Nitric oxide-melatonin crosstalk as a stress-state-dependent signaling module in plants.

Nitric oxide (NO) and melatonin (MT) are ubiquitous plant stress signals, yet their interplay is often reduced to simple antioxidant additivity or fixed synergy. In this review, we synthesize current evidence into an evidence-supported working framework in which NO-MT crosstalk functions as a reversible, stress-state-dependent signaling module governed by NO set-point control, redox-encoded post-translational modifications, and phase-, tissue-, and network-gated feedback. We distinguish three operational states: feed-forward activation, buffering restraint, and network integration. We further define minimum diagnostic criteria for state assignment, including NO kinetics, blockability, turnover/denitrosation capacity, and target-selective S-nitrosation. This framework clarifies when NO-MT coupling is causal, when it is buffered, and when it is redirected by broader reactive oxygen species/calcium/hormone/hydrogen sulfide.