Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 20

(activator of Xa21 -mediated immunity; Lee et al.
2009). Ax21 protein is conserved across a micro-
bial genus, and a sulfated 17-amino acid syn-
thetic peptide of the N-terminal region of Ax21,
axY S 22, which is 100% conserved in some
pathogens including Xoo and Xoc , is sufficient
for Ax21 activity (Lee et al. 2009). Ax21 protein
is consistent with the definition of PAMPs. Thus
Xa21 protein is also considered to be a PRR (Lee
et al. 2009). This is a typical example of PRR and
R protein not being strictly separate (Lee et al.
2009; Thomma et al. 2011).
Several key components of Xa21 protein-
initiated defense signaling pathway have been
identified. Five Xa21 binding proteins - E3
ubiquitin ligase/XB3, WRKY62/XB10, protein
phosphatase 2C (PP2C)/XB15, ATPase/XB24,
and Bip3 (also known as glucose-regulated pro-
tein 78) - were identified (Fig. 2). The E3 ubiq-
uitin ligase interacts with the kinase domain of
Xa21 protein and acts as a substrate for the Xa21
serine and threonine kinase; it is necessary for
full accumulation of the Xa21 protein and Xa21 -
mediated immunity (Wang et al. 2006). Xa21
protein binds to WRKY62 when its juxtamem-
brane motif and serine/threonine kinase catalytic
activity are present. WRKY62 functions as a
negative regulator in basal resistance and Xa21 -
mediated resistance (Peng et al. 2008). Another
WRKY transcriptor, WRKY76, also functions as
a negative regulator of Xa21 -mediated defense
(Seo et al. 2011). PP2C, which interacts with
the juxtamembrane motif and kinase domain of
Xa21 protein, can dephosphorylate autophos-
phorylated Xa21; it negatively regulates Xa21 -
mediated resistance (Park et al. 2008). ATPase
is physically associated with the juxtamembrane
motif and kinase domain of Xa21 protein in vivo ,
and it can enhance autophosphorylation of Xa21
protein by its enzymatic activity. Transgenic rice
plants overexpressing ATPase are compromised
for Xa21 -meditated resistance because Xa21
protein is degraded via endoplasmic reticulum-
associated degradation in the presence of Ax21
protein (Chen X et al. 2010). The endoplasmic
reticulum chaperone Bip3 can interact with Xa21

protein in vivo . Rice plants overexpressing Bip3
have decreased Xa21 protein accumulation and
inhibited Xa21 protein processing, which results
in compromised Xa21 -mediated resistance (Park
et al. 2010a).
Xa21 and Xa3/Xa26 are the same type of pro-
teins and have 53% sequence similarity (Song
et al. 1995; Sun et al. 2004). Domain swap anal-
yses have revealed that the defense signaling
pathways initiated by Xa21 and Xa3/Xa26 pro-
teins may partially overlap; the LRR domains are
important determinants of race-specific recogni-
tion of the Xa21 and Xa3/Xa26 proteins (Zhao
et al. 2009). Furthermore, the juxtamembrane
motifs of the two proteins may also influence the
pathogen recognition specificity, in addition to
being important for protein stability (Xu et al.
2006; Zhao et al. 2009).

xa25

The recessive gene, xa25 , localized on the
centromeric region of chromosome 12, confers
resistance to Philippine Xoo race 9 (PXO339). It
also encodes a plasma membrane protein of the
MtN3/saliva family similar to xa13 (Liu et al.
2011). The xa25 gene was isolated from indica
rice cultivar Minghui 63 by a map-based cloning
strategy. The encoding proteins of recessive
xa25 and its dominant allele Xa25 have eight
amino acid differences. Furthermore, there are
nucleotide differences in their promoter regions.
The expression of dominant Xa25 , but not
recessive xa25 , was rapidly induced by PXO339
but not other Xoo strains that are compatible with
recessive xa25 . The nature of the xa25 -encoding
protein and its expression pattern in comparison
with its dominant allele Xa25 in rice-PXO339
interaction suggest that the dominant Xa25 may
be a race-specific susceptible gene and the reces-
sive xa25 may be a Xoo -induced expressional
non-reaction mutant similar to the recessive
xa13 . The rice MtN3/saliva family contains
more than 20 paralogs. Some MtN3/saliva pro-
teins from different species can mediate glucose
transport (Chen L. et al. 2010, 2012). Further