Rice (Oryza sativa L.) plants with substantially increased Rubisco content were obtained by Agrobacterium-mediated transformation with the rice rbcS sense gene under the control of the rice rbcS promoter. The primary transformants were screened for the ratio of Rubisco to leaf-N content, and the transformants with >120% wild-type levels of Rubisco were selected. In the progeny of the selected lines of the transformants, the mRNA levels of one member of the rbcS gene family were increased from 3.9- to 6.2-fold, whereas those of other members of the rbcS gene family were unchanged. The total levels of rbcS mRNA were increased from 2.1- to 2.8-fold. The levels of rbcL mRNA were increased from 1.2- to 1.9-fold. Rubisco protein content was significantly increased by 30% on a leaf area basis. The ratio of Rubisco-N to leaf-N was also increased by 10-20%, irrespective of N treatment. The specific activity of Rubisco per unit of enzyme protein was not different. However, light-saturated photosynthesis was not enhanced even when the rate was measured at low [CO2] where Rubisco becomes limiting for photosynthesis. Some lines showed lower photosynthesis at high [CO2] (>60 Pa). We conclude that introduction of additional sense rbcS leads to overexpression of rbcS and that this overexpression slightly up-regulates the gene expression of rbcL at the transcript level and enhances the amount of Rubisco holoenzyme. However, overproduction of Rubisco protein does not improve photosynthesis.
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