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 CO2 assimilation in C3 plants. In order to find useful Rubisco for improvement of photosynthesis in rice under elevated CO2, we analyzed the catalytic turnover rate (kcat) of Rubisco in Poaceae including C3 alpine plants, C3 cold-resistant plants and C4 plants. Rubisco in these plants showed 1.1- to 2.8-fold higher kcat than that in rice. However, the most of high kcat Rubisco also showed a higher km for CO2 (Kc) than that of rice, indicating that increase in kcat led to decrease in the affinity for CO2. Rubisco in Festuca ovina, Phleum pratense and Sorghum bicolor showed relatively high kcat to Kc. Although the kcat of Rubisco in F. ovina 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 kcat (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 kcat Rubisco of F. ovina and S. bicolor in rice could significantly enhance CO2 assimilation at Ci of 50 Pa, the level assumed to be reached by the middle of this century