Regulation of starch biosynthetic enzymes in wheat
Starch is the major carbohydrate reserve in plant storage tissues and constitutes up to 80% of the daily human calorific intake. It is important both as an energy source, and also has many industrial usages. Despite its importance, we still do not understand the regulation of starch synthesis. Moderate elevated temperatures (>25°C) cause reduced starch deposition and lowered crop yield in developing wheat endosperm. Wheat plants grown from anthesis at 27°C compared to 20°C show decreased starch synthesis per endosperm, lowered starch synthase (SS) activity (most notably SSI and SSII), and loss of interaction of these SSs with starch branching enzymes (SBEs). It is unclear whether the loss of SS activity and related protein-protein interactions are due to a direct effect of elevated temperature on the enzymes, or indirect effects via regulatory mechanisms. As temperature increases, the dissolved oxygen concentration in the tissues decreases leading to lowered ATP concentrations and changes in cellular redox state. Evidence suggests that in some dicot plants, redox modulation may regulate some enzymes in the starch synthetic pathway. Unlike dicots, wheat (and other cereals) contains two isoforms (plastidial and cytosolic) of ADP-glucose pyrophosphorylase (AGPase). The data presented in this thesis indicates that both isoforms are redox modulated as 80% of total cellular AGPase activity was lost upon oxidation, more than can be accounted for by changes in one isoform. Immunoblotting indicated the presence of a disulphide bond between the two small subunits of AGPase which is lost under reducing and denaturing conditions. Further investigations showed that in maize, all SS isoforms were redox sensitive. In wheat, only SSI, SSII and granule-bound SS were redox sensitive with reducing conditions increasing activity. The SBEs were not shown to be redox sensitive. The results reported in this thesis provide further evidence that the starch synthetic pathway may be under redox control in vivo. Coupled with the study on temperature response, this implies that regulation of starch synthesis is highly complex and can be influenced by environmental conditions in a number of ways.