High level expression of C-4-specific NADP-malic enzyme in leaves and impairment of photoautotrophic growth in a C-3 plant, rice

The chloroplastic NADP-malic enzyme (NADP-ME) is a key enzyme of the C-4 ph otosynthesis pathway in NADP-ME type C-4 plants such as maize. To express t he 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 o f the enzyme were expressed under the control of the rice Cab promoter, Tra nsformants carrying the rice cDNA showed the NADP-ME activities in the leav es less than several-fold that of non-transformants, while those carrying t he 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 t hat expression of the rice C-3-specific NADP-ME is suppressed at co- and/or post-transcriptional levels by some regulation mechanisms intrinsic to ric e, while that of the foreign C-4-specific isoform can escape from such supp ression. The accumulation of the maize C-4-specific NADP-ME led to bleachin g of leaf color and growth hindrance in rice plants under natural light. Th ese deteriorative effects resulted from enhanced photoinhibition of photosy nthesis due to an increase in the level of NADPH inside the chloroplast by the action of the maize enzyme.