Screening of High kcat Rubisco among Poaceae for Improvement of Photosynthetic CO2 Assimilation in Rice

The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a major limitation of photosynthetic CO 2 assimilation in C 3 plants. In order to find useful Rubisco for improvement of photosynthesis in rice under elevated CO 2 , we analyzed the catalytic turnover rate (k cat ) of Rubisco in Poaceae including C 3 alpine plants, C, cold-resistant plants and C 4 plants. Rubisco in these plants showed 1.1- to 2.8-fold higher k cat than that in rice. However, the most of high k cat Rubisco also showed a higher km for CO 2 (Kc) than that of rice, indicating that increase in k cat led to decrease in the affinity for CO 2 . Rubisco in Festuca ovina, Phleum pratense and Sorghum bicolor showed relatively high k cat to Kc. Although the k cat of Rubisco in F. ouina and P. pratense was not so high (1.5-1.6 fold relative to rice), the Kc was comparable to that in rice and the amino acid sequence of RbcL shared higher identity to that in rice than that in S. bicolor. By contrast, Rubisco of S. bicolor showed considerably high k cat (2.5-fold relative to rice), which is considered to be the most important factor for improvement of photosynthesis. In our estimation, the expression of high k cat Rubisco of F. ovina and S. bicolor in rice could significantly enhance CO 2 assimilation at Ci of 50 Pa, the level assumed to be reached by the middle of this century.