ABSTRACT
This chapter highlights five major mechanisms that operate under salt stress to improve salinity tolerance in plants. These are osmolyte metabolism, antioxidant-detoxification metabolism, phenolic/cell wall metabolism, ion homeostasis, and signal transduction. The content of proline and glycine betaine increases under salt stress as a result of higher synthesis and decreased catabolism. Betaine aldehyde dehydrogenase, the biosynthetic enzyme for glycine betaine, serves as detoxification enzyme under salinity stress. Proline and glycine betaine improve salt tolerance through activating antioxidant metabolism and decreasing oxidative toxicity under salt stress. Although glycine betaine and proline may result in high level of antioxidant metabolism under salt stress but recent researches have indicated that all constituents of antioxidant metabolism may not be required to be upregulated for salt tolerance. Plants could use phenolics, ascorbate, and glutathione either nonenzymatically or enzymatically to detoxify deleterious reactive oxygen species (ROS). Lignin present in cell wall has been found to increase under abiotic and biotic stresses. Ion homeostasis regulation can occur in root of plant, in vacuoles to keep ion concentration low in cytosol, and may help to retrieve ions from xylem stream. Salt tolerance has been related to higher ion exclusion, sequestration of ions in vacuoles and higher ratios of K+/Na+, K+/Ca+2, Ca+2/Na+ in different plant species. Chapter discusses the role of several signaling mechanisms, namely, calcium signaling, phytohormonal signaling, ROS signaling, ROS/NO signaling, ethylene signaling, and refers to role of polyamines in signaling. While the calcium signaling is involved in SOS1/SO2/SOS3 pathway for ion homeostasis and proline
accumulation, phytohormonal signaling and ROS/NO-signaling are found to regulate salt responses in plants. Polyamines, ethylene, and glycine betaine share common precursor of S-adenosyl methionine for their synthesis, hence could interact with one another under salinity. The review provides the readers a good insight into the salt tolerance mechanisms that come into play when plants are subjected to salt stress.
