Metabolite-level carbon-flow analysis reveals that starch synthesis from hexose is a limiting factor in a high-yielding rice cultivar

Understanding the limiting factors of grain filling is essential for the further improvement of rice (Oryza sativa L.) grain yields. The slower grain growth of Momiroman, a high-yielding rice cultivar, is not improved by increasing the carbon supply. Thus, a low sink activity, which is the metabolic activity of assimilate consumption/storage in sink organs, may be a limiting factor of grain filling. However, there is no metabolic evidence corroborating this hypothesis, partly because there is no consensus on how to define and quantify sink activity. In this study, we investigated the carbon flows, at a metabolite level, from photosynthesis in leaves to starch synthesis in grains of three high-yielding cultivars using a stable isotope of carbon, 13C. The large amount of newly fixed carbon assimilates in Momiroman was stored as hexose instead of being converted to starch. Additionally, the activity of ADP-glucose pyrophosphorylase and the expression of AGPS2b, encoding an ADP-glucose pyrophosphorylase protein, in the superior grains of Momiroman were lower than in the other two rice cultivars. Thus, the slower starch synthesis from hexose, which is partly explained by the low expression level of AGPS2b, may be the primarily metabolic reason for the lower sink activity in Momiroman. (199/200 words).