Functional Incorporation of Sorghum Small Subunit Increases the Catalytic Turnover Rate of Rubisco in Transgenic Rice

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) limits photosynthetic CO2 fixation because of its low catalytic turnover rate (kcat) and competing oxygenase reaction. Previous attempts to improve the catalytic efficiency of Rubisco by genetic engineering have gained little progress. Here we demonstrate that the introduction of the small subunit (RbcS) of high kcat Rubisco from the C4 plant sorghum (Sorghum bicolor) significantly enhances kcat of Rubisco in transgenic rice (Oryza sativa). Three independent transgenic lines expressed sorghum RbcS at a high level, accounting for 30, 44 and 79% of the total RbcS. Rubisco was likely present as a chimera of sorghum and rice RbcS, and showed 1.32-1.50 fold higher kcat than in nontransgenic rice. Rubisco from transgenic lines showed a higher Km for CO2 and slightly lower specificity for CO2 than nontransgenic controls. These results suggest that Rubisco in rice transformed with sorghum RbcS partially acquires the catalytic properties of sorghum Rubisco. Rubisco content in transgenic lines was significantly increased over wild type levels but Rubisco activation was slightly decreased. The expression of sorghum RbcS did not affect CO2 assimilation rates under a range of CO2 partial pressures. The Jmax/Vcmax ratio was significantly lower in transgenic line compared to the nontransgenic plants. These observations suggest that the capacity of electron transport is not sufficient to support the increased Rubisco capacity in transgenic rice. Although the photosynthetic rate was not enhanced, the strategy presented here opens the way to engineering Rubisco for improvement of photosynthesis and productivity in the future.