High-level expression of C4 photosynthetic genes in transgenic rice
This work was supported in part by a PROBRAIN grant from the Bio-Oriented Technology Research Advancement Institution (BRAIN) of Japan.
Attempts have been made to transfer C4 traits to C3 plants by introducing a chimeric gene construct containing cDNAs for C4 enzymes under the control of active promoters in C3 plants. However, the levels of transcripts and proteins in these transformants were far below those in C4 plants. Our studies have demonstrated that the promoters for maize C4-specific genes encode phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK), which can drive the high-level expression of a reporter gene in transgenic rice plants in an organ-specific, mesophyll-specific, and light-dependent manner as in maize. These results suggest that the rice plant possesses the regulatory factors necessary for high-level expression of the C4-specific genes, and imply that the introduction of the intact maize genes would lead to the high-level expression of the C4 enzymes in rice leaves. The introduction of the intact maize C4-specific genes containing all exons and introns and its own promoter and terminator sequences led to the high-level expression of the PEPC and PPDK proteins in the leaves of transgenic rice plants. The activities of PEPC and PPDK were 110- and 40-fold higher than those of nontransgenic rice, respectively. The high-level expression of each C4 enzyme altered metabolism slightly but did not seem to increase the photosynthetic efficiency of transgenic rice leaves.