Rice Genetics IV - IRRI books - International Rice Research Institute

High-level expression ofC 4photosynthetic genesin transgenic riceM. Matsuoka, H. Fukayama, M.S.B. Ku, and M. MiyaoAttempts have been made to transfer C 4traits to C 3plants by introducing achimeric gene construct containing cDNAs for C 4enzymes under the controlof active promoters in C 3plants. However, the levels of transcripts and proteinsin these transformants were far below those in C 4plants. Our studieshave demonstrated that the promoters for maize C 4-specific genes encodephosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphatedikinase (PPDK), which can drive the high-level expression of a reporter genein transgenic rice plants in an organ-specific, mesophyll-specific, and lightdependentmanner as in maize. These results suggest that the rice plantpossesses the regulatory factors necessary for high-level expression of theC 4-specific genes, and imply that the introduction of the intact maize geneswould lead to the high-level expression of the C 4enzymes in rice leaves. Theintroduction of the intact maize C 4-specific genes containing all exons andintrons and its own promoter and terminator sequences led to the high-levelexpression of the PEPC and PPDK proteins in the leaves of transgenic riceplants. The activities of PEPC and PPDK were 110- and 40-fold higher thanthose of nontransgenic rice, respectively. The high-level expression of eachC 4enzyme altered metabolism slightly but did not seem to increase the photosyntheticefficiency of transgenic rice leaves.Land plants, including many important crops such as rice, wheat, and barley, assimilateatmospheric CO 2 through the C 3 photosynthetic pathway and are classified as C 3plants. In C 3 plants, CO 2 is fixed into a C 3 compound, phosphoglycerate, in mesophyllcells, through carboxylation of ribulose bisphosphate (RuBP) catalyzed by RuBPcarboxylase–oxygenase (Rubisco) (Fig. 1). This reaction is inhibited up to 50%by atmospheric O 2 that competes with CO 2 at the active site of Rubisco, and O 2 fixedin this way wastes energy via the process of photorespiration. C 4 plants such as maizeand sugarcane have evolved a novel biochemical mechanism to overcome photorespiration.C 4 plants have succeeded in eliminating photorespiration by splittingphotosynthetic reactions between two morphologically distinct cell types, the bundlesheath and mesophyll cells (Furbank and Taylor 1995). In a C 4 plant, maize, CO 2 isHigh-level expression of C 4photosynthetic genes in transgenic rice 439