Rice blast fungus hijacks a rice RALF peptide to initiate infection
Plant-derived peptides regulate immunity and host-microbe interactions, but whether fungal pathogens exploit endogenous plant peptides for infection-site selection remains largely unknown. Rapid alkalinization factors (RALFs) are a family of plant peptide hormones that contribute to plant development, reproduction, stress responses, and cell-wall organization. Here, we show that OsRALF7, which is an immune-activating peptide in rice (Oryza sativa), is co-opted by the rice blast fungus Magnaporthe oryzae as a host-derived cue for appressorium formation, specialized infection structures required for host penetration. While synthetic OsRALF7 peptide induced defense responses in rice suspension cells, the same peptide induced appressorium formation in M. oryzae. OsRALF7-induced appressorium formation was associated with activation of the cAMP/PKA and Pmk1 MAPK pathways and required a hydrophobic surface. M. oryzae inoculated onto Osralf7 knockout plants formed fewer appressoria, and the positioning of these appressoria relative to the boundaries between two adjacent plant epidermal cells was altered. These findings suggest that OsRALF7 provides positional information for infection initiation for M. oryzae. OsRALF7 localized to the plasma membrane and cell wall in rice, and its appressorium-inducing activity required a conserved YISY motif and cysteine residues. This activity was not conserved in Colletotrichum higginsianum, as RALF23 from Arabidopsis thaliana did not induce appressorium formation but instead showed antifungal activity. Together, these findings uncover a dual function of endogenous plant immune-modulating RALF peptides at the plant-pathogen interface, where they can be exploited by M. oryzae as a host-derived cue for infection initiation while also retaining antifungal activity against other fungal species.