OsRALF26 Serves as an Endogenous Signal Recognised by <scp>XA21</scp> to Promote Robust and Distal Resistance in Rice

Plant immune receptors detect both microbe-derived and endogenous signals to activate defences. XA21, a rice immune receptor, confers strong race-specific resistance to a subset of Xanthomonas oryzae pv. oryzae (Xoo) strains by recognising the microbial sulphated peptide RaxX. However, the molecular basis for the notably robust XA21-mediated immune response has remained unclear. Here, we report that the small secreted peptide OsRALF26, previously identified as an Oryza-specific ligand for FERONIA-like receptor 1 (OsFLR1), is also directly perceived by XA21. Recognition of OsRALF26 by XA21 triggers a pronounced reactive oxygen species (ROS) burst, pathogenesis-related (PR) gene induction, and enhanced resistance to Xoo. Notably, silencing OsRALF26 leads to a spatially biased reduction in XA21-mediated resistance, particularly in distal tissues. These findings identify OsRALF26 as a host-derived ligand of XA21 that is required for full activation of XA21-mediated immunity in distal tissues, consistent with a role for OsRALF26 in spatial propagation of XA21-dependent defence. By integrating microbe-derived and endogenous signals, XA21 exemplifies a versatile immune strategy in rice. This dual recognition may have arisen through the introgression of XA21, which unintentionally conferred OsRALF26 responsiveness-thereby reinforcing immune robustness in rice varieties.