High-level expression of C₄ photosynthetic genes in transgenic rice

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.