Rhizobacteria–Plant Interaction, Alleviation of Abiotic Stresses

Abstract

At the present scenario, climate change became the potential threat to growers with rise in temperature, inconsistent rainfall, and salinization of agricultural land. However, the microbes more specifically plant growth-promoting rhizobacteria (PGPR) play a significant role to mitigate the abiotic stresses. Rhizobacteria act as bioprotectants against drought, salt, heavy metals, high temperature, and cold stress. During drought condition, PGPR intensifies osmolytes (proline, glycine, betaine) and acts as an osmoprotectant. The drought-related enzyme ACC deaminases were regulated by the PGPR, which also regulates the stomatal physiology during the water deficit conditions. The salt stress in plants was also a complex process to understand. During salt stress condition, PGPR acts as an activator of antioxidant enzymes and polyamines and also acts as a modulator of abscisic acid. Inoculation of PGPR affects the expression of 14 genes (four upregulated and two downregulated) related to salt stress. The effect of heavy metal toxicity is also found in plants, which is due to the improper fertilizer applications, industrial waste, sludge, etc. The main site for accumulation of heavy metals is the root nodule. At present many PGPR sp., i.e., Bacillus sp., Pseudomonas sp., Azotobacter sp., Enterobacter sp., and Rhizobium sp., were proposed to speed up the phytoremediation process of nodules. Bacterial metallothioneins (MTs) of the family Bmt, a family with low-molecular proteins, play a significant role to absorb heavy metals. High temperature also acts as a constraint of normal plant root nodulation and rhizobial growth. The strains of PGPRs evolve during the heat stress period against the raised temperature with the production of extra LPS, EPS, and special class of proteins, i.e., heat shock proteins (HSPs). Cold tolerance can also be derived by PGPR as the accumulation of more carbohydrate, regulation of stress-related genes for osmolytes expression, and enhancement of specific protein synthesis, which helps plant to fight against cold stress.