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

Fusarium oxysporum, through the production of specific enzymes (Ting et

al., 2012). During the pathogenic attack fungal endophytes helps plant to

produce phytoalexins, secondary metabolites and phytohormones providing

defensive response in plants (Dastogeer et al., 2020). Beside fungal endo­

phytes, bacterial microorganism also helped plants by providing protection

against several pathogenic attacks. Pseudomonas putida and Serratia plymu­

thica can inhibit two fungal pathogen Rhizoctonia solani and Verticillium

dahilae by secreting antifungal biochemicals (Zhang et al., 2019).

3.5 PGPR AND PLANT GROWTH

Rhizosphere is the area in the soil that is unique in nature as it supports the

highest microbial diversity and activity. Rhizosphere is greatly influenced by

the activity of plant roots. A great proportion of microorganisms occupies the

root rhizosphere area that promotes plant growth are known as plant growth

promoting rhizobacteria (PGPR). Roots secrete several organic solutes,

sugars, and chemicals which are needed for the survival of PGPR and in

return PGPR promotes plant growth by regulating different biochemical,

physiological, and anatomical processes (Nadeem et al., 2013). In addition to

plant growth, these beneficial bacteria also provide tolerance against adverse

environmental conditions like drought, salinity, and heavy metals stress.

PGPR-mediated plant growth includes both direct and indirect methods.

Direct mechanism involves the production of beneficial compounds by the

bacterium that helps in nutrients uptake and fixing biological nitrogen. An

indirect mechanism occurs when PGPR inhibits pathogenic attacks in the

plant. They reduce the impact of pathogen attack by producing substances

that induce resistance in plants and also produce siderophores which limits

iron availability for the pathogens. Beside these two mechanisms, PGPR has

a striking ability to produce different enzymes including ACC deaminase

and chitinase. During the plant growth PGPR enhances production of IAA,

cytokinin, ABA, siderophores, hydrogen cyanide and rhizobitoxine which

not only induce plant growth but also provide resistance against biotic and

abiotic stresses (Nadeem et al., 2013; Kumar et al., 2019). Rhizobitoxine

induce nodule formation by inhibiting the level of ethylene which has

a negative impact on nodulation. In addition to this ACC deaminase is a

very important enzyme for plant growth under different stress conditions

like drought, salinity, and heavy metal stress. Bacteria those produce ACC

deaminase promote plant growth under abiotic stress condition by cleaving