Summary
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.