Starch Content in Leaf Sheath Controlled by CO2-Responsive CCT Protein is a Potential Determinant of Photosynthetic Capacity in Rice.

CO2-responsive CCT protein (CRCT) is the suggested positive regulator of starch synthesis in vegetative organs, particularly the leaf sheath of rice. In this study, we analyzed the effects of the starch level in the leaf sheath on the photosynthetic rate in the leaf blade using CRCT overexpression and RNA interference (RNAi) knockdown transgenic rice grown under ambient (38 Pa) or elevated (100 Pa) CO2 conditions. In leaf sheath, the starch content was markedly changed in relation to CRCT expression levels under both CO2 conditions. In contrast, the soluble sugar and starch contents of the leaf blade were markedly increased in the knockdown line grown under elevated CO2 conditions. The overexpression or RNAi knockdown of CRCT did not cause large effects on the photosynthetic rate of the transgenic lines grown under ambient CO2 condition. However, the photosynthetic rate of the overexpression line was enhanced, while that of the knockdown line was substantially decreased under elevated CO2 conditions. These photosynthetic rates were weakly correlated with the nitrogen contents and negatively correlated with the total nonstructural carbohydrate contents. Thus, the capacity for starch synthesis in leaf sheath, which is controlled by CRCT, can indirectly affect the carbohydrate content, and then the photosynthetic rate in the leaf blade of rice grown under elevated CO2 conditions.