The capacity for regeneration of the substrate for Rubisco, ribulose 1,5-bisphosphate (RuBP), is one of the candidates for the bottleneck in photosynthesis in transgenic rice plants with overproduction of Rubisco. As the overproduction of chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was suggested to slightly enhance RuBP regeneration capacity, transgenic rice plants with co-overproduction of Rubisco and GAPDH were generated to improve photosynthesis. The amount of Rubisco increased by 8% to 18%, whereas GAPDH activity increased to 2.1- to 3.2-fold that of the wild-type. However, the rate of CO2 assimilation did not increase under conditions of high irradiance and different [CO2] partial pressures. The possibility that photosynthesis in the co-overproduced plants was co-limited by Rubisco, GAPDH, and other unknown factors is discussed.
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