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

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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.