Final technical report

This project sought to develop new means of creating variation in the structure of starch that accumulates in maize seeds, through manipulation of the enzyme starch synthase III (SSIII). The central hypothesis was that SSIII is responsible for construction of certain lengths of linear glucan chains within the major starch component amylopectin, and that manipulation of this enzyme could create new varieties of starch that might have novel utilities as a renewable resource. The hypothesis was proven to be true through analysis of the effects of maize du1- mutations, which affect the structure and function of SSIII. SSIII was found to be required for the formation for two distinct groups of chain lengths in maize amylopectin, specifically those containing 7-9 glucose units and those containing 37-55 glucose units. Decrease in the frequency of these chains, as compared to wild type, is accompanied by an increase in chains of 11-15 glucose units. A hypothesis consistent with these data is that one of the other SS isoforms produces chains in the range of 11-15 units, and these are then elongated by SSIII to the range of 37-55 units. In order to try to manipulate the activity of SSIII in novel ways, transgenic maize plants were constructed in which the presumed regulatory part of the protein was detached from the known catalytic region responsible for synthesis of linear glucan chains within starch. Three different transgenes were introduced into maize, each containing different truncated versions of SSIII. Transgenic plants were followed over several generations, and their structure of their starches were analyzed. Novel structures were in fact observed. Specifically, there was a large increase in the frequency of chains containing 9-15 glucose units as compared two wild type maize starch, and a decrease in the frequency of those with 18-30 units. These structures of starch are distinct from those that caused by null mutations that eliminate SSIII, indicated a novel functional effect on starch synthesis in the transgenic plants. The novel starches produced by this method can now be tested for any advantageous functional properties