Genetically-clustered antifungal phytocytokines and receptor proteins function together to trigger plant immune signaling

Summary

• Phytocytokines regulate plant immunity via cell-surface receptors. Populus trichocarpa RUST INDUCED SECRETED PEPTIDE 1 (PtRISP1) exhibits an elicitor activity in poplar, as well as a direct antimicrobial activity against rust fungi. PtRISP1 gene directly clusters with a gene encoding a leucine-rich repeat receptor protein (LRR-RP), that we termed RISP- ASSOCIATED LRR-RP (PtRALR).

• In this study, we used phylogenomics to characterize the RISP and RALR gene families, and functional assays to characterize RISP/RALR pairs.

• Both RISP and RALR gene families specifically evolved in Salicaceae species (poplar and willow), and systematically cluster in the genomes. Two divergent RISPs, PtRISP1 and Salix purpurea RISP1 (SpRISP1), induced a reactive oxygen species (ROS) burst and mitogen- activated protein kinases (MAPKs) phosphorylation in Nicotiana benthamiana leaves expressing the respective clustered RALR. PtRISP1 triggers a rapid stomatal closure in poplar, and both PtRISP1 and SpRISP1 directly inhibit rust pathogen growth.

• Altogether, these results suggest that plants evolved phytocytokines with direct antimicrobial activities, and that the genes coding these phytocytokines co-evolved and physically cluster with their cognate receptors.