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.