Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 21

study is needed to elucidate the biochemical
function of Xa25 protein in rice- Xoo interaction.
Genetic studies have revealed that recessive
xa25 gene has the nature of dominance reversal;
it mediates Xoo resistance recessively at seeding
stage but dominantly at adult stage (thus it was
named Xa25(t) in Chen et al. 2002; Liu et al.
2011). However, transgenic rice plants carrying
both recessive xa25 and its dominant allele Xa25
as a transgene are susceptible to Xoo at both
seedling and adult stages, confirming that xa25
is a recessive gene. This dominance reversal of
xa25 -carrying plants is associated with reduction
of Xoo -induced expression of dominant Xa25 at
the adult stage as compared to Xa25 expression
at seedling stage.

including Xa2 , Xa4 , Xa7 , Xa10 , xa24 , Xa30 ,
Xa31(t) , and xa34(t) , have been fine-mapped.
The Xa2 gene is mapped to an approximately
190-kb region on the long arm of chromosome
4 (He et al. 2006). The Xa4 gene is defined by
a 47-kb DNA fragment on the long arm of chro-
mosome 11, and the Xa4 locus is linked or tightly
linked to the Xa3/Xa26 locus (Sun et al. 2003,
2004). Xa7 is located in a 118.3-kb region on the
long arm of chromosome 6 (Chen et al. 2008).
Xa10 is mapped to a 74-kb region on the long
arm of chromosome 11 (Gu et al. 2008). The
Xa22(t) is localized to a 100-kb region of chro-
mosome 11, and this locus is also tightly linked
to Xa3/Xa26 locus (Wang et al. 2003). The reces-
sive xa24 is mapped to a 71-kb DNA fragment
on the long arm of chromosome 2 (Wu X. et al.
2008). The Xa30 is mapped to a 38-kb region
on the long arm of chromosome 4 (Cheema et al.
2008). The Xa31(t) is limited to a length of about
100 kb on the long arm of chromosome 11 (Wang
et al. 2009). The recessive xa34(t) is defined to
a 204-kb DNA fragment near the centromeric
region of chromosome 1 (Chen et al. 2011). The
fine-mapping information of these genes will
facilitate breeding programs by marker-assisted
selection (MAS). Furthermore, the isolation of
these MR genes will deepen understanding of
molecular mechanism underlying BB disease in
general, and the opportunity to develop func-
tional markers for more precise breeding.

Xa27

Xa27 , localized on the long arm of chromosome
6, mediates resistance to diverse strains of Xoo ,
including Chinese Xoo strains and Philippine
Xoo races 2 to 6. It was isolated from indica
rice line IRBB27 by map-based cloning (Gu
et al. 2005). Xa27 encodes an apoplast protein
of 113 amino acids that has no distinguishable
sequence similarity to proteins from organisms
other than rice (Wu L. et al. 2008). The resis-
tant and susceptible alleles of Xa27 encode an
identical protein, whereas the promoters of this
pair of alleles have crucial sequence differences
that determine the specific recognition of Xoo
(Gu et al. 2005). The resistance of Xa27 is dose
dependent. The TAL effector AvrXa27 from Xoo
induces Xa27 expression by binding to the UPA
box (upregulated by AvrBs3) of the Xa27 pro-
moter (Boch et al. 2009). However, the recessive
MR gene xa5 can attenuate the Xa27 -mediated
resistance in rice, which suggests that Xoo TA L
effector could not use protein encoded by the
recessive xa5 as a transcription machinery to
activation of Xa27 (Gu et al. 2009).

Quantitative Resistance to Xoo

Researchers commonly study quantitative resis-
tance by identifying disease resistance QTLs. At
least 74 QTLs against Xoo have been identified
in different rice cultivars interacting with differ-
ent Xoo strains (Figure 2.3; Li et al. 1999; Luo
et al. 1998; Yu et al. 2003; Wang et al. 2005;
Li et al. 2006; Yang CD et al. 2006; Hu et al.
2008; Kou et al. 2010; Fu et al. 2011; Deng et al.
2012). These QTLs are distributed on all 12 chro-
mosomes. Several resistance QTLs span a large
segment of a chromosome, indicating the poor
quality of the data. However, resistance QTLs

Fine-MappedMR Genes

In addition to the fine-mapping of the seven
characterized MR genes, eight other MR genes,