Cloning of a QTL Increasing Leaf Photosynthesis Rate in Rice

Rice yield is determined by the balance of a number of factors, including sink size, source strength, and carbohydrate translocation. Recent progress in rice genomics has promoted cloning genes underlying quantitative trait loci (QTLs) associated with sink size such as grain number and grain size. However, sink size-enhanced-lines do not often perform expected yield due to the decline of grain filling. Therefore, enhancement of source strength such as leaf photosynthesis is expected to be important to increase rice yield potential. In this study, we cloned and characterized GPS (GREEN FOR PHOTOSYNTHESIS), a QTL controlling leaf photosynthesis rate, using mapping populations derived from a cross between Takanari, one of the high-yielding and high-photosynthesis cultivars, and Koshihikari, a leading cultivar, both grown in Japan. The Farquhar photosynthesis model elucidated that Takanari allele increased photosynthesis rate by regulating carboxylation efficiency. Map-based cloning revealed that GPS was identical to NAL1 (NARROW LEAF1), a gene previously reported to control lateral leaf growth. The high-photosynthesis allele of GPS was found to be a partial loss-of-function allele of NAL1. This allele increased mesophyll cell number between vascular bundles, which led to thickened green leaves and consequently enhanced photosynthesis rate. A near isogenic line (NIL) carrying the Koshihikari allele of GPS in the Takanari genetic background performed lower yield potential than Takanari. These results indicate that utilization of the Takanari allele of GPS can enhance future high-yield breeding and provides a new strategy for increasing rice productivity.