Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 25

Xa21 ), was released in the Philippines (Toen-
niessen et al. 2003). “Angke” and “Cone,” which
carry Xa4

example, WRKY13 , GH3-2 , GH3-8 , OsDR8 ,or
C3H12 can be used alone for the improvement
of rice BB resistance by manipulating its expres-
sion with appropriate promoters (Hu et al. 2008;
Kou and Wang 2010; Deng et al. 2012). The
transgenic approach may especially enable the
use of resistance QTLs whose functions depend
on upstream signaling in an unfavorable genetic
background.

Xa7 , respectively,
are derived from an existing popular variety by
using MAS; both cultivars are well accepted by
farmers and consumers in Indonesia (Jena and
Mackill 2008). An aromatic resistance cultivar
developed by MAS is Pusa 1460 (Improved Pusa
Basmati 1), and this Indian aromatic germplasm
harbors xa13 and Xa21 (Gopalakrishnan et al.
2008). Another successful application of MAS
to improve resistance to BB is RP BIO 226
(Improved Samba Mahsuri). It was derived from
introgression of xa5 , xa13 , and Xa21 into a pre-
mium quality rice variety (Samba Mahsuri) in
India (Sundaram et al. 2008). QTLs are valuable
resources for durable and broad-spectrum resis-
tance. MAS can provide an efficient approach to
using major resistance QTLs for rice improve-
ment. However, the success of this approach
depends on the genetic backgrounds (Kou and
Wang 2010) and the nature of the effect being
introgressed. MAS also may not be applicable
for employing minor resistance QTLs in breed-
ing because it is costly; thus it might not be worth
to pursue QTLs with small effects. In addition,
pyramiding multiple QTLs may bring undesired
traits into an improved cultivar due to linkage
drag.
Genetic engineering provides another oppor-
tunity for rice breeding with capacity to break
the reproductive isolation between species and
make the best use of germplasm resources.
Genetic engineering can also make the best
use of a gene to improve BB resistance. For
example, Xa3/Xa26 -mediated resistance is influ-
enced by the developmental stage and the genetic
background. Expression of Xa3/Xa26 using a
constitutive promoter can enhance Xa3/Xa26 -
mediated resistance in an unfavorable genetic
background and generates resistant rice plants
at both seedling and adult stages but without
influencing agronomic performance (Cao et al.
2007a; Gao et al. 2010). A transgenic approach
may also be applied in improving BB resistance
by using a single minor resistance QTL. For

+ xa5 and Xa4 +

Conclusion and Future Prospects

In the last decade, researchers have made sub-
stantial progress in trying to understand the
interaction between rice and Xoo . A number of
genes contributing to qualitative and quantitative
resistance against Xoo have been characterized.
These genes provide a basis for further explor-
ing the defense signaling network and eluci-
dating the molecular mechanisms of rice resis-
tance. These genes also provide multiple choices
for rice resistance improvement by different
approaches.

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