanced nutrient

uptake. Oscimum basilicum inoculated with Cronobacter dublinensis,

Bacillus sp. and Pseudomonas monteilii improved nutrient uptake under

stress condition (Rakshapal et al., 2013).

3.5.1 PGPR-MEDIATED ABIOTIC STRESS TOLERANCE IN PLANTS

Application of PGPR to manage abiotic stress in plant is now a widely followed

strategy. The first evidence of PGPR mediated stress tolerance was found

in Arabidopsis thaliana when Paenibacillus polymyxa improved drought

tolerance in the plant (Timmusk & Wagner, 1999). After that, various studies

have well established the fact that PGPR have several beneficial effects on

plants under abiotic stress condition. The main mechanisms through which

they promote stress tolerance are producing growth hormones, inhibiting the

effects of ethylene, enhanced nutrients uptake, facilitate nitrogen fixation,

producing antimicrobial molecules and minimizes effects of ROS (Ndeem

et al., 2013). During stress condition ethylene level needs to be maintained

normal to achieve normal growth. PGPR maintains ethylene level by

producing ACC deaminase enzyme. PGPR binds to the root surface and

produce IAA which helps to elevate the concentration of ACC deaminase in

the root vicinity and promote root growth under drought situation (Glick et

al., 1998). Under saline environment plants cope with nutritional imbalance

and ion toxicity. It is very essential to maintain a high ratio of K⁺/Na⁺ in the

plant to overcome salinity stress. Exopolysaccharides produced by certain

strains of PGPR limits the accumulation of excess Na⁺ ions, thus maintaining

a high K⁺/Na⁺ ratio inside the cell (Glick et al., 2007). PGPR improved plants