Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 19

The xa5 showed a constitutive expression pat-
tern in different tissues, and the resistance of xa5
is not dose dependent (Iyer and McCouch 2004;
Jiang et al. 2006).

sion of recessive xa13 and the copper level in
xylem vessels can suppress Xoo growth, which
results in rice resistance to Xoo infection (Yuan
et al. 2010).
Suppressing the expression of dominant Xa13
can result in the same level of resistance to
PXO99 as mediated by recessive xa13 in rice;
suppressing recessive xa13 can generate plants
that are immune to PXO99 (Chu et al. 2006).
Since xa13 recessively regulates rice resistance,
suppressing Xa13/xa13 is one choice to improve
rice Xoo resistance in hybrid rice-breeding
programs. However, both dominant Xa13 and
xa13 are required for reproductive development.
Pathogen-induced or tissue-specific promoters
can be used to specifically suppress Xa13/xa13
in the infection sites.

xa13

The xa13 gene localized on the long arm of chro-
mosome 8, originally identified in cultivar BJ1,
recessively confers resistance to Philippine Xoo
race 6 (PXO99) (Ogawa et al. 1987). The indica
rice line IRBB13 carrying only xa13 against Xoo
is resistant to

50% of Xoo strains/isolates col-
lected from major rice-growing areas of China
and India (Shanti et al. 2001; Singh et al. 2003;
Li et al. 2009). This gene was isolated from
IRBB13 by a map-based cloning strategy (Chu
et al. 2006). The xa13 and its dominant allele
Xa13 , which is also named Os8N3 and OsS-
WEET11 (Yang B et al. 2006; Chen L et al.
2010), encode polytopic plasma membrane pro-
teins of the MtN3/saliva family (Figure 2.2;
Yuan et al. 2010). Promoter swap analysis con-
firmed that dominant Xa13 is a susceptibility
gene, and the cause of the functional difference
of recessive xa13 and dominant Xa13 in rice-
Xoo interaction is their promoter regions (Yuan
et al. 2009). The expression of dominant Xa13
is induced by the direct binding of the TAL
effector PthXo1 of Xoo strain PXO99 to the cis -
element, the UPT PthXol box, on the Xa13 pro-
moter (Yang B. et al. 2006; R omer et al. 2010;
Yuan et al. 2011). PXO99 is more sensitive to
copper, an essential micronutrient of plants and
an important element for a number of pesticides
in agriculture, than other Xoo strains (Yuan et al.
2010). The Xa13 protein cooperates with two
plasma membrane-localized copper transporter-
type proteins, COPT1 and COPT5, to promote
removal of copper from xylem vessels, where
Xoo multiplies and spreads to cause disease (Fig-
ure 2.2). Thus, the redistribution of copper in rice
plants facilitates Xoo spread in rice and results in
disease. Promoter mutations in dominant Xa13
result in recessive xa13 whose promoter lacks the
UPT PthXol box. PXO99 cannot induce the expres-

>

Xa21

Xa21 , localized on the long arm of chromo-
some 11, was first identified in wild rice Oryza
longistaninata . It confers resistance to diverse
races of Xoo from eight different countries and
has been used for breeding programs since the
1970s (Wang et al. 1996). This gene was cloned
by using a map-based cloning strategy in indica
rice line IRBB21 (Song et al. 1995). It encodes
a plasma membrane-localized LRR receptor
kinase protein (Figure 2.2). Xa21 -mediated resis-
tance is not expressed in the early developmental
stages and gradually increases from the seedling
stage to later stages, with 100% resistance at the
adult stage (Century et al. 1999). The gradually
increased expression of Xa21 gene during rice
development is associated with development-
controlled Xa21 -mediated resistance (Zhao et al.
2009). Ectopic expression of Xa21 gene can gen-
erate rice plants with a high level of resistance
to Xoo at both seedling and adult stages (Zhao
et al. 2009; Park et al. 2010b).
Xa21 gene is a single polymorphic deter-
minant that confers resistance to Xoo strains
expressing avirXa21 (Lee et al. 2008). Thus,
Xa21 is considered to be an R gene. The avirXa21
gene was isolated and later renamed Ax21