Gibberellin/abscisic acid antagonism in barley aleurone cells: Site of action of the protein kinase PKABA1 in relation to gibberellin signaling molecules

The antagonism between gibberellins (GA) and abscisic acid (ABA) is an impo rtant factor regulating the developmental transition from embryogenesis to seed germination. In barley aleurone layers, the expression of genes encodi ng alpha -amylases and proteases is induced by GA but suppressed by ABA. It has been shown that an ABA-induced protein kinase, PKABA1, mediates the AB A suppression of alpha -amylase expression. Using a barley aleurone transie nt expression system, we have now localized the site of action of PKABA1 re lative to other signal transduction components governing the expression of alpha -amylase. The expression of alpha -amylase can be transactivated by t he transcription factor GAMyb, which is itself induced by GA. A truncated G AMyb containing the DNA binding domain but lacking the transactivation doma in prevents the GA induction of cu-amylase, further supporting the notion t hat GAMyb mediates the GA induction of alpha -amylase expression. Although ABA and PKABA1 strongly inhibit the GA induction of alpha -amylase, they ha ve no effect on GAMyb-transactivated alpha -amylase expression. Using a GAM yb promoter-beta -glucuronidase construct, we also show that both ABA and P KABA1 repress the GA induction of GAMyb. In the slender mutant, GAMyb and a lpha -amylase are highly expressed, even in the absence of GA. However, thi s constitutive expression can still be inhibited by ABA, PKABA1, or an inhi bitor of cGMP synthesis. On the basis of these observations, we suggest tha t PKABA1 acts upstream from the formation of functional GAMyb but downstrea m from the site of action of the Slender gene product. Because PKABA1 inhib its the GA induction of the GAMyb promoter-beta -glucuronidase construct, i t appears that at least part of the action of PKABA1 is to downregulate GAM yb at the transcriptional level.