Regulation of Adp-glucose Pyrophosphorylase Subunit Expression –-a Key Enzyme for Starch Biosynthesis in Plants

INTRODUCTION Starch is the predominant carbon sink of storage organs and leaves of many plants. Substantial evidence from the analysis of starch-deficient mutants, transgenic plants, and enzyme kinetics suggest that ADP-glucose pyrophosphorylase (AGPase) catalyzes a key step for starch biosynthesis in both photosynthetic and nonphotosynthetic tissues. Fruit quality in tomato is largely a function of the soluble solids content of the fruit, and there is a direct relationship between starch levels early in fruit development and the soluble solids content at maturity. Moreover, the ability to synthesize starch shortly after fruit set, may be critical for successful organ development. Increased starch biosynthesis in tomato fruit may also lead to increased assimilate partitioning into fruit, and a decrease in non-edible biomass, reducing waste processing needs in the BLSS. Thus, AGPase appears to be an attractive biotechnological target for increasing starch synthesis in plants, both on earth and in space. The enzyme is a heterotetramer containing two S and two B subunits. We have identified three distinct genes for the S subunit and one gene for the B subunit (Chen et al., Plant Science 136:59-676, 1998). The purpose of this research is to determine the biochemical basis for regulation of AGPase activity in tomato, and to design novel ways of increasing starch content in plants by biotechnological manipulation of the gene(s) for this enzyme.