Photosynthesis and Biomass Improvement in Rice

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Transgenic rice (Oryza sativa L.) plants with different amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were obtained by the Agrobacterium-mediated transformation with the rice rbcS sense or antisense gene under the control of the rice rbcS promoter. Although Rubisco content was enhanced up to 150% wild type Rubisco in some sense-rbcS transformant, the analyses of the response of CO_2 assimilation to intercellular CO_2 partial pressure (A : Ci curve) did not show that enhancement of Rubisco content leads to an increase in CO_2-limited photosynthesis. In addition, several the sense transformant lines show decreased photosynthesis at 100 Pa CO_2. This was caused by partial deactivation of Rubisco in vivo.For the anti-rbcS plants, changes in the levels of protein and rbcL and rbcS mRNA of Rubisco and in the activities other several Calvin-cycle enzymes were examined in the leaves at different leaf positions. A decrease in Rubisco content always led to decreased photosynthesis, especially under low CO_2 conditions. Although no differences the activities of other enzymes and chlorophyll content were found between genotypes, only Rubisco content in leaves at lower positions remained relatively higher levels in antisense plants. Since rbcS mRNA levels were not detected in their leaves, these results indicate that suppression of Rubisco degradation occurred in the senescent leaves of the antisense plants. Thus, the existence of a specific degradation mechanism(s) of Rubisco during leaf senescence is suggested to be in vivo.