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Role of Endophytes, Plant Growth Promoting Rhizobacteria

impact and maintain their growth under such condition. It was found that in

most of the stress condition overproduction of ROS like hydrogen peroxide

(H2O2), hydroxyl radicals and superoxides occur which has a number of

detrimental effects in plants (Nadeem et al., 2013). To minimize the effects

of ROS plant developed a complex network of antioxidant enzymes system.

Antioxidant enzymes like super oxide dismutase (SOD), catalase, ascorbate

peroxidase (APX) and glutathione reductase (GR) improved plant tolerance

against environmental stresses. It was reported that plants under stress condi­

tion contains much more antioxidant enzymes than plants that are susceptible

to stress (Gill & Tuteja, 2010). The action of antioxidant enzymes increases

with the increase of stress tolerance in a plant species. Not only the anti­

oxidant enzymes, during the stress period plant also accumulated excess

amount of antioxidant metabolites like tocopherols, carotenoids, flavonoids,

phenols, etc. (Apel & Hirt, 2004). Increase concentration of salt in the soil

creates barriers to nutrients uptake and ion toxicity. To mitigate salt stress

plant produces different compatible solutes like trehalose, proline, glycine

betaine and many other which enhance tolerance in plant under saline

environment (Ashraf et al., 2013). Accumulation of these compatible solutes

helps in osmotic adjustment in the plant body that protects water loss and

dilutes the concentration of toxic ions (Verma et al., 2021). Plants under

temperature stress expressed several stress responsive genes and initiate a

complex network of signaling cascade resulting in leaf rolling, alteration

of leaf orientation, early maturation and changing membrane structure

by altering lipid composition (Hasanuzzaman et al., 2013). Beside biotic

factors, biotic stress also causes several damages to plants. Plant protects

them from pathogenic attack by producing different hydrolytic enzymes

and phytoalexins. Hypersensitive response is another strategy plant adopted

during disease progression (Gonzalez-Teuber et al., 2010).

Plant enables different signaling molecules like phytohormones and

protein kinases which helps in receiving the stress stimuli and protects

plants from different stresses. Beside phytohormones, nitric oxides (NO),

Ca²⁺ and inositol phosphates also help in initiating signaling cascade under

stress environment (Hilleary & Gilroy, 2018). These signaling molecules

along with the stress stimuli enhance production of different stress-related

phytohormones including jasmonic acids (JA), abscisic acid (ABA), sali­

cylic acid (SA) and ethylene. In recent studies, it was reported that NO

and Ca²⁺-mediated signaling has a crucial role in stress tolerance of plants.

ABA-dependent stomata closure and drought stress response is mediated

through NO and Ca²⁺-dependent signaling pathway (Gulyani et al., 2018).