Amount of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) Protein and levels of mRNAs of rbcS and rbcL in the leaves at different positions in transgenic rice plants with decreased content of Rubisco

The differences in the amounts of various photosynthetic components, including ribulose- 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and chlorophyll (Chl), and the activities of chloroplastic fructose-1,6-bisphosphatase (Cp-FBPase), NADP-glyceraldehyde-3- phosphate dehydrogenase (NADP-G3PDH), ribulose-5-phosphate kinase (Ru5P kinase), and sucrose-phosphate synthase (SPS) enzymes were examined in leaf blades at different leaf positions on the main culms of transgenic rbcS antisense rice plants. The transcript levels of rbcS and rbcL mRNAs were also determined. All the leaves were collected when the flag leaves became fully expanded. Although the Rubisco content was much lower in the upper leaves in the rbcS antisense plants than in the wild-type plants, the difference was less pronounced in lower leaves. Whereas the levels of rbcS mRNA in all the leaves were depressed in the antisense plants, the levels of rbcL mRNA did not differ. Both rbcS and rbcL mRNA levels in lower leaves were very low in the wild-type and antisense plants. Chl content and the activities of Cp-FBPase, NADP-G3PDH, Ru5P kinase, and SPS were slightly higher in the antisense plants. However, the changes among the leaves at different positions were similar to those in the wild-type plants. These results suggest that a selectively slow decrease in the Rubisco content in lower leaves occurs in the rbcS antisense plants and that it is caused by the attenuation of degradation and not by the maintenance of Rubisco synthesis.