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identified by different research groups are fre-
quently colocalized, which suggests the possi-
bility of a real QTL and/or a QTL with a broad
spectrum of resistance against
Xoo
.
ing that accumulation of thiamine regulated by
OsDR8
may be required for resistance to
Xoo
.
Mitogen-activated protein kinase (MAPK) cas-
cades have a pivotal role in PTI and ETI. Sev-
eral rice MAPKs have been reported to function
either as an activator or a suppresser in rice resis-
tance to
Xoo
. The MPK6 is a two-faced player in
rice-
Xoo
interactions; it functions as a positive
regulator in local resistance to
Xoo
, whereas it
is a negative regulator for SAR after
Xoo
infec-
tion (Shen et al. 2010). Auxin facilitates
Xoo
invasion of rice (Ding et al. 2008; Fu et al.
2011), and indole-3-acetic acid (IAA) is a major
form of auxin in most plants.
GH3-2
and
GH3-
8
encode IAA-amido synthetases that deactivate
IAA by conjugating it to an amino acid, which
prevents IAA-induced loosening of the cell wall,
the natural protective barrier of plant cells against
pathogens. Thus,
GH3-2
and
GH3-8
contribute
to basal resistance.
GH3-1
may function in the
same way as
GH3-2
and
GH3-8
in rice resis-
tance against
Xoo
(Domingo et al. 2009; Kou
et al. 2010).
More than one gene may contribute to a resis-
tance QTL.
WRKY13
and
GH3-2
, which are
tightly linked on rice chromosome 1, may col-
lectively contribute to a minor resistance QTL
conferring resistance to
Xoo
(Fu et al. 2011).
CharacterizedGenesContributingto
ResistanceQTLs
Recently, great progress has been made in char-
acterizing the genes contributing to resistance
QTLs against
Xoo
in rice. At least one major
QTL (
WRKY45
) that explains more than 10%
phenotypic variation and eight minor QTLs
(
NRR
,
WRKY13
,
OsDR8
,
MPK6
,
GH3-1
,
GH3-
2
,
GH3-8
, and
C3H12
) that explain less than
10% phenotypic variation against
Xoo
have been
characterized (Hu et al. 2008; Kou et al. 2010;
Fu et al. 2011; Deng et al. 2012). These genes
provide the preliminary information for under-
standing the molecular basis of rice quantitative
resistance to
Xoo
.
All the characterized genes contributing to
resistance QTLs against
Xoo
belong to defense-
responsive genes based on the features of their
encoding proteins and biochemical functions or
predicted functions in rice-
Xoo
interaction. The
encoding proteins of these genes appear to func-
tion in the MR protein-mediated defense path-
way or a basal defense pathway either as posi-
tive or negative regulators in rice resistance to
Xoo
.
WRKY45
locus has at least two alleles,
WRKY45-1
and
WRKY45-2
, which encode pro-
teins with a 10-amino acid difference (Tao et al.
2009).
WRKY45-1
acts as a negative regulator,
whereas
WRKY45-2
is a positive regulator in
rice resistance to
Xoo
. As described previously,
WRKY45-2
,
WRKY13
, and
C3H12
all function
in
Xa3/Xa26
-initiated defense signaling pathway
and
NRR
functions in
Xa21
-initiated defense sig-
naling pathway in rice-
Xoo
interactions.
OsDR8
encodes an enzyme-like protein involved in thi-
amine biosynthesis (Wang et al. 2006).
OsDR8
-
suppressing plants showed compromised resis-
tance to
Xoo
accompanied by reduced thiamine
level; exogenous application of thiamine restored
the resistance of the transgenic plants, suggest-
OtherGenesContributingtoQuantitative
Resistance
A number of defense-responsive genes have
been reported to positively or negatively regu-
late partial resistance to
Xoo
, such as
NH1
,
XB3
,
TGA2.1
,
Spl11
,
WRKY
62,
WRKY71
,
WRKY76
,
MPK5
,
MPK12
, and
Rac1.
However, their asso-
ciation with resistance QTLs to
Xoo
has not
been reported (Kou et al. 2010). As previously
described,
NH1
and
TGA2.1
are involved in
SAR, and
XB3
,
WRKY62
, and
WRKY76
function
in a
Xa21
-mediated defense pathway. The mutant
of
Spl11
confers broad-spectrum resistance to
both
Xoo
and
M. oryzae
(Zeng et al. 2004).
Overexpression of
WRKY71
in rice resulted in
constitutive expression of
NH1
and
PR1b
and
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