Analysis of chloroplastic malate valve by using mutants of metabolite transporters

由于拟南芥质体2-酮戊二酸/苹果酸转运蛋白（OMT）基因敲除突变体显示出轻微的生长延迟，我们测量了叶组织中的代谢物水平。与野生型植物相比，2-酮戊二酸和柠檬酸的水平在敲除植物中更高。虽然一些氨基酸（Glu，Asp，Ala和Ser）在突变体中减少，但Gln在突变体中大量积累。在强光胁迫下，基因敲除植株表现出比野生型植株更明显的光抑制。从这些结果中，我们得出结论，OMT是重要的碳和氮代谢的连接，苹果酸阀，运输基质还原当量的细胞质。与此相反，过表达线，有几十倍高积累的OMT mRNA和两倍高活性的底物转运表现出与野生型植物相比，在表型上没有差异。拟南芥OMT基因对无机氮补充的响应是瞬时表达。我们发现，该基因的表达是由硝酸根离子触发的。在某些C_4植物中（高粱、小米和绿色粟），OMT基因以叶肉细胞特异性方式表达。

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.