Overproduction of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; E.C. 18.104.22.168) in rice did not necessarily improve photosynthesis. The reason for this was that a partial deactivation of Rubisco occurred in RBCS-overexpressed rice plants. Since a negative correlation between the amounts of Rubisco activase (RCA) and Rubisco has been reported for plants with overproduced or decreased RCA, the possibility that RCA content declines in RBCS-overexpressed rice plants was considered. The relationship between RCA and Rubisco contents was examined in RBCS-overexpressed and RBCS-antisense rice plants. Whereas the ratio of RCA to Rubisco contents in RBCS-antisense plants increased three- to fourfold as compared with that of the wild-type levels, this ratio decreased 60–70% of the wild-type levels in RBCS-overexpressed rice plants. Thus, an apparent trade-off between the amounts of RCA and Rubisco was observed in RBCS-transgenic rice plants. However, the amounts of several Calvin–Benson cycle enzymes changed in a similar manner to that of RCA in both types of RBCS-transgenic rice plants. When the relationships between the amounts of these enzymes, including RCA, and those of total leaf-N minus Rubisco-N and trichloroacetic acid (TCA)-soluble N were examined, there were no differences between them irrespective of genotypes. These results indicate that the negative correlation between the amounts of RCA and Rubisco in RBCS-transgenic rice plants is the result of a change in N allocation to Rubisco in transgenic rice plants rather than a trade-off. Such a negative correlation was also found for other Calvin–Benson cycle enzymes. In addition, since the amounts of these Calvin–Benson cycle enzymes and RCA were highly correlated with their mRNA levels irrespective of genotype, it is suggested that changes in the amounts of these proteins are regulated at their transcript levels by a change in N allocation to Rubisco.
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