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 (k_cat) 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 k_cat and K_m 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. [ABSTRACT FROM AUTHOR]