Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 15

which is a sequence and functional ortholog of
Arabidopsis NPR1, results in enhanced resis-
tance to Xoo (Chern et al. 2005). In rice, NH1
interacts with TGA2.1 transcription factor and
negative regulator of resistance (NRR). TGA2.1
negatively regulates basal defense responses to
Xoo (Fitzerald et al. 2005). Rice NRR nega-
tively regulates SAR in Arabidopsis and basal
and Xa21 -mediated Xoo resistance in rice (Chern
et al. 2005, 2008). It is also known that a
rice mitogen-activated protein kinase, MPK6,
negatively regulates SAR in rice- Xoo interac-
tion (Shen et al. 2010). ISR of plants against
pathogens is a widespread phenomenon that acti-
vates multiple defense mechanisms including
increased activity of pathogenesis-related gene
(PR) proteins. Attenuated UV-mutant Xoo strains
have been documented to induce rice ISR against
BB (Thein and Prathuangwong 2010).

tified MR genes against Xoo have been isolated.
Most of the characterized MR genes encode pro-
teins that are different from the most common R
protein, such as nucleotide-binding site (NBS)-
leucine-rich repeat (LRR) protein (Liu et al.
2010). This feature suggests that the molecular
mechanisms of qualitative resistance in rice- Xoo
system are more complicated than in other plant-
pathogen systems.

Xa1

Xa1 , localized on the long arm of chromosome 4,
was used in Japanese rice breeding for BB resis-
tance from 1967. It confers resistance to Japanese
Xoo race I, which is the most dominant race in
Japan. Xa1 , which was cloned by a map-based
cloning strategy from the japonica rice cultivar
Kogyoku and indica rice line IRBB1, encodes a
cytoplasmic NBS-LRR protein (Yoshimura et al.
1998) (Figure 2.2). The expression of Xa1 can
be induced by Xoo and wounding. The induc-
tion of expression is speculated to be involved
in enhanced resistance to Xoo (Yoshimura et al.
1998).

Qualitative Resistance to Xoo

Asian-cultivated rice (AA genome) consists of
two major subspecies, indica ( O. sativa L. ssp.
indica ) and japonica ( O. sativa L. ssp. japon-
ica) .Atleast37 MR genes against Xoo have been
identified and designated in a series from Xa1 to
Xa36 , with one symbol having been used for two
different genes (Table 2.1). Most of these genes
were identified in Asian-cultivated rice while
only a few were identified from wild rice species,
which were then introgressed into cultivated rice.
It is generally accepted that R proteins encoded
by dominant R genes recognize specific pathogen
effectors and initiate defense signal transduction
leading to rapid and race-specific disease resis-
tance in most plant-pathogen systems, includ-
ing rice R gene-mediated resistance to fungal
pathogen Magnaporthe oryzae (Dangl and Jones
2001; Martin et al. 2003; Liu et al. 2010). How-
ever, more than one-third of identified MR genes
against Xoo confer recessive resistance, namely
xa5 , xa8 , xa9 , xa13 , xa15 , xa19 , xa20 , xa24 ,
xa25/Xa25(t) , xa26(t) , xa28(t) , xa31(t) , xa33(t) ,
and xa34(t) (Table 2.1). Only 7 ( Xa1 , Xa3/Xa26 ,
xa5 , xa13 , Xa21 , xa25 , and Xa27 ) of the 37 iden-

Xa3/Xa26

Xa3/Xa26 gene, localized on the long arm of
chromosome 11, was isolated as Xa26 from an
indica rice cultivar Minghui 63 (AA genome)
with a map-based cloning strategy. It encodes
a plasma membrane-localized LRR receptor
kinase-type protein with an extracellular LRR
domain, a transmembrane motif, and a cytoplas-
mic kinase domain (Sun et al. 2004). Further
study revealed that Xa3 , a previously named MR
gene, and Xa26 are actually the same gene, which
was then renamed as Xa3/Xa26 (Xiang et al.
2006) (Figure 2.2). Xa3/Xa26 gene confers rela-
tively broad-spectrum resistance to different Xoo
races; rice cultivars carrying Xa3/Xa26 gene have
been widely used in rice production in China
for a long period of time (Xu et al. 2004; Gao
et al. 2010; Li et al. 2012). The Xa3/Xa26 alle-
les, Xa3/Xa26-2 from wild rice Oryza officinalis
(CC genome) and Xa3/Xa26-3 from the CC