Convergent evolution of distinct immune sensor systems for fungal polygalacturonases in Brassicaceae

Plant pattern recognition receptors (PRRs) facilitate recognition of microbial surface patterns and mediate activation of plant immunity. Arabidopsis thaliana RLP42, a leucine-rich repeat (LRR) receptor protein (LRR-RP), senses fungal endopolygalacturonases (PGs) through a ternary complex comprising RLP42, the adapter kinase SOBIR1, and SERK proteins. Several fungal PGs harbor a conserved 9-amino acid fragment pg9(At), which is sufficient to activate RLP42-dependent plant immunity. Domain swap experiments using RLP42 and paralogous RLP40 sequences revealed a dominant role of the island domain (ID) for ligand binding and PRR complex assembly. Involvement of the ID in plant receptor function is reminiscent of plant phytosulfokine (PSK) perception through the receptor, PSKR, a LRR receptor kinase. Sensitivity to pg9(At), which is restricted to A. thaliana, exhibits notable accession specificity as active RLP42 alleles were found in only 16 of 52 accessions tested. Arabidopsis arenosa and Brassica rapa, two Brassicaceae species closely related to A. thaliana, perceive plant immunogenic PG fragments pg20(Aa) or pg36(Bra), which are distinct from pg9(At). Our study unveils unprecedented complexity and dynamics of PG pattern recognition receptor evolution within a single plant family. PG perception systems may have evolved rather independently as a result of convergent evolution even among closely related species.