Effects of overexpression of the Rubisco small subunit gene under the control of the Rubisco activase promoter on Rubisco contents of rice leaves at different positions

Photosynthetic activity is expected to be maintained during leaf senescence if the amount of Rubisco is maintained. As the mRNA level of the Rubisco gene decreases rapidly just before the stage of full expansion, its overexpression after full expansion would be effective. We previously found that the mRNA level of the Rubisco activase gene (RCA) is maintained during leaf senescence. In the present study, the Rubisco small subunit gene (RBCS) was overexpressed under the control of the promoter of RCA in rice. Although Rubisco content increased in the uppermost, fully expanded leaves of transgenic plants in comparison with that in wild-type plants, in the penultimate leaves it decreased to a level similar to that of the wild-type plants. In expanding leaves in transgenic plants, the mRNA levels of the transgene RBCS increased further, leading to increases in the mRNA levels of total RBCS and the Rubisco large subunit gene (RBCL). In the uppermost, fully expanded leaves and the penultimate fully expanded leaves, however, the mRNA levels of RBCS2 and total RBCS decreased drastically, and those of RBCL were similar to the wild-type levels. These results indicate that Rubisco content was not maintained during leaf senescence in the transgenic plants. One reason for this is that the expression of RBCS by the RCA promoter was weak in leaves at low positions. We discuss the possible reasons for this unexpected phenomenon.