Transgenic rice plants with an antisense gene inserted via Agrobacterium tumefaciens were used to explore the impact of the reduction of Rubisco activase (RCA) on Rubisco and photosynthesis. In this study, transformants containing 15% to 35% wild type Rubisco activase were selected, which could survive in ambient CO2 concentration but grew slowly compared with wild type controls. Gas exchange measurements indicated that the rate of photosynthesis decreased sig- nificantly, while stomatal conductance and transpiration rate did not change; and that the intercellular CO2 concentration even increased. Rubisco determination showed that these plants had approximately twice as much Rubisco as the wild types, although they showed 70% lower rate of photosynthesis, which was likely an acclimation response to the reduction in Rubsico activase and/or the reduction in carbamylation.
The cellular localization of Rubisco and Rubisco activase (RCA) in rice (Oryza sativa subsp. indica cv. Zhenong 952) leaf was investigated with immunogold-labeled electron microscope techniques on the basis of determining the diurnal changes of photosynthetic rate (Pn), Rubisco and RCA activities, and quantifying two enzyme contents in the leaf with immuno-diffusion method in order to understand why RCA activity decreased in the midday when its contents was high. The results showed that Rubisco mainly was located in chloroplast, and RCA were found both in chloroplast and mitochondria. The lowering of Rubisco in chloroplast as well as Rubisco activity at noon could be one of good reasons to explain the photosynthetic midday depression in leaf. The density of RCA in chloroplast reached the maximum at 14:00 and a valley at 11:00. The result much coincided with the activity of RCA in leaf. In mitochondria, the density of RCA changed abruptly in one day with the highest at 13:00 and it can well elucidate why the activities of Rubisco declined at noon when its amount was increasing. Therefore the cellular localization and/or distribution of Rubisco and RCA during a day is more important for Pn, Rubisco and RCA activities.
