Crosstalk Between Gibberellin and Abscisic Acid in Cereal Aleurone

The antagonism between gibberellins (GA) and abscisic acid (ABA) is an important factor regulating the developmental transition from embryogenesis to seed germination. In cereal aleurone layers, the expression of genes encoding hydrolytic enzymes needed for seedling growth, such as α-amylases and proteases, is induced by GA but suppressed by ABA. In addition, ABA induces the expression of genes that may play a role in the establishment of stress tolerance. Because of these well-defined biochemical and molecular markers, the cereal aleurone layers have been used as a convenient system for studying GA/ABA actions. Both gain- and loss-of-function approaches have been followed by the constitutive or the RNAi-mediated knockdown expression of specific regulatory molecules. The GA signaling pathway is anchored by the transcription factor GAMyb, which interacts with a specific region in the promoter of GA up-regulated genes, and an upstream regulatory molecule SLN1 (SLR1) that appears to be a functional homolog of the Arabidopsis GAI/RGA regulatory proteins. It is established that the ABA induction and suppression of gene expression follow two distinct signaling pathways, with the former requiring a transcription factor ABI5, but inhibited by a protein phosphatase 2C, and the latter mediated by a protein kinase PKABA1 and an unknown factor. The ABA suppression action has been pinpointed to be upstream from the formation of functional GAMyb but downstream from the site of action of SLN1 (SLR1).