Exploiting the self-regulatory circuit of nitrate assimilation in plants for improved nitrogen use efficiency and crop sustainability.

联合国粮食及农业组织预计，在今后30至40年内，世界人口将增加三分之一。全球人口的增加将给农业系统带来巨大压力。要满足已经不堪重负的粮食、纤维和燃料需求，就需要同时提高农业生产。然而，对于主要的种植制度，实际平均产量在潜力的20%到80%之间。因此，开发新的策略来缩小收益差距具有重要的经济和社会意义。目前，作物生产力在很大程度上依赖于商业肥料的使用，可归因产量百分比高达90%。特别是，无机离子硝酸盐（陆地植物氮的主要来源）的供应是作物产量的主要瓶颈。由于其在水中的高流动性，硝酸盐离子经常从土壤中流出，最终导致环境影响。因此，目前的施肥战略往往效果有限，对人类健康和环境有潜在危害，同时每年仍造成数十亿美元的经济损失。相反，植物初级代谢的遗传和生化改良是提高作物产量同时更有效利用自然资源的一种安全、可持续的替代方法。为了应对土壤中硝酸盐的时空波动，植物进化出了根据自身的氮状态和土壤中硝酸盐浓度调节硝酸盐获取的能力。植物通过复杂的运输系统在根的质膜上主动运输硝酸盐。在有限的可用性下，硝酸盐的获取依赖于高亲和力的转运体，其对硝酸盐的招募和活性是通过翻译后修饰介导的。硝酸盐一旦被根系吸收，主要被运送到茎部，通过连续的同化反应将N原子进一步并入碳骨架，形成含N的有机分子，如氨基酸、蛋白质和核苷酸。作为自然界中最耗能的生化途径之一，硝酸盐的同化受到严格控制，以保证植物的正常发育和生长。多项证据表明，硝酸盐同化途径的通量与活性氮的产生有关。特别是，我们最近发现氧化还原活性分子一氧化氮（NO）是氮代谢的最终产物之一，通过硝酸盐同化途径反馈调节通量。NO的生物活性主要通过蛋白质翻译后修饰s -亚硝基化介导，即蛋白质sno的共价附着。我们的研究结果表明，细胞内蛋白质- sno的积累与硝酸盐转运蛋白的表达减少和同化反应的抑制有关。因此，与硝酸盐同化相关的反馈循环机制限制了植物的养分同化。值得注意的是，蛋白质- sno水平的遗传操纵显著影响植物的活力，表明这种反馈机制可以用来提高植物的生产力。然而，氮同化途径中氧化还原反应节点的身份以及它们如何控制植物适应性尚不清楚。在这里，我建议使用一种创新的、遗传的、基因组的和跨学科的成像技术来识别和综合操纵植物中反馈硝酸盐同化的代谢节点。此外，一氧化氮介导的氧化还原信号的遗传和生化管理有可能最终揭示新的化学和遗传靶点，可用于未来的作物改良策略。

The Food and Agriculture Organization of the United Nations projects that in the next 30-40 years the world will be one third more populous. This increase in the global population will put extraordinary pressure on agricultural systems. A concurrent boost in agricultural production will be required to meet already overburdened food, fiber and fuel demands. However, for major cropping systems, the actual average yield range between 20% and 80% of potential. Thus, the development of new strategies to reduce the yield gap is of great economic and social importance. Currently, crop productivity relies heavily on the use of commercial fertilizers, with attributable yield percentage reaching values as high as 90%. Particularly, supply of the inorganic ion nitrate, the primary source of nitrogen for land plants, represents a major bottleneck in crop yield. Due to its high mobility in water, nitrate ions are often runoff from the soil, eventually leading to environmental impact. Thus, current fertilization strategies often offer limited efficacy and are potentially hazardous to human health and the environment, while still leading to economic losses of billions every year. Conversely, genetic and biochemical improvement of plant primary metabolism represents a safe and sustainable alternative to increasing crop yield while making more efficient use of natural resources. To cope with fluctuations in its availability in time and space, plants have evolved the ability to modulate nitrate acquisition according to their N status and nitrate concentration in soil. Plants actively transport nitrate across the plasma membrane of the roots through the sophisticated transport systems. Under limiting availability, nitrate acquisition relies on high-affinity transporters, which recruitment and activity in response to nitrate is mediated by post-translational modifications. Once taken up by roots, nitrate is mainly transported to shoots for further incorporation of N atoms into carbon skeleton through sequential assimilatory reactions to form N-containing organic molecules, such as amino acids, proteins and nucleotides. As one of the most energy-consuming biochemical pathways in nature, nitrate assimilation is tightly controlled to ensure proper plant development and growth. Several lines of evidence indicate that flux in nitrate assimilation pathway is associated with production of reactive nitrogen species. Particularly, we have recently shown that the redox active molecule nitric oxide (NO), one of the end products of nitrogen metabolism, feedback regulates flux through nitrate assimilation pathway. NO bioactivity is mediated mainly through the protein post-translational modification S-nitrosylation, i.e. the covalent attachment of a NO moiety form protein-SNO. Our findings revealed that intracellular protein-SNO accumulation is associated with reduced expression of the nitrate transporters and inhibition of assimilatory reactions. Thus, a feedback loop mechanism associated with nitrate assimilation limits nutrient assimilation in plants. Remarkably, genetic manipulation of protein-SNO levels markedly impacted plant vigour, suggesting that this feedback mechanism can be harnessed to improve plant productivity. It remains unclear, however, the identity of the redox-responsive nodes in nitrogen assimilation pathway and how they operate to control plant fitness. Here I propose to use a innovative, genetic, genomic and inter-disciplinary imaging techniques to identify and synthetically manipulate metabolic nodes that feedback nitrate assimilation in plants. Moreover, the proposed genetic and biochemical management of NO-mediated redox signalling has the potential to ultimately reveal novel chemical and genetic targets that can be used in future crop improvement strategies.