Research to Dramatically Improve Rice Leaf Photosynthesis with Marker Assisted Selection

Improved individual-leaf photosynthesis (P_n) may be a key component of increased crop yield. P_n of young, fully expanded flag leaves ranges from ~20 to 30 µmol m^-2 s^-1 in rice cultivars; an indica cultivar, ‘Takanari’, has the highest recorded P_n. However, we recently found two high-P_n rice lines (HP lines) with higher P_n than ‘Takanari’ in backcrossed inbred lines derived from a cross between ‘Takanari’ and ‘Koshihikari’, a japonica cultivar with lower P_n. The P_n values of the HP lines were comparable to those of maize, a C₄ plant. This suggests that ‘Koshihikari’ has important genetic factors for further improvement of rice P_n. The research project described in this paper has three objectives: identification of quantitative trait loci (QTLs) and genes that contribute to the greatly increased P_n in ‘Takanari’, clarification of the molecular mechanisms and physiological functions of each gene, and development of high-P_n rice lines by stacking several genes. Using chromosome-segment-substitution lines, we have identified four QTL regions related to the high P_n of the HP lines. We are currently conducting map-based cloning and gene pyramiding together with examination of the physiological functions of each QTL. This project will potentially lead to greatly increased photosynthesis and productivity in rice.