Regulation of replenishment of carbon skeletons for nitrogen assimilation in roots.

氮素同化酶活性的增加和碳骨架作为氮素受体的补充都是氮素继续同化进入根系和同化物输出到地上部所必需的。本研究的目的是为了明确根系暗碳固定对碳骨架补充所必需的调控。小麦、大麦和番茄根部的磷酸烯醇式丙酮酸羧化酶（PEPC）活性随着NH_4^+的增加而增加，并且在供氮开始后7天，饲喂NH_4^+的植物的PEPC活性比饲喂NO_3的植物高2-2.5倍。蛋白质印迹分析表明，PEPC亚基蛋白的量增加，反映在可提取的PEPO活性。苹果酸、柠檬酸和酸性氨基酸对番茄根中PEPC的活性有抑制作用。这些结果表明，PEPC活性是由从头蛋白质合成和相关代谢物水平的变化。NH_4^+处理1d的小麦根系暗碳固定率显著提高。小麦根中PEPC活性增加，PEPC蛋白质NH_4^+积累。这些结果表明，根中NH_4^+的初级同化是促进暗碳固定的关键事件，这与供应NH_4 ^+时根中PEPC活性的增加相一致。

Both increased activity of nitrogen assimilating enzymes and replenishment of carbon skeletons as acceptors of nitrogen are required to continue the assimilation of nitrogen that enters into roots and export of assimilates to shoot. The objective of this study is to make clear regulation of dark carbon fixation in roots necessary for replenishment of carbon skeletons. The activity of phosphoenolpyruvate carboxylase (PEPC) in wheat, barley and tomato roots increased in response to NH_4^+ , and the NH_4^+-fed plants exhibited a 2-2.5-fold higher PEPC activity than NO_3 -fed plants at 7 days after the onset of nitrogen supply. Western blot analysis revealed that the amounts of PEPC subunit proteins increased as reflected in the extractable PEPO activity. Malate, citrate and acidic amino acids inhibited the activity of PEPC purified from tomato roots. These results suggest that the PEPC activity is regulated by de novo protein synthesis and by changes in related-metabolite levels. The dark carbon fixation rate increased remarkably in roots of wheat plants that had been grown with NH_4^+ for 1 day. In response to, PEPC activity increased and PEPC proteins NH_4^+ accumulated in wheat roots. Neither activity nor amounts of PEPC in roots did not increase in the presence of MSX.These findings suggest that the primary assimilation of NH_4^+ in roots is a crucial event for stimulation of dark carbon fixation, which coincides with the increased root PEPC activity when NH_4^+ is supplied.