Novel receptor‐like kinases in cacao contain PR ‐1 extracellular domains

Summary Members of the pathogenesis‐related protein 1 ( PR ‐1) family are well‐known markers of plant defence responses, forming part of the arsenal of the secreted proteins produced on pathogen recognition. Here, we report the identification of two cacao ( T heobroma cacao   L .) PR ‐1s that are fused to transmembrane regions and serine/threonine kinase domains, in a manner characteristic of receptor‐like kinases ( RLKs ). These proteins ( TcPR ‐1f and TcPR ‐1g) were named PR ‐1 receptor kinases ( PR ‐1 RK s). Phylogenetic analysis of RLKs and PR ‐1 proteins from cacao indicated that PR ‐1 RK s originated from a fusion between sequences encoding PR ‐1 and the kinase domain of a L ec RLK ( L ectin R eceptor‐ L ike K inase). Retrotransposition marks surround TcPR ‐1f , suggesting that retrotransposition was involved in the origin of PR ‐1 RK s. Genes with a similar domain architecture to cacao PR ‐1 RK s were found in rice ( O ryza sativa ), barrel medic ( M edicago truncatula ) and a nonphototrophic bacterium ( H erpetosiphon aurantiacus ). However, their kinase domains differed from those found in L ec RLK s, indicating the occurrence of convergent evolution. TcPR ‐1g expression was up‐regulated in the biotrophic stage of witches' broom disease, suggesting a role for PR ‐1 RK s during cacao defence responses. We hypothesize that PR ‐1 RK s transduce a defence signal by interacting with a PR ‐1 ligand.