Nitrogen-Centric Molecular Mechanisms in Crop Plants Under Elevated CO₂: Interactions, Responses, and Implications

Increasing atmospheric carbon dioxide (CO₂) is transforming the climate space in which plants grow, severely affecting crop physiology and crop productivity. Elevated CO₂ enhances photosynthesis and biomass; however, it can nitrogen (N) metabolism, inhibiting the nutritional value and the yield capacity of crops. The most important central N-regulated protein machineries are: transporters, nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, and urease which control N assimilation, distribution and remobilization in crop plants. With high CO₂, these molecular components exhibit altered expression and activities mostly due to the reduction in N concentration. Complex systemic plant responses under N control like adaptation of photosynthetic capacity, flowering time, reproductive development and seed nutrient profiles further support the complexity of the interactions between C and N signaling. The high CO₂ environment requires a more holistic analysis of the regulatory networks of N metabolism and anticipative crop improvement strategies. Future breeding and crop improvement strategies should focus on enhancing the resilience of N assimilation by optimizing the N transporter function and maintaining C-N stoichiometry, thereby sustaining crop performance and nutritive quality under changing climatic conditions. The present review identifies the molecular processes that regulate N responses in crops grown under elevated CO₂, highlighting the differences between legume versus non-legume and C₃ versus C₄ plant responses and providing details that can ensure mitigation against negative impacts and outline future perspectives on crop improvement.