Biology Reference
In-Depth Information
Xoo
Resistance in Rice
responses. Defense-responsive genes are char-
acterized by their response to a pathogen attack
via changed expression levels or posttransla-
tional modifications of their encoding proteins
(Kou and Wang 2010). In general, PTI is a rel-
ative weak defense response and ETI is a high-
level defense response. However, strong PTI and
weak ETI have also been reported (Thomma
et al. 2011). Furthermore, PAMPs and effec-
tors as well as PRRs and R proteins cannot be
strictly maintained, because there is a continuum
between PTI and ETI (Thomma et al. 2011). For
example, rice
Xa21
-mediated
Xoo
resistance is
triggered by a narrowly conserved PAMP, Ax21,
and Xa21 protein is considered to be both a PRR
and an R protein (Lee et al. 2009). In addition,
the defense signaling pathways initiated by PRRs
and R proteins are partially overlapping (Kou and
Wang 2010).
According to the speed and strength of the
plant response to pathogen invasion, plant resis-
tance can be divided into two major categories:
qualitative or complete resistance and quantita-
tive or partial resistance. Qualitative resistance is
a rapid and high level of defense response medi-
ated by
MR
genes, including
R
and
PRR
genes
that confer a high level of resistance. More than
30
MR
genes that mediate qualitative resistance
and have different resistance spectra against
Xoo
have been named. Quantitative resistance is con-
trolled by multiple genes or resistance QTLs and
can be broad spectrum and/or durable (Kou and
Wang 2010). A large number of resistance QTLs
have been identified in the interactions of dif-
ferent rice varieties and
Xoo
strains (Kou and
Wang 2012).
In addition to innate immunity, plants have
different types of induced resistance, including
systemic acquired resistance (SAR) and induced
systemic resistance (ISR). Genetic studies in
Arabidopsis
revealed that NPR1 (non-expressor
of pathogenesis-related genes 1) is important for
SAR, and TGA transcription factors are repres-
sors of SAR (Vlot et al. 2009). Some evidence
supports rice having a similar SAR pathway
for
Xoo
resistance. Overexpression of rice NH1,
Overview of Disease Resistance
Mechanism in Plants
Physical and biochemical barriers provide a first
line of defense against potential pathogen attack.
These constitutive defenses include the presence
of many preformed barriers such as waxy epi-
dermal cuticles, cell wall, bark, antimicrobial
enzymes, and secondary metabolites. However,
pathogens have evolved strategies to breach these
passive defense barriers. When
Xoo
enters a
leaf apoplast through hydathodes or wounds, the
plant relies on its innate immune system to detect
the invading organisms and activate inducible
defenses.
The current view of plant-pathogen interac-
tions has revealed that the innate immune system
consists of a two-branched defense response. The
first branch is pathogen (microbe)-associated
molecular patterns (PAMPs/MAMPs)-triggered
immunity (PTI) or basal resistance, which is
initiated by the direct recognition pathogen
PAMPs through plant pattern-recognition recep-
tors (PRRs) (Jones and Dangl 2006; Boller and
Felix 2009). PRRs are plasma membrane pro-
teins. PAMPs, which are essential for microbe
fitness or survival, are relatively conserved
molecules within a class of microbes dur-
ing evolution, such as flagellin, peptidogly-
can, and lipopolysaccharides. The other branch
is effector-triggered immunity (ETI) or race-
specific resistance that is activated on direct or
indirect detection of pathogen effectors by plant
proteins encoded by
R
genes (Jones and Dangl
2006; Thomma et al. 2011). R proteins are either
intracellular, plasma membrane, or extracellular,
and each of these R proteins recognizes one or
a few specific effectors. Pathogen effectors are
rapid evolving, which results in loss of function
of R proteins.
After the presence of PAMPs or effectors
activates PPRs or R protein, the plant recep-
tors transfer the defense signal to downstream
components encoded by defense-responsive or
defense-related genes, which leads to defense
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