| Many plant genes that are not expressed under normal growth conditions are activated in response to low temperature, drought, or salt stress. Plants must sense the stress they are under, then transmit the signal to the cellular machinery and activate stress-regulated genes. To help understand the signal events involved in the process, we used the firefly luciferase reporter gene driven by the stress-responsive RD29A promoter to screen for Arabidopsis mutants defective in stress signaling. In this study, the identification of several genetic loci is reported. Mutations in the FIERY1 locus resulted in increased gene expression under low temperature, drought, salt, and abscisic acid (ABA) treatments. FIERY1 thus underlies a connecting point of these diverse signaling pathways. FIERY1 encodes an inositol polyphosphate 1-phosphatase and is proposed to mediate the degradation of the second messenger inositol 1,4,5-trisphosphate. On the other hand, mutation in the SAD1 (s&barbelow;upersensitive to A&barbelow;BA and d&barbelow;rought 1) locus rendered the mutant plants more sensitive in gene expression, seed germination and seedling growth to ABA and salt/drought stress, but the response to cold was not changed. sad1 is also defective in drought-induced ABA biosynthesis and is impaired at the last step of ABA biosynthesis, i.e. the conversion of ABA aldehyde to ABA. SAD1 encodes an Sm-like U6 snRNP and is predicted to participate in mRNA processing. Two other loci defined in this study were found to encode enzymes in the ABA biosynthetic pathways. LOS5 encodes a molybdenum cofactor sulfurase and LOS6 encodes a zeaxanthin epoxidase. Mutations in these loci diminished osmotic stress-induced gene expression, suggesting that osmotic stress signaling does require ABA. Through studies with SAD1, LOS5 and LOS6 loci, a feedback regulatory loop was also identified. In this regulatory loop, ABA stimulates the expression of ABA biosynthetic genes, and this self-regulation may confer a rapid response to osmotic stress by speeding up ABA biosynthesis. These genetic, molecular, and biochemical studies provide many new insights into the signal transduction mechanisms in response to environmental stresses, and present successful examples of a molecular genetic approach to understand complex processes such as stress signal transduction. |
