Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 17

Fig. 2.2. Molecular mechanisms of characterized major disease resistance gene-mediated resistance to Xoo . For a color
version of this figure, please refer to the color plate.

genome of wild rice Oryza minuta (BBCC
genome), encode proteins with high sequence
similarity to the Xa3/Xa26 protein and can medi-
ate a similar spectrum of resistance against Xoo
(Li et al. 2012). The speciation of the AA and CC
genomes is approximately 7.5 million years ago.
These characteristics suggest that the Xa3/Xa26
locus may confer a durable resistance.
Xa3/Xa26 -mediated resistance is influenced
by the genetic background and the developmen-
tal stage of a plant. This gene confers higher
level of resistance in a japonica background than
in an indica background, and rice plants carry-
ing Xa3/Xa26 gene have full resistance to some
Xoo strains at both seedling and adult stages,
but have full resistance to other Xoo strains at

adult stage (Yang et al. 2003; Sun et al. 2004;
Cao et al. 2007a). Further study has demon-
strated that the expression level of Xa3/Xa26
gene is associated with genetic background- and
development-controlled resistance (Cao et al.
2007a; Zhao et al. 2009). Xa3/Xa26 -mediated
resistance is dose dependent: as the expression of
Xa3/Xa26 gene increases, the plant's resistance
increases. A japonica background facilitates the
expression of Xa3/Xa26 gene compared with
an indica background. In addition, the expres-
sion of Xa3/Xa26 gene gradually increases with
development and reaches the highest level at the
maximum tillering to booting (panicle develop-
ment) stages. Rice plants constitutively overex-
pressing Xa3/Xa26 have a high level and broad