Expression of the Granule-Bound Starch Synthase I (Waxy) Gene from Snapdragon Is Developmentally and Circadian Clock Regulated

The granule-bound starch synthase I (GBSSI or waxy) enzyme catalyzes one of the enzymatic steps of starch synthesis. This enzyme is responsible for the synthesis of amylose and is also involved in building the final structure of amylopectin. Little is known about expression of GBSSI genes in tissues other than storage organs, such as seeds, endosperm, and tuber. We have isolated a gene encoding the GBSSI from snapdragon (Antirrhinum majus). This gene is present as a single copy in the snapdragon genome. There is a precise spatial and developmental regulation of its expression in flowers. GBSSI expression was observed in all floral whorls at early developmental stages, but it was restricted to carpel before anthesis. These results give new insights into the role of starch in later reproductive events such as seed filling. In leaves the mRNA level of GBSSI is regulated by an endogenous circadian clock, indicating that the transition from day to night may be accompanied by abolition of expression of starch synthesis genes. This mechanism does not operate in sink tissues such as roots when grown in the dark.