Engineering starch biosynthesis for increasing rice seed weight: the role of the cytoplasmic ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase (AGPase) controls a rate-limiting step in starch biosynthesis. In cereals, manipulation of this enzyme is a prime target to increase starch production during seed development as a means to increase sink strength and, in turn, crop yields. The Escherichia coli glgC triple mutant (TM) gene, which encodes a highly active and allosterically insensitive AGPase, was introduced into rice and expressed during endosperm development. The mutated enzyme was targeted to either the amyloplast or cytoplasm to determine the relationship between intracellular location of ADP-glucose formation and starch synthesis. Transgenic rice seeds expressing the amyloplast or cytoplasmic AGPase-TM showed up to 13-fold higher levels of AGPase activity compared to untransformed plants when assayed in the presence of inorganic phosphate to suppress the endogenous activity. Plants having elevated cytoplasmic AGPase activity under Pi-inhibitory conditions showed increases in (14)C-sucrose labeling into starch and, in turn, increases (up to 11%) in seed weight over the wt. In contrast, transgenic plants expressing the amyloplast-targeted AGPase activity showed small to moderate increases in (14)C-sucrose-labeling rates into starch and either a moderate increase in seed weight or, in several instances, a reduction in seed weight. Our results demonstrate that the intracellular location of AGPase has a marked effect on the capacity of the enzyme to increase starch synthesis and, in turn, seed weight. (C) 2004 Elsevier Ireland Ltd. All rights reserved.