Hos5 - a negative regulator of osmotic stress-induced gene expression in Arabidopsis thaliana

Osmotic stress activates the expression of many plant genes through ABA-dep endent as well as ABA-independent signaling pathways. We report here the ch aracterization of a novel mutant of Arabidopsis thaliana, hos5-1, which exh ibits increased expression of the osmotic stress responsive RD29A gene. The expression of several other stress genes are also enhanced by the hos5-1 m utation. The enhanced expression is specific to ABA and osmotic stress beca use low temperature regulation of these genes is not altered in the mutant. Genetic analysis indicated that hos5-1 is a recessive mutation in a single nuclear gene on chromosome ill. Double mutant analysis of hos5-1 and the A BA-deficient aba1-1 as well as the ABA-insensitive abi1-1 mutant indicated that the osmotic stress hypersensitivity of hos5-1 is not affected by ABA d eficiency or insensitivity. Furthermore, combined treatments of hos5-1 with ABA and osmotic stress had an additive effect on RD29A-LUC expression. The se results suggest that the osmotic stress hypersensitivity in hos5-1 may b e ABA-independent. The germination of hos5-1 seeds was more resistant to AB A. However, the hos5-1 mutation did not influence stomatal control and only slightly affected the regulation of growth and proline accumulation by ABA . The hos5-1 mutation reveals a negative regulator of osmotic stress-respon sive gene expression shared by ABA-dependent and ABA-independent osmotic st ress signaling pathways.