Whole-plant growth and N utilization in transgenic rice plants with increased or decreased rubisco content under different CO 2 partial pressures

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) strongly limits photosynthesis at lower CO₂concentration [CO₂] whereas 10 Rubisco limitation is cancelled by elevated [CO₂]. Therefore, increase or reduction in Rubisco content by transformation with a sense or an antisense RBCS construct are expected to alter the biomass production under different CO₂levels. RBCS-sense (125% Rubisco of wild-type) and -antisense (35% Rubisco of wild-type) rice (Oryza sativa L.) plants were grown for 63 days at three different CO₂levels: low [CO₂] (28 Pa), normal [CO₂] (40 Pa) and elevated [CO₂] (120 Pa). The biomass of RBCS-sense plants was 32% and 15% greater at low [CO₂] and normal [CO₂] than that of the wild-type plants, respectively, but did not differ at elevated [CO₂]. Conversely, the biomass of RBCS-antisense plants was the smallest at low [CO₂]. Thus, overproduction of Rubisco was effective for biomass production at low [CO₂]. Greater biomass production at low [CO₂] in RBCS-sense plants was caused by an increase in the net assimilation rate, and associated with an increase in the amount of N uptake. Furthermore, Rubisco overproduction in RBCS-sense plants was also promoted at low [CO₂]. Although it seems that low [CO₂]-growth additionally stimulates the effect of RBCS overexpression, such a phenomenon observed at low [CO₂] was mediated through an increase in total leaf N content. Thus, the dependence of the growth improvement in RBCS-sense rice on growth [CO₂] was closely related to the degree of Rubisco overproduction which was accompanied not only by leaf N content but also by whole plant N content.