Amelioration of thermal stress in crops by plant growth-promoting rhizobacteria

Crops are affected by various types of abiotic stress, resulting in a decrease in agricultural productivity thereby challenging food-security issues. Among the different types of abiotic stresses, the temperature has been regarded as a major one followed by salt and drought stress. The increasing temperature causes an alteration in the period of growth and the distribution of crop plants. High-temperature also damages the membrane, some proteins, inactivate the main enzymes and disturb the synthesis of biomolecules. Moreover, heat stress has been found to restrict the process of cell division as well. On the other hand, low-temperature is the main determinant of freezing which results in the reduction of plant cellular metabolism. Freezing stress forms ice-crystals in the cells that further leads to cell death due to dehydration and leakage of electrolyte. Generally, the stress tolerance in crops has been determined based on the cultivar's ability to withstand freezing temperatures; however, with advancements in microbiological techniques, several stress-tolerant microbes have been identified which either through direct or indirect mechanisms alleviate temperature-stress in plants. Furthermore, the function of cryoprotectant compounds and their signaling mechanisms in plants have been also discussed in detail for signifying their role in the alleviation of cold stress in plants. This review highlights the responses of plant growth-promoting rhizobacteria (PGPR) that are metabolically active under thermal stress conditions, and result in the production of metabolites that promotes plant growth and facilitate the uptake of nutrients under thermal stress in agro-ecosystems.