A small decrease in rubisco content by individual suppression of RBCS genes leads to improvement of photosynthesis and greater biomass production in rice under conditions of elevated CO₂

Rubisco limits photosynthesis at low CO₂ concentrations ([CO₂]), but does not limit it at elevated [CO₂]. This means that the amount of Rubisco is excessive for photosynthesis at elevated [CO₂]. Therefore, we examined whether a small decrease in Rubisco content by individual suppression of the RBCS multigene family leads to increases in photosynthesis and biomass production at elevated [CO₂] in rice (Oryza sativa L.). Our previous studies indicated that the individual suppression of RBCS decreased Rubisco content in rice by 10-25%. Three lines of BC2F2 progeny were selected from transgenic plants with individual suppression of OsRBCS2, 3 and 5. Rubisco content in the selected lines was 71-90% that of wild-type plants. These three transgenic lines showed lower rates of CO₂ assimilation at low [CO₂] (28 Pa) but higher rates of CO₂ assimilation at elevated [CO₂] (120 Pa). Similarly, the biomass production and relative growth rate (RGR) of the two lines were also smaller at low [CO₂] but greater than that of wild-type plants at elevated [CO₂]. This greater RGR was caused by the higher net assimilation rate (NAR). When the nitrogen use efficiency (NUE) for the NAR was estimated by dividing the NAR by whole-plant leaf N content, the NUE for NAR at elevated [CO₂] was higher in these two lines. Thus, a small decrease in Rubisco content leads to improvements of photosynthesis and greater biomass production in rice under conditions of elevated CO₂.