Salicylic acid mediates graft incompatibility by repressing auxin-dependent vascular reconnection in tomato

Graft incompatibility limits the combination of scion and rootstock, yet the signals that block vascular reconnection remain poorly understood. Using incompatible tomato-pepper grafts, we show that salicylic acid (SA) over accumulates at the graft junction, predominantly in the rootstock, while incompatible scions exhibit enhanced pattern-triggered immunity. SA-deficient nahG tomato failed to rescue incompatibility and instead showed severely impaired self-graft healing, indicating that successful tissue reunion requires an optimal, rather than minimal, SA response. Defense-associated genes remained activated in failed grafts regardless of SA accumulation, placing SA downstream of incompatibility determination. Exogenous SA application phenocopied incompatibility, blocking xylem reconnection, inducing cell death, and suppressing auxin signaling. Incompatible grafts similarly displayed reduced auxin accumulation and response at the graft junction. Exogenous auxin partially rescued xylem reconnection in incompatible grafts but failed to activate the cambial regulator or fully restore compatibility. We propose that graft compatibility depends on SA-auxin homeostasis, where moderate SA supports wound-associated defense and regeneration, whereas excessive SA suppresses the auxin-dependent program required for xylem differentiation. These findings identify SA as a dose-dependent regulator of graft healing and link excessive immune activation to failed vascular regeneration.