Photosynthetic Performance Of Transgenic Rice Plants Overexpressing Maize C-4 Photosynthesis Enzymes

Transgenic rice plants overexpressing maize C-4-specific phosphoenolpyruvate carboxylase (PEPC) exhibit a higher photosynthetic rate (up to 30%) and a more reduced O-2 inhibition of photosynthesis than untransformed plants. There is a small increase in the amount of atmospheric CO2 being directly fixed by PEPC. Similarly, transgenic rice plants overexpressing the maize chloroplastic pyruvate. orthophosphate dikinase (PPDK), also have higher photosynthetic rates (up to 35%) than untransformed plants. This increased photosynthetic capacity is at least in part due to an enhanced stomatal conductance and a higher internal CO2 concentration. Using conventional hybridization, we have integrated maize PEPC and PPDK genes into the same transgenic rice plants. In the segregating population, the photosynthetic rates of plants with high levels of both maize enzymes are up to 35% higher than those of untransformed plants.Under full-sunlight conditions, the photosynthetic capacity of field-grown PEPC transgenic rice plants is twice as high as that of untransformed plants. PEPC transgenic plants consistently have a higher photosynthetic quantum yield by photosystem II and a higher capacity to dissipate excess energy photochemically and nonphotochemically. Preliminary data from field tests show that the grain yield is about 10-30% higher in PEPC and 30-35% higher in PPDK transgenic rice plants relative to untransformed plants. Taken together. these results suggest that introduction of C-4 photosynthesis enzymes into rice has a good potential for enhancing the crop's photosynthetic capacity and yield.