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

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 (Theobroma 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‐1RKs). Phylogenetic analysis of RLKs and PR‐1 proteins from cacao indicated that PR‐1RKs originated from a fusion between sequences encoding PR‐1 and the kinase domain of a LecRLK (Lectin Receptor‐Like Kinase). Retrotransposition marks surround TcPR‐1f, suggesting that retrotransposition was involved in the origin of PR‐1RKs. Genes with a similar domain architecture to cacao PR‐1RKs were found in rice (Oryza sativa), barrel medic (Medicago truncatula) and a nonphototrophic bacterium (Herpetosiphon aurantiacus). However, their kinase domains differed from those found in LecRLKs, 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‐1RKs during cacao defence responses. We hypothesize that PR‐1RKs transduce a defence signal by interacting with a PR‐1 ligand.