Photosynthèse et Métabolisme (Unité de Recherche Associée Centre National de la Recherche Scientifique D 1128), Bâtiment 430, Université Paris-Sud, F-91405 Orsay cedex, France.
Plant Physiol. 1992 Sep;100(1):7-12. doi: 10.1104/pp.100.1.7.
The regulation of carbon partitioning between carbohydrates (principally sucrose) and amino acids has been only poorly characterized in higher plants. The hypothesis that the pathway of sucrose and amino acid biosynthesis compete for carbon skeletons and energy is widely accepted. In this review, we suggest a mechanism involving the regulation of cytosolic protein kinases whereby the flow of carbon is regulated at the level of partitioning between the pathways of carbohydrate and nitrogen metabolism via the covalent modulation of component enzymes. The addition of nitrate to wheat seedlings (Triticum aestivum) grown in the absence of exogenous nitrogen has a dramatic, if transient, impact on sucrose formation and on the activities of sucrose phosphate synthase (which is inactivated) and phosphoenolpyruvate carboxylase (which is activated). The activities of these two enzymes are modulated by protein phosphorylation in response to the addition of nitrate, but they respond in an inverse fashion. Sucrose phosphate synthase in inactivated and phosphoenolpyruvate carboxylase is activated. Nitrate functions as a signal metabolite activating the cytosolic protein kinase, thereby modulating the activities of at least two of the key enzymes in assimilate partitioning and redirecting the flow of carbon away from sucrose biosynthesis toward amino acid synthesis.
碳水化合物（主要是蔗糖）与氨基酸之间碳分配的调控在高等植物中研究得还很不充分。蔗糖和氨基酸生物合成途径竞争碳骨架和能量的假说已被广泛接受。在这篇综述中，我们提出了一个涉及胞质蛋白激酶调控的机制，其中通过对组成酶的共价修饰，在碳水化合物和氮代谢途径之间的分配水平上调节碳流。在不存在外源氮的情况下生长的小麦幼苗（Triticum aestivum）中添加硝酸盐对蔗糖的形成以及蔗糖磷酸合酶（失活）和磷酸烯醇丙酮酸羧激酶（激活）的活性有显著的、如果是短暂的影响。这两种酶的活性通过蛋白磷酸化来调节，以响应硝酸盐的添加，但它们的反应方式相反。蔗糖磷酸合酶失活，磷酸烯醇丙酮酸羧激酶被激活。硝酸盐作为信号代谢物激活胞质蛋白激酶，从而调节至少两种同化产物分配关键酶的活性，并将碳流从蔗糖生物合成重新定向到氨基酸合成。
