Aberrant chloroplasts in transgenic rice plants expressing a high level ofmaize NADP-dependent malic enzyme

NADP-dependent malic enzyme (NADP-ME) is a major decarboxylating enzyme in NADP-ME-type C-4 species such as maize and Flaveria. In this study, chlorop lastic NADP-ME was transferred to rice (Oryza sativa L.) using a chimeric g ene composed of maize NADP-ME cDNA under the control of rice light-harvesti ng chlorophyll-a/b-binding protein (Cab) promoter. There was a 20- to 70-fo ld increase in the NADP-ME activity in leaves of transgenic rice compared t o that in wild-type rice plants. Immunocytochemical studies by electron mic roscopy showed that maize NADP-ME was mostly localized in chloroplasts in t ransgenic rice plants, and that the chloroplasts were agranal without thyla koid stacking. Chlorophyll content and photosystem II activity were inverse ly correlated with the level of NADP-ME activity. These results suggest tha t aberrant chloroplasts in transgenic plants may be caused by excessive NAD P-ME activity. Based on these results and the known fact that only bundle s heath cells of NADP-ME species, among all three C4 subgroups, have agranal chloroplasts, we postulate that a high level of chloroplastic NADP-ME activ ity could strongly affect the development of chloroplasts.