Abstract
SummaryThe rice XA21‐mediated immune response is activated on recognition of the RaxX peptide produced by the bacterium Xanthomonas oryzae pv. oryzae (Xoo). The 60‐residue RaxX precursor is post‐translationally modified to form a sulfated tyrosine peptide that shares sequence and functional similarity with the plant sulfated tyrosine (PSY) peptide hormones. The 5‐kb raxX‐raxSTAB gene cluster of Xoo encodes RaxX, the RaxST tyrosylprotein sulfotransferase, and the RaxA and RaxB components of a predicted type I secretion system. To assess raxX‐raxSTAB gene cluster evolution and to determine its phylogenetic distribution, we first identified rax gene homologues in other genomes. We detected the complete raxX‐raxSTAB gene cluster only in Xanthomonas spp., in five distinct lineages in addition to X. oryzae. The phylogenetic distribution of the raxX‐raxSTAB gene cluster is consistent with the occurrence of multiple lateral (horizontal) gene transfer events during Xanthomonas speciation. RaxX natural variants contain a restricted set of missense substitutions, as expected if selection acts to maintain peptide hormone‐like function. Indeed, eight RaxX variants tested all failed to activate the XA21‐mediated immune response, yet retained peptide hormone activity. Together, these observations support the hypothesis that the XA21 receptor evolved specifically to recognize Xoo RaxX.
Highlights
Host receptors activate innate immunity pathways upon pathogen recognition (Ronald & Beutler, 2010)
These results suggest that RaxX recognition by XA21 is restrained by different sequence and length requirements compared to its recognition by the root growth promoting receptor(s) for plant sulfated tyrosine (PSY) hormone(s)
The raxX-raxSTAB gene cluster is present in a subset of Xanthomonas spp
Summary
Host receptors activate innate immunity pathways upon pathogen recognition (Ronald & Beutler, 2010). Oryzae (Xoo) (Wang et al, 1996). Four Xoo genes that are required for activation of XA21-mediated immunity, are located in the raxX-raxSTAB gene cluster (Fig. 1). The 60-residue RaxX predicted precursor protein undergoes sulfation by the RaxST tyrosylprotein sulfotransferase at residue Tyr-41 (Pruitt et al, 2015). Located outside the raxX-raxSTAB gene cluster, the raxC gene, an ortholog of the tolC gene, encodes the predicted outer membrane channel for this secretion complex (da Silva et al, 2004). The raxPQ genes encode enzymes to assimilate sulfate into 3'-phosphoadenosine 5'-phosphosulfate (PAPS) (Shen et al, 2002), the sulfodonor for the RaxST sulfotransferase (Han et al, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
