High level expression of C4-specific NADP-malic enzyme in leaves and impairment of photoautotrophic growth in a C3 plant, rice

The chloroplastic NADP-malic enzyme (NADP-ME) is a key enzyme of the C-4 photosynthesis pathway in NADP-ME type C-4 plants such as maize. To express the chloroplastic NADP-ME in leaves of a C-3 plant, rice, full-length cDNAs encoding the rice C-3-specific isoform and the maize C-4-specifrc isoform of the enzyme were expressed under the control of the rice Cab promoter, Transformants carrying the rice cDNA showed the NADP-ME activities in the leaves less than several-fold that of non-transformants, while those carrying the maize cDNA showed activities up to 30-fold that of non-transformants or about 60% of the NADP-ME activity of maize leaves. These results indicate that expression of the rice C-3-specific NADP-ME is suppressed at co- and/or post-transcriptional levels by some regulation mechanisms intrinsic to rice, while that of the foreign C-4-specific isoform can escape from such suppression. The accumulation of the maize C-4-specific NADP-ME led to bleaching of leaf color and growth hindrance in rice plants under natural light. These deteriorative effects resulted from enhanced photoinhibition of photosynthesis due to an increase in the level of NADPH inside the chloroplast by the action of the maize enzyme.