Integration of carbon and nitrogen signalling: A central framework for plant metabolism and development.

Carbon (C) and nitrogen (N) availability must be continuously integrated to sustain plant growth and development, yet the molecular mechanisms underlying this integration have long remained elusive. In this review, we highlight emerging evidence that C and N signaling converge on a regulatory network involving SnRK1, NLP7, TOR, and potentially nitric oxide. The clearest conceptual advance is the demonstration that SnRK1 directly phosphorylates the nitrate sensor and transcriptional regulator NLP7, providing an integration node through which energy limitation and C deficiency can suppress the primary nitrate response. SnRK1 activity is modulated by C status and the sucrose-specific signaling metabolite trehalose 6-phosphate, and by nitrate availability through HOS1-dependent regulation of KIN10 abundance. In parallel, nitric oxide is emerging as a key signal at the interface of nitrate assimilation, mitochondrial activity, sucrose responses, and TOR activation. The discovery of additional regulatory layers suggests that C/N integration is organized as interconnected signaling modules rather than single linear pathways. We further discuss how these signaling interactions are involved in plant metabolism and development. By coordinating nitrate assimilation with tricarboxylic acid cycle activity, redox metabolism, and C skeleton availability, C/N signaling aligns C anabolic capacity with N supply. These effects extend to developmental outputs, including meristem activity, root growth, shoot branching, and flowering. Together, these findings support a shift from viewing C and N signaling as parallel pathways to considering them as a unified regulatory system for metabolic and developmental plasticity.