MOLECULAR ASPECTS OF OSMOTIC-STRESS IN PLANTS

Plant molecular responses to osmotic stress are complex as evidenced b y the isolation of numerous OR (osmotic stress-regulated) genes. Altho ugh functions including osmolyte biosynthesis, membrane transport, sig nal transduction, and cellular protection have been predicted for OR g enes, few of them have been established. Current efforts toward isolat ing and analyzing the expression of individual OR genes should be repl aced by systematic approaches to analyze all OR genes simultaneously i n 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 AB A-independent pathways have been identified. Functional genetic approa ches using yeast and plant model systems are expected to complement cu rrent molecular analysis of overexpression of OR genes in transgenic p lants. These systems will help to establish functions of OR genes, to dissect osmotic stress-signaling pathways, and to determine critical a nd rate-limiting cellular processes for osmotic stress tolerance. In t his regard, initial results obtained through mutational analysis in Ar abidopsis thaliana are promising and have identified novel salt-tolera nt as well as salt-hypersensitive mutants.