Abstract
We have previously reported that, in addition to typical cytosolic isoforms, rice has a novel type of PEPC targeted to the chloroplast (Osppc4). It was highly expressed in mesophyll cells in the leaves, being one of major PEPCs in the leaf blade. To study the functions of Osppc4, we compared non-transgenic rice and the knockdown lines in which Osppc4 expression was suppressed by the RNAi technique. Growth analysis showed that the knockdown suppressed nitrogen uptake/assimilation and resulted in stunting at the vegetative stage, which was more marked when rice plants were grown with ammonium than with nitrate as the nitrogen source. Comparison of leaf metabolomes suggested that knockdown suppressed the GOGAT reaction by reducing levels of organic acids, carbon skeleton donors for the ammonium assimilation and the subsequent amino acid synthesis. These results indicate that, in addition to glycolysis, rice has a unique route for the synthesis of organic acids that involves a chloroplastic PEPC, and this route functions as a major pathway for provision of carbon skeletons when rice plants are grown with ammonium.
