**4.6 Microbial induction of antioxidant machinery in plants**

The production of reactive oxygen species (ROS) is induced in plants under osmotic stress. These include hydrogen peroxide (H2O2), superoxide, singlet oxygen, radicals, alkoxy radicals, and superoxide anion radicals. The ROS production triggers irreversible damages to lipids, proteins, and DNA, affecting the redox regulation [66]. Plants induce antioxidant defense systems involving enzymatic and non-enzymatic pathways to protect against the oxidative damage during osmotic stress. Several enzymes (e.g., glutathione reductase, superoxide dismutase (SOD), catalase, and ascorbate peroxidase) and non-enzymatic components (e.g., ABA, cysteine, and glutathione) catalyze the biosynthetic pathways of antioxidant pathways [67, 68]. The microbial inoculation of plants triggers the antioxidant biosynthetic pathways in the soil and, thus, confers drought stress tolerance. Kaushal and Wani [69] and Ilyas et al. [70] reported the secretion of phenolic components by various microbes (e.g., algae, zatinomycetes, and fungi) upon exposure of plants to stress condition. The microbeplant interaction confers stress tolerance through regulating the ROS biosynthesis and maintaining a homeostatic balance between ROS and their removal. Overall, the ROS scavenging ability of PGPR regulates the antioxidant enzymes and may provide a solid barrier against abiotic stress.

#### **4.7 Stimulation of stress-response genes by plant-microbe interactions**

The microbial inoculation of plants confers stress tolerance by triggering the expression of genes involved in plant defense against abiotic stresses. Under drought stress in plants, the expression of stress-responsive genes is modulated, which is necessary for optimization of plant growth and development. There are numerous stress-responsive genes and proteins, which are involved in plant-microbe interactions and the resulting stress tolerance. These include sHSP, CaPR-10, dehydrin-like protein (Cadhn), 11-pyrroline-5-carboxylate reductase (P5CR), pyrroline-5-carboxylate dehydrogenase (P5CDH), and vacuolar ATPases [71]. Depending on the role of the encoded proteins, the expression of these stressresponsive genes can be categorized into functional or regulatory proteins as revealed by microarray studies. The stress-responsive functional proteins include water channel transporters, detoxification enzymes, osmolyte biosynthesis enzymes, macromolecule protection factors, and proteases [71, 72]. The stress-responsive genes encoding regulatory proteins include transcription factors, ABA biosynthetic factors, and phosphate kinases. The microbial-plant interaction-based expression and upregulation of stress-responsive genes can be harnessed as a powerful tool for enhancing plant drought stress tolerance.
