Arabidopsis basic leucine zipper transcription factors involved in an abscisic acid-dependent signal transduction pathway under drought and high-salinity conditions

The induction of the dehydration-responsive Arabidopsis gene, rd29B, is med iated mainly by abscisic acid (ABA), Promoter analysis of rd29B indicated t hat two ABA-responsive elements (ABREs) are required for the dehydration-re sponsive expression of rd29B as cis-acting elements. Three cDNAs encoding b asic leucine zipper (type)type ABRE-binding proteins were isolated by using the yeast one-hybrid system and were designated AREB1, AREB2, and AREB3 (A BA-responsive element binding protein). Transcription of the AREB1 and AREB 2 genes is up-regulated by drought, NaCl, and ABA treatment in vegetative t issues. In a transient transactivation experiment using Arabidopsis leaf pr otoplasts, both the AREB1 and AREB2 proteins activated transcription of a r eporter gene driven by ABRE, AREB1 and AREB2 required ABA for their activat ion, because their transactivation activities were repressed in aba2 and ab i1 mutants and enhanced in an era1 mutant. Activation of AREBs by ABA was s uppressed by protein kinase inhibitors. These results suggest that both ARE B1 and AREB2 function as transcriptional activators in the ABA-inducible ex pression of rd29B, and further that ABA-dependent posttranscriptional activ ation of AREB1 and AREB2, probably by phosphorylation. is necessary for the ir maximum activation by ABA, Using cultured Arabidopsis cells, we demonstr ated that a specific ABA-activated protein kinase of 42-kDa phosphorylated conserved N-terminal regions in the AREB proteins.