Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 18

spectrum of resistance to Xoo at both seedling
and adult stages, without any effects on mor-
phology and agronomic performance (Gao et al.
2010). Furthermore, other factors may also con-
tribute to genetic background-controlled resis-
tance conferred by Xa3/Xa26 gene in addition of
the one influencing Xa3/Xa26 expression (Zhou
et al. 2009).
Domain swap analyses suggest that the LRR
domain of Xa3/Xa26 protein is an important
determinant of race-specific recognition during
rice- Xoo interaction; in addition, the juxtamem-
brane region of this protein also appears to
contribute to resistance specificity (Zhao et al.
2009). Four components in Xa3/Xa26 protein-
initiated defense-signaling pathway have been
identified (Figure 2.2). Although they function
downstream of Xa3/Xa26 protein in the defense
signaling leading to Xoo resistance, these com-
ponents can mediate a broad-spectrum resis-
tance compared with Xa3/Xa26 protein. For
example, WRKY45-2, a WRKY-type transcrip-
tion factor, positively regulates rice resistance to
Xoo , Xanthomonas oryzae pv. oryzicola ( Xoc )
causing bacterial streak, and M. oryzae caus-
ing fungal blast (Tao et al. 2009). WRKY13,
which is also a transcription factor and functions
upstream of WRKY45-2 in the rice- Xoo inter-
action, positively controls rice resistance to Xoo
and M. oryzae (Qiu et al. 2007; Tao et al. 2009).
C3H12, a nucleic acid-binding protein upstream
of WRKY45-2 in the rice- Xoo interaction, pro-
motes rice resistance against Xoo and Xoc (Deng
et al. 2012; Deng H. and Wang S. unpublished
data). OsDR10 , a gene of de novo origin and
encoding an unknown protein, negatively regu-
lates rice resistance to Xoo , and transgenic plants
with suppressed expression of OsDR10 gene
have been shown to have broad-spectrum resis-
tance to Xoo , including the Xoo strain that is com-
patible with Xa3/Xa26 gene (Xiao et al. 2009).
OsDR10 protein appears to function upstream of
WRKY13 in the rice- Xoo interaction.
Xa3/Xa26 gene belongs to a tandem clustered
multiple gene family, and paralogs of this family
have a similar tissue-specific expression pattern

(Sun et al. 2006; Xu S et al. 2007; Xu L et al.
2008). One paralog of this family, MRKa gene,
can mediate partial resistance to Xoo when it
is overexpressed (Cao et al. 2007b). The kinase
domain of MRKa protein can partially replace
the function of the kinase domain of Xa3/Xa26
protein in Xoo resistance, suggesting that the
functions of the paralogs in this family may be
partially conserved. This hypothesis is also sup-
ported by a recent report that another paralog of
this family, NRKe gene, regulates rice response
to raised temperature (Zhang et al. 2011). The
kinase domain of Xa3/Xa26 protein can replace
the function of the kinase domain of NRKe pro-
tein in response to temperature change.

xa5

The recessive xa5 , localized on the short arm of
chromosome 5, was first identified in varieties
of the DZ192 group in 1977 (Iyer and McCouch
2004). It mediates specific resistance to Japanese
races and Philippine races 1, 2, 3, and 5 by restric-
tion of bacterial movement, but not multipli-
cation (Iyer and McCouch 2004; Iyer-Pascuzzi
et al. 2008). This gene was cloned by a map-
based cloning approach combined with allele
sequence analysis (Iyer and McCouch 2004),
and further complementation testing confirmed
this gene (Jiang et al. 2006). The xa5 encodes
a typical gamma subunit of transcription factor
IIA (TFIIA ), which is one of general transcrip-

tion factors required for transcription by RNA
polymerase II (Iyer and McCouch 2004). There
are two nucleotide substitutions in the recessive
allele, which results in an amino acid substitu-
tion of dominant (susceptible) Xa5 gene. It is
speculated that Xoo TAL effectors usurp parts of
plant basal transcription machinery to regulate
rice gene expression; the missense mutation of
xa5 allele does not compromise its general func-
tion in transcription, but it may evade TAL viru-
lence functions (Gu et al. 2009; Boch et al. 2010).
Thus, xa5 displays resistance to Xoo . Xoo avrXa5
is an avirulence gene, which encodes a TAL-type
protein, corresponding to xa5 (Zou et al. 2010).

γ