2 TRANSCRIPTION FACTORS, DREB1 AND DREB2, WITH AN EREBP AP2 DNA-BINDING DOMAIN SEPARATE 2 CELLULAR SIGNAL-TRANSDUCTION PATHWAYS IN DROUGHT-RESPONSIVE AND LOW-TEMPERATURE-RESPONSIVE GENE-EXPRESSION, RESPECTIVELY, IN ARABIDOPSIS/

Plant growth is greatly affected by drought and low temperature. Expre ssion of a number of genes is induced by both drought and low temperat ure, although these stresses are quite different. Previous experiments have established that a cia-acting element named DRE (for dehydration -responsive element) plays an important role in both dehydration- and low-temperature-induced gene expression in Arabidopsis. Two cDNA clone s that encode DRE binding proteins, DREB1A and DREB2A, were isolated b y using the yeast one-hybrid screening technique. The two cDNA librari es were prepared from dehydrated and cold-treated rosette plants, resp ectively. The deduced amino acid sequences of DREB1A and DREB2A showed no significant sequence similarity, except in the conserved DNA bindi ng domains found in the EREBP and APETALA2 proteins that function in e thylene-responsive expression and floral morphogenesis, respectively. Both the DREB1A and DREB2A proteins specifically bound to the DRE sequ ence in vitro and activated the transcription of the beta-glucuronidas e reporter gene driven by the DRE sequence in Arabidopsis leaf protopl asts. Expression of the DREB1A gene and its two homologs was induced b y low-temperature stress, whereas expression of the DREB2A gene and it s single homolog was induced by dehydration. Overexpression of the DRE B1A cDNA in transgenic Arabidopsis plants not only induced strong expr ession of the target genes under unstressed conditions but also caused dwarfed phenotypes in the transgenic plants. These transgenic plants also revealed freezing and dehydration tolerance. In contrast, overexp ression of the DREB2A cDNA induced weak expression of the target genes under unstressed conditions and caused growth retardation of the tran sgenic plants. These results indicate that two independent families of DREB proteins, DREB1 and DREB2, function as trans-acting factors in t wo separate signal transduction pathways under low-temperature and deh ydration conditions, respectively.