Effects of introduction of sorghum RbcS with rice RbcS knockdown by RNAi on photosynthetic activity and dry weight in rice

Rubisco is the key enzyme responsible for photosynthetic CO2 fixation. Previously, we succeeded to increase the catalytic rate (kcat) of Rubisco by the overexpression of sorghum C4-type Rubisco small subunit (RbcS) in rice. These Rubisco are present as a chimera of sorghum RbcS and rice RbcS. In this study, we introduced an RNAi construct for rice RbcS into a sorghum RbcS overexpression line (SS10). Obtained double-transgenic lines iSS4 and iSS5 dominantly expressed sorghum RbcS and accumulated 35% and 20% of Rubisco content of non-transgenic rice (WT), respectively. Rubisco in these double-transgenic lines showed higher kcat and Km for CO2 than WT and SS10. Moreover, iSS4 showed higher photosynthetic nitrogen use efficiency under high CO2 conditions. These results demonstrate the usefulness of C4-type RbcS to improve photosynthetic performance under elevated CO2 conditions.