Analysis of chloroplastic malate valve by using mutants of metabolite transporters

由于拟南芥可塑性2-氧戊二酸/苹果酸转运体（OMT）基因的敲除突变体表现出轻微的生长延迟，我们测量了叶组织中的代谢物水平。基因敲除植株中2-氧葡萄糖酸盐和柠檬酸盐的含量高于野生型植株。虽然一些氨基酸（Glu, Asp， Ala和Ser）减少，但Gln在突变体中大量积累。在强光胁迫条件下，基因敲除植株比野生植株表现出更明显的光抑制。从这些结果，我们得出结论，OMT在碳和氮代谢的连接中很重要，苹果酸阀将基质还原等价物运输到细胞质中。相比之下，OMT mRNA积累量高出10倍、底物运输活性高出2倍的过表达系在表型上与野生型植物没有差异。拟南芥OMT基因对无机氮补充的响应是短暂的。我们发现这个基因的表达是由硝酸盐离子触发的。在一些c4植物（高粱、谷子和绿草）中，OMT基因以叶肉细胞特异性的方式表达。

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.