Hormonal regulation of α‐amylase gene expression in germinating wheat (Triticum aestivum) grains

The diffusion of GA₁and GA₃from the embryo, and the decline in ABA content of endosperm, were associated with the induction of α‐amylase (EC 3.2.1.1) gene expression in aleurone of intact wheat (Triticum aestivum L. cv. Maris Huntsman) grains germinated at 25°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‐oxidase. (2) the increase in content of GAs belonging to the early 13‐hydroxylation GA pathway, and (3) the accumulation of ent‐kaurene in grains imbibed in the presence of an ent‐kaurene oxidase inhibitor. Again, the initiation of GA biosynthesis in the scutellum was closely associated with the induction of α‐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 imbibition in order to induce α‐amylase activity in aleurone, and the response could be replicated by low doses of GA₁applied to de‐embryonated grains. After‐ripened wheat aleurone was relatively unresponsive to applied ABA in terms of suppression of GA‐induced α‐amylase production. Subtle differences were observed in the temporal pattern of α‐amylase gene expression between intact germinated grains and de‐embryonated grains challenged with GA₁. It appears that endogenous GAs are an important component of the embryo stimulus initiating α‐amylase gene expression in aleurone of germinating wheat grain, as originally proposed for barley. Their role in the synthesis of α‐amylase in scutellar epithelium remains to be clarified.