Growth of Rice Overexpressing ADP-Glucose Pyrophosphorylase in Leaf and Seed Tissue Under Varying Levels of Nitrogen

Starch is an important plant metabolite, playing a role in both sink and source strength. Leaf and seed starch biosynthesis are both important to cereal yield since carbohydrates are stored transiently as starch in leaves and long term as starch in seeds. The rate limiting step in starch biosynthesis is controlled by the enzyme ADP-glucose pyrophosphorylase (AGPase). The lack of AGP, and starch production, in leaves or seeds diminishes plant productivity while AGP up-regulation increases yield. Overexpression of AGPase in both leaves and seeds leads to greater yield increases than observed with expression of the leaf or seed AGP transgene alone. Examination of global changes in metabolites and gene transcripts revealed that carbon metabolism was additively upregulated in plants with expression of both the leaf and seed AGPase transgenes. In this study, we examined growth and metabolism of rice plants with expression of the AGPase transgene in leaves and seeds compared to the WT when grown under varying levels of nitrogen. Preliminary data indicates the largest increase in plant yield and upregulation of carbon metabolism in response to AGPase overexpression occurs at optimal levels of nitrogen (100-150 ppm). At low nitrogen, yield increases and carbon upregulation were not observed. Together, these results indicate that yield advances in cereals are possible by targeting starch biosynthesis in both leaves and seeds. Our results further indicate that yield increases conditioned by increased leaf and seed starch are highly interdependent with nitrogen level.