Abscisic acid and water-stress induce the expression of a novel rice gene

We have identified a novel rice gene, called RAB 21, which is induced when plants are subject to water-stress. This gene encodes a basic, glycine-rich protein (mol. wt 16,529) which has a duplicated domain structure. Immunoblots probed with antibodies raised against beta-galactosidase/RAB 21 fusion protein detect RAB 21 protein only in cytosolic cell fractions. RAB 21 mRNA and protein accumulate in rice embryos, leaves, roots and callus-derived suspension cells upon treatment with NaCl (200 mM) and/or the plant hormone abscisic acid (10 microM ABA). The effects of NaCl and ABA are not cumulative, suggesting that these two inducers share a common response pathway. Induction of RAB 21 mRNA accumulation by ABA is rapid (less than 15 min in suspension cells) and does not require protein synthesis, indicating that preformed nuclear and/or cytosolic factors mediate the response to this hormone. We have characterized the RAB 21 gene by determining the complete nucleotide sequence of a nearly full-length cDNA and corresponding genomic copy, and by mapping the start site of its major transcript. The proximal promoter region contains various GC-rich repeats.