Regulation of phytosterol biosynthetic pathway during drought stress in rice.

Plants respond to drought stress in the form of various physio-biochemical and molecular changes at both cellular and molecular levels. Drought stress causes the destruction of cell membranes by disintegration of membrane lipids. One of the major groups of membrane lipids that plays important role in preserving the integrity of cell membranes is phytosterols. HMG-CoA reductase (HMGR) is the principal enzyme in the biosynthesis of plant sterols, synthesized via mevalonic acid pathway. Phospholipid: sterol acyltransferase (PSAT) is another important enzyme that plays an important role in turnover of phytosterols into steryl esters and helps maintain homeostasis of membrane lipids. In this study, the expression of both HMGR and PSAT genes in drought sensitive (IR64) and drought tolerant (N22) rice cultivars under applied drought conditions were found to be elevated. The increase in expression of these genes was proportional to the level of severity of applied drought stress. This is substantiated by the negative correlation of HMGR and PSAT expression to relative water content (RWC) and membrane stability index (MSI). Expression of PSAT was also found to be positively correlated to ABA content and HMGR expression.

Cloning and characterization of a water deficit stress responsive transcription factor gene from Oryza sativa L.

Understanding the biochemical and molecular basis of drought mechanism in rice is important as drought is one of the major causes affecting rice crop adversely. A 1017 bp gene sequence encoding AP2/ERF family TF was isolated from Oryza sativa sp. Indica cv N22 encoding a protein of 338 amino acid residues, with a molecular weight of 36.58 kDa, and no intron in the ORF. The gene was named as AP2/ERF-N22(2) different from the drought responsive gene AP2/ERF-N22 that we reported earlier. AP2/ERF-N22(2) has entirely different characteristics from that of AP2/ERF-N22. It has a single AP2 domain of 55 amino acid residues and a cluster of acidic amino acid residues at the C-terminal region, which could function as a trans-activation domain. Presence of NLS indicates that it is a nuclear localized transcription factor encoding gene. It falls in group VI L, sharing characteristic similarities. Arabidopsis members of group VI L have been shown to be involved in response to cytokinin under drought stress.