IDENTIFICATION OF A NEGATIVE REGULATOR OF GIBBERELLIN ACTION, HVSPY, IN BARLEY

To broaden our understanding of the molecular mechanisms of gibberelli n (GA) action, we isolated a spindly clone (HvSPY) from barley cultiva r Himalaya and tested whether the HvSPY protein would modulate GA acti on in barley aleurone. The HvSPY cDNA showed high sequence identity to Arabidopsis SPY along its entire length, and the barley protein funct ionally complemented the spy-3 mutation. HvSPY and SPY proteins showed sequence relatedness with animal O-linked N-acetylglucosamine transfe rases (OGTs), suggesting that they may also have OGT activity. HVSPY h as a locus distinct from that of Sin, a mutation that causes the const itutive GA responses of slender barley, which phenotypically resembles Arabidopsis spy mutants. The possibility that the HVSPY gene encodes a negative regulator of GA action was tested by expressing HvSPY in a barley aleurone transient assay system. HvSPY coexpression largely abo lished GA(3)-induced activity of an alpha-amylase promoter. Surprising ly, HvSPY coexpression increased reporter gene activity from an abscis ic acid (ABA)-inducible gene promoter (dehydrin), even in the absence of exogenous ABA. These results show that HvSPY modulates the transcri ptional activities of two hormonally regulated promoters: negatively f or a GA-induced promoter and positively for an ABA-induced promoter.