Nitrogen assimilation plays a vital role in plant metabolism. Assimilation of nitrate, the primary source of nitrogen in soil, is linked to the generation of the redox signal nitric oxide (NO). An important mechanism by which NO regulates plant development and stress responses is through S-nitrosylation, that is, covalent attachment of NO to cysteine residues to form S-nitrosothiols (SNO). Despite the importance of nitrogen assimilation and NO signalling, it remains largely unknown how these pathways are interconnected. Here we show that SNO signalling suppresses both nitrate uptake and reduction by transporters and reductases, respectively, to fine tune nitrate homeostasis. Moreover, NO derived from nitrate assimilation suppresses the redox enzyme S-nitrosoglutathione Reductase 1 (GSNOR1) by S-nitrosylation, preventing scavenging of S-nitrosoglutathione, a major cellular bio-reservoir of NO. Hence, our data demonstrates that (S)NO controls its own generation and scavenging by modulating nitrate assimilation and GSNOR1 activity.

Transcriptional reprogramming in response to developmental changes or environmental inputs is regulated by a wide variety of transcription factors and cofactors. In plants, the stability of many transcriptional regulators is mediated by the ubiquitin-mediated proteasome. Recent reports suggest that additional post-translational modifications modulate the ubiquitination and thus stability of transcriptional regulators. In addition to well-recognized phosphorylative control, particularly conjugation to the ubiquitin-like protein SUMO as well as thiol modification by nitric oxide to yield S-nitrosothiols, are emerging as key regulatory steps for governing protein ubiquitination in the nucleus. Complex interplay between these different post-translational modifications may provide robust control mechanisms to fine tune developmental and stress-responsive transcriptional programs.

Water deficit is a major environmental constraint on crop productivity and performance and nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. This study aims to test the hypothesis that leaf spraying ofS‐nitrosoglutathione (GSNO), anNOdonor, improves the antioxidant defense in both roots and leaves of sugarcane plants under water deficit, with positive consequences for photosynthesis. In addition, the roles of key photosynthetic enzymes ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in maintainingCO₂assimilation ofGSNO‐sprayed plants under water deficit were evaluated. Sugarcane plants were sprayed with water orGSNO100 μMand subjected to water deficit, by adding polyethylene glycol (PEG‐8000) to the nutrient solution. Sugarcane plants supplied withGSNOpresented increases in the activity of antioxidant enzymes such as superoxide dismutase in leaves and catalase in roots, indicating higher antioxidant capacity under water deficit. Such adjustments induced byGSNOwere sufficient to prevent oxidative damage in both organs and were associated with better leaf water status. As a consequence,GSNOspraying alleviated the negative impact of water deficit on stomatal conductance and photosynthetic rates, with plants also showing increases in Rubisco activity under water deficit.