Send to

Choose Destination
Mol Plant Pathol. 2019 May;20(5):656-672. doi: 10.1111/mpp.12783. Epub 2019 Feb 18.

Variation and inheritance of the Xanthomonas raxX-raxSTAB gene cluster required for activation of XA21-mediated immunity.

Author information

Department of Plant Pathology and the Genome Center, University of California, Davis, CA, 95616, USA.
Research School of Biology, Australian National University, Canberra, 0200, Australia.
Department of Microbiology & Molecular Genetics, University of California, Davis, CA, 95616, USA.


The 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.


XA21; Xoo; plant immunity; raxX-raxSTAB gene cluster


Supplemental Content

Full text links

Icon for Wiley
Loading ...
Support Center