Abstract
The rice disease resistance (
R
) gene
Xa3/Xa26
(having also been named
Xa3
and
Xa26
) against
Xanthomonas oryzae
pv.
oryzae
(
Xoo
), which causes bacterial blight disease, belongs to a multiple gene family clustered in chromosome 11 and is from an AA genome rice cultivar (
Oryza sativa
L.). This family encodes leucine-rich repeat (LRR) receptor kinase-type proteins. Here, we show that the orthologs (alleles) of
Xa3/Xa26
,
Xa3/Xa26-2
, and
Xa3/Xa26-3
, from wild
Oryza
species
O. officinalis
(CC genome) and
O. minuta
(BBCC genome), respectively, were also
R
genes against
Xoo
.
Xa3/Xa26-2
and
Xa3/Xa26-3
conferred resistance to 16 of the 18
Xoo
strains examined. Comparative sequence analysis of the
Xa3/Xa26
families in the two wild
Oryza
species showed that
Xa3/Xa26-3
appeared to have originated from the CC genome of
O. minuta
. The predicted proteins encoded by
Xa3/Xa26
,
Xa3/Xa26-2
, and
Xa3/Xa26-3
share 91–99% sequence identity and 94–99% sequence similarity. Transgenic plants carrying a single copy of
Xa3/Xa26
,
Xa3/Xa26-2
, or
Xa3/Xa26-3
, in the same genetic background, showed a similar resistance spectrum to a set of
Xoo
strains, although plants carrying
Xa3/Xa26-2
or
Xa3/Xa26-3
showed lower resistance levels than the plants carrying
Xa3/Xa26
. These results suggest that the
Xa3/Xa26
locus predates the speciation of A and C genome, which is approximately 7.5 million years ago. Thus, the resistance specificity of this locus has been conserved for a long time.