Dissection of abscisic acid signal transduction pathways in barley aleurone layers

Abscisic acid (ABA) induces genes that are highly expressed during late emb ryogenesis, but suppresses gibberellin (GA)-responsive genes essential for seed germination and seedling growth. Promoter elements necessary and suffi cient for ABA up- and down-regulation of gene expression have been previous ly defined in barley aleurone layers. We have studied the effect of a prote in phosphatase 2C, ABI1, an ABA-inducible protein kinase, PKABA1, and a tra nscription factor, VP1, on ABA action in a barley aleurone transient expres sion system. The observations have allowed us to dissect ABA signal transdu ction pathways leading to either induction or suppression of gene expressio n. The ABA induction of embryogenesis genes is highly inhibited in the pres ence of a mutated protein phosphatase 2C, encoded by the abi1-1 dominant mu tant gene that is known to block ABA responses in Arabidopsis. However, the abi1-1 gene product has no effect on the ABA suppression of a GA-responsiv e alpha -amylase gene. On the other hand, PKABA1 suppresses the expression of alpha -amylase genes, but has little effect on ABA up-regulated genes. T herefore, it appears that ABA induction and suppression follow two separate signal transduction pathways with the former inhibited by ABI1 and the lat ter modulated by PKABA1. The presence of VP1 enhances the ABA induction of late embryogenesis genes, but also suppresses germination specific genes. A schematic model based on these observations is presented to explain the ef fect of these regulatory proteins on ABA-mediated gene expression.