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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

unfavorable conditions in order to counteract detrimental effects (Gupta et

al., 2013). Aside from PGPM, another crucial aspect of growth and develop­

ment is the reciprocal action of fungi with the roots present in the higher

plants. AM is the most frequent type of mycorrhizae found in agricultural

fields. These fungi are crucial players in the nutrient cycle, absorption, and

translocation.

2.3 METHOD AND MODE OF DROUGHT STRESS TOLERANCE

The microorganisms that are tolerant to drought conditions have the potential

to improve plant survivability and development when there is a lack of water.

To survive in low water potential environments, microorganisms have trans­

formed, adapted, and/or developed a mode for tolerance (Table 2.1). They

can gather osmolytes and create exo-polysaccharides, and they can form

thick walls or go inactive (EPS). These plant-associated microorganisms

have a variety of ways for dealing with the obstructive effects of drought

on both the plants and soil. They give nutrients and superior environmental

conditions for ongoing plant growth, regardless of water content. Beneficial

microorganisms that colonize the rhizosphere enhance plant growth and

development in a variety of ways, both directly and indirectly: (i) synthesis

of phytohormones such as cytokinins, indole-3-acetic acid (IAA) as well

as abscisic acid (ABA); (ii) exopolysaccharides from bacteria; (iii) ACC

deaminase; and (iv) elevated systemic toleration are all possible mechanisms.

Plant-produced phytohormones play an important role in maturation and

survivability (Farooq et al., 2009; Porcel et al., 2014). Furthermore, PGPR has

the potential to generate plant-related hormones that promote plant division

and maturity in stressful situations. During drought stress, IAA, a highly active

auxin, governs differentiation in the vascular tissues along with adventitious

and/or lateral differentiation of roots, division of cells, and growth of shoot

(Goswami et al., 2015). ABA is a key growth-related regulator in drought-

tensile plants. As PGPR is introduced into a seed or plant, the concentration

of ABA rises, regulating the physiology of the plant to resist drought stress.

Drought strain is alleviated by ABA through modulating the drought-related

genes transcription and hydraulic conductivity through roots (Jiang et al.,

2013). Azospirillum brasilense recovers Arabidopsis thaliana’s drought

response mostly through increasing levels of ABA (Cohen et al., 2015) (Table

2.1). During times of stress, 1-aminocyclopropane-1-carboxylate (ACC) acts

as an instant antecedent of the ethylene. Hydrolyzation of ACC is done by

ACC deaminase from the bacteria into ammonia and alpha-ketobutyrate (Bal