Biochemical and molecular characterisations of salt tolerance components in rice varieties tolerant and sensitive to NaCl: the relevance of Na+ exclusion in salt tolerance in the species

Abstract

Soil salinisation is a major abiotic stress in agriculture, and is especially a concern for rice production because among cereal crops, rice is the most salt-sensitive. However, the production of rice must be increased substantially by the year 2050 to meet the demand of the ever growing population. Hence, understanding the biochemical events determining salt tolerance in rice is highly desirable so that the trait can be introduced in cultivars of interest through biotechnological intervention. In this context, an initial study on NaCl response in four Indica rice varieties showed a lower uptake of Na+ in the salt-tolerant Nona Bokra and Pokkali than in the salt-sensitive IR64 and IR29, indicating Na+ exclusion as a primary requirement of salt tolerance in the species. This was also supported by the following features in the salt-tolerant, but not in the -sensitive varieties: (1) highly significant NaCl-induced increase in the activity of PM-H+ATPase, (2) a high constitutive level and NaCl-induced threonine phosphorylation of PM-H+ATPase, necessary to promote its activity, (3) a high constitutive expression of 14-3-3 protein that makes PM-H+ATPase active by binding with the phosphorylated threonine at the C-terminal end, (4) a high constitutive and NaCl-induced expression of SOS1 in roots, and (5) significant NaCl-induced expression of OsCIPK 24, a SOS2 that phosphorylates SOS1. The vacuolar sequestration of Na+ in seedlings was not reflected from the expression pattern of NHX1/NHX1 in response to NaCl. NaCl-induced downregulation of expression of HKTs in roots of Nona Bokra, but upregulation in Pokkali also indicates that their role in salt tolerance in rice could be cultivar specific. The study indicates that consideration of increasing exclusion of Na+ by enhancing the efficiency of SOS1/PM-H+ATPase Na+ exclusion module could be an important aspect in attempting to increase salt tolerance in the rice varieties or cultivars of interest.