HORMONAL-REGULATION OF ALPHA-AMYLASE GENE-EXPRESSION IN GERMINATING WHEAT (TRITICUM-AESTIVUM) GRAINS

The diffusion of GA(1) and GA(3) from the embryo, and the decline in A BA content of endosperm, were associated with the induction of alpha-a mylase (EC 3.2.1.1) gene expression in aleurone of intact wheat (Triti cum aestivum L. cv. Maris Huntsman) grains germinated at 25 degrees C. The scutellum appeared to be the main site of de novo GA biosynthesis based on (1) the abundance of transcripts of a cloned wheat GA 20-oxi dase, (2) the increase in content of GAs belonging to the early 13-hyd roxylation GA pathway, and (3) the accumulation of ent-kaurene in grai ns imbibed in the presence of an ent-kaurene oxidase inhibitor. Again, the initiation of GA biosynthesis in the scutellum was closely associ ated with the induction of alpha-amylase gene expression in scutellar epithelium, although the two events may not have been causally linked. The embryo was required to be present for 36 h from the start of imbi bition in order to induce alpha-amylase activity in aleurone, and the response could be replicated by low doses of GA(1) applied to de-embry onated grains. After-ripened wheat aleurone was relatively unresponsiv e to applied ABA in terms of suppression of GA-induced alpha-amylase p roduction, Subtle differences were observed in the temporal pattern of alpha-amylase gene expression between intact germinated grains and de -embryonated grains challenged with GA(1). it appears that endogenous GAs are an important component of the embryo stimulus initiating alpha -amylase gene expression in aleurone of germinating wheat grain, as or iginally proposed for barley. Their role in the synthesis of alpha-amy lase in scutellar epithelium remains to be clarified.