Biology Reference
In-Depth Information
KACC10331, and Philippine strain PXO99A,
have been sequenced (Lee et al. 2005; Ochiai
et al. 2005; Salzberg et al. 2008). The
Xoo
genome is a single circular chromosome of
about 50 million bases (Mb), and it contains
nearly 5,000 open reading frames (ORFs). It fea-
tures remarkable plasticity and evolves rapidly.
There are large numbers of major rearrange-
ments and indels between the three strains, which
contributes to the genomic variation in
Xoo
.
This genomic variation explains the diversity of
Xoo
genotypes and pathotypes (Salzberg et al.
2008).
activator-like domain (Boch et al. 2009; Kay
and Bonas 2009; Yuan and Wang 2012). TAL
effectors function as specific transcriptional acti-
vators in the plant cell nucleus. The speci-
ficity of DNA recognition by the TAL effec-
tor is determined by the variable amino acids
at residues 12 and 13 of each repeat. How-
ever, some TAL effectors have been identified
as avirulence (Avr) proteins in disease resistance
(
R
) gene-mediated
Xoo
resistance (Boch and
Bonas 2010).
In addition to the effectors translocated by
the type III secretion system, the other impor-
tant virulence factors of
Xoo
are extracellular
enzymes and polysaccharide and a diffusible
signal factor (Feng et al. 1996; B uttner and
Bonas, 2010; He et al. 2010). The extracellu-
lar enzymes, such as endoglucanases, xylanase,
cellobiosidase, and esterase, are secreted by the
type II secretion system of
Xoo
to degrade
the plant cell wall (B uttner and Bonas 2010).
The extracellular polysaccharide protects bac-
teria against environmental stress. Null muta-
tion of
rpfC
in
Xoo
strain T3000 substan-
tially influences the synthesis of extracellular
polysaccharide and virulence in rice (Feng et al.
1996). The diffusible signal factor is a cell-
cell communication signal, and it can affect the
expression of virulence genes (He et al. 2010).
Repeats in the structural toxin (RTX toxin),
which has functions in biofilm development, cel-
lular adherence, and eukaryotic cell targeting,
represent another type of important virulence
factors among gram-negative bacteria (Coote
1992; Satchell 2011). Several RTX toxins,
including phenylacetic acid, trans-3-methylthio-
acrylic acid, and 3-methylthio-propionic acid,
have been identified in
Xoo
(Noda et al. 1989).
Thus, RTX toxins may also be virulence fac-
tors of
Xoo
. In addition, the
rax
genes (such
as
raxA
,
raxB
,
raxC
, and
raxST
)of
Xoo
are
involved in secretion by the type I secre-
tion system and sulfation of peptide Ax21
(activator of
Xa21
-mediated immunity), which
elicit rice Xa21 protein-mediated resistance (Lee
et al. 2009).
Factors Affecting Pathogenicity
of
Xoo
Key to
Xoo
pathogenicity is the type III secre-
tion system that is encoded by hypersensitive
response and pathogenicity (
Hrp
) genes (Boch
and Bonas 2010). The
Hrp
gene cluster is nec-
essary for pathogenicity in susceptible hosts
and for a hypersensitive response in resistance
plants and nonhost plants. In the
Xoo
genome,
the
Hrp
gene cluster includes 26 genes that
have a high sequence similarity (Ochiai et al.
2005). These genes are regulated by two cru-
cial components, HrpG and HrpX, in the
Xan-
thomonas
genus. The expression of
HrpX
gene
is upregulated by HrpG protein (Koebnik et al.
2006).
The type III secretion system translocates
effector proteins into plant cells to support bac-
terial virulence, proliferation, and dissemina-
tion. The largest effector family of
Xoo
is the
transcription activator-like (TAL) effector family
(also called the avrBs3/pthA family) (Boch and
Bonas 2010). A common feature shared by TAL
effectors is the central repeat region that con-
sists of 1.5 to 28.5 repeats, with each repeat con-
taining 33-34 amino acids, and contributes to
binding the
cis
-elements named UTP (upregu-
lated by TAL effector) boxes of plant gene pro-
moters, the amino-terminal translocation region,
the carboxyl-terminal nuclear localization sig-
nal, and carboxyl-terminal acidic transcription
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