Analysis of chloroplastic malate valve by using mutants of metabolite transporters

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Because an Arabidopsis knockout mutant of plastidic 2-oxoglutarate/malate transporter (OMT) gene showed slightly delay of growth, we measured metabolite levels in the leaf tissues. The levels of 2-oxoglutarate and citrate were higher in the knockout plant compared with wild-type plant. Although some amino acids (Glu, Asp, Ala and Ser) were decreased, Gln was abundantly accumulated in the mutant. The knockout plant showed more prominent photoinhibition than wild-type plant under the high-light stress condition. From these results, we concluded that OMT is important in the connection of carbon and nitrogen metabolisms, and malate valve that transports stromal reducing equivalents to cytosol. In contrast, overexpression lines that had several ten-fold higher accumulation of OMT mRNA and two-fold higher activities of substrate transport showed no difference in phenotype compared with the wild-type plant. The Arabidopsis OMT gene expresses transiently in response to inorganic nitrogen supplementation. We revealed that this gene expression is triggered by nitrate ion. In some C_4 plants (sorghum, finger millet and green panic), OMT genes express in a mesophyll cell-specific manner.