Post-Translational Control of Nitrate Reductase Under Elevated CO2 in Solanum lycopersicum: Carbon–Nitrogen Signaling and Photosynthetic Acclimation
Rising atmospheric CO2 is altering carbon–nitrogen interactions in C3 crops, with tomato (Solanum lycopersicum L.) showing enhanced carbon assimilation but frequently reduced nitrogen acquisition and assimilation. Nitrate reductase (NR), the rate-limiting enzyme in nitrate reduction, plays a central role by integrating nitrate assimilation with carbon metabolism and nitric oxide (NO) signaling. This review summarizes current knowledge of NR regulation in tomato under elevated CO2 (eCO2), focusing on post-translational mechanisms and their contribution to photosynthetic acclimation. Elevated CO2 modulates NR activity through interconnected changes in photorespiration, carbohydrate-mediated feedback, redox regulation, source–sink dynamics, and nitrogen availability. While eCO2 generally suppresses leaf nitrate assimilation by reducing photorespiratory support, root-zone CO2 enrichment can transiently stimulate root NR activity, highlighting tissue-specific regulation. Multi-omics studies further demonstrate extensive metabolic and molecular reprogramming affecting carbon skeleton supply, amino acid biosynthesis, and nitrogen assimilation. In addition, NR-dependent NO production links nitrogen metabolism with stomatal regulation through ABA-independent H2O2–NO signaling. Despite these advances, the roles of NR phosphorylation, 14-3-3 protein interactions, and redox-mediated regulation under eCO2 remain poorly understood. Overall, NR functions as a key metabolic and signaling hub coordinating carbon and nitrogen metabolism under future climate conditions. Understanding these regulatory mechanisms will facilitate strategies to improve nitrogen-use efficiency, sustain photosynthesis, and enhance tomato productivity under elevated atmospheric CO2 while identifying priorities for future physiological, molecular, and multi-omics research.