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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

1
Department of Plant Pathology and the Genome Center, University of California, Davis, CA, 95616, USA.
2
Research School of Biology, Australian National University, Canberra, 0200, Australia.
3
Department of Microbiology & Molecular Genetics, University of California, Davis, CA, 95616, USA.

Abstract

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.

KEYWORDS:

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

PMID:
30773771
DOI:
10.1111/mpp.12783

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