MODULAR NATURE OF ABSCISIC-ACID (ABA) RESPONSE COMPLEXES - COMPOSITE PROMOTER UNITS THAT ARE NECESSARY AND SUFFICIENT FOR ABA INDUCTION OF GENE-EXPRESSION IN BARLEY

The modular nature of the abscisic acid response complex (ABRC), the p romoter unit necessary and sufficient for abscisic acid (ABA) inductio n of gene expression in barley, is defined in this study. We investiga ted ABA induction of a barley late embrogenesis abundant (Lea) gene, H VA1, and found that the ABRC of this gene consists of a 10-bp box with an ACGT core (ACGT-box) and the 11 bp directly upstream, named coupli ng element 3 (CE3). Only one copy of this ABRC is sufficient to confer ABA induction when linked to a minimal promoter, Because we previousl y reported another ABRC in the barley HVA22 gene, which consists of an ACGT-box with a distal coupling element (CE1), exchange experiments w ere conducted to study the interaction among modular elements in these ABRCs. We show that ACGT-boxes in these ABRCs are interchangeable, in dicating that an ACGT-box can interact with either a distal or a proxi mal coupling element to confer ABA response, However, the two coupling elements are not fully exchangeable, Although CE3 can function either proximal or distal to the ACGT-box, CE1 is only functional at the dis tal position, The presence of both the distal and the proximal couplin g elements has a synergistic effect on the absolute level of expressio n as well as on ABA induction, These ABRCs function in both seed and v egetative tissues, In seeds, ABA induction of the ABRC containing the proximal CE3, but not the ABRC with the distal CE1, is enhanced in the presence of the transcription regulator Viviparous1, indicating that these two ABRCs are mediated by different ABA signal transduction path ways.