Molecular aspects of osmotic stress in plants

Plant molecular responses to osmotic stress are complex as evidenced by the isolation of numerous OR (osmotic stress-regulated) genes. Although functions including osmolyte biosynthesis, membrane transport, signal transduction, and cellular protection have been predicted for OR genes, few of them have been established. Current efforts toward isolating and analyzing the expression of individual OR genes should be replaced by systematic approaches to analyze all OR genes simultaneously in selected plant species. Both transcriptional and posttranscriptional regulation of OR genes have been described. Cis-elements that respond to osmotic stress through abscisic acid (ABA)-dependent as well as ABA-independent pathways have been identified. Functional genetic approaches using yeast and plant model systems are expected to complement current molecular analysis of overexpression of OR genes in transgenic plants. These systems will help to establish functions of OR genes, to dissect osmotic stress-signaling pathways, and to determine critical and rate-limiting cellular processes for osmotic stress tolerance. In this regard, initial results obtained through mutational analysis in Arabidopsis thaliana are promising and have identified novel salt-tolerant as well as salt-hypersensitive mutants.