Biology Reference
In-Depth Information
laboratories from several countries, in the
collective effort aiming at multiple coverage of
the pathogen genome. Chromosomal and mito-
chondrial (haplotypes Ia, IIa, and IIb) genome
sequences are available for download/analysis
html
(
Phytophthora infestans
Sequencing
Project, Broad Institute of Harvard and MIT).
Use of such resources extends the LB-related
research and contributes to fundamental
research as well, in terms of comparative
genomics, genetics, gene/protein annotation,
plant resistance investigations, and many others.
An example of speeding-up the race with the
highly variable
P. infestans
is the recent strat-
egy for effectoromics, that is, cloning the genes,
whose products display the characteristics typi-
cal for other known
P. infestans Avr
genes. Thus
far, all cloned effector genes of this pathogen
encode one class of proteins: The modular
secreted proteins with a RXLR motif for translo-
cation into the host cell, followed by diverse,
rapidly evolving C-terminal effector domains
(Vleeshouwers et al. 2011). Effectoromics, a
high-throughput functional genomics approach
that uses effectors to probe plant germplasm for
specific recognition by R-proteins, has recently
emerged as a powerful tool for identification of
Avr
and
R
genes (Jia et al. 2010; Vleeshouw-
ers et al. 2011). The availability of genome
sequence resources for
P. infestans
has enabled
the generation of effector libraries cloned in vec-
tors designed for
in planta
expression (Haas
et al. 2009). Effectors are transiently expressed
in
Solanum
germplasm by agro-infiltration with
Agrobacterium tumefaciens
and/or a virus vec-
tor such as Potato virus X (PVX), and plants
are monitored for the occurrence of macroscopic
cell death responses to the individual effectors
(Vleeshouwers et al. 2011). Effectors trigger-
ing cell death represent candidate
Avr
genes.
These are subsequently validated for
Avr
activ-
ity by genetic analyses. Co-segregation of the
cell death response to the effectors correlates
with HR-based resistance against
P. infestans
isolates in genetic populations. If the match-
ing
R-
gene has been cloned, additional verifi-
cation of R-AVR pairs can be obtained by co-
expression of
R
-gene and
Avr
-gene candidates
in leaves of tester plants such as
Nicotiana ben-
thamiana
(Jia et al. 2010; Vleeshouwers et al.
2011). Similar studies with tomato are yet to
be undertaken. Nevertheless, as exemplified by
potato, the potential exists for the R-AVR pairs
discovery of tomato proteins interacting with the
pathogen AVR protein being over-expressed, for
instance by means of fixation, immunoprecipi-
tation, and subsequent analyses. Such a strategy
should considerably enhance the tomato-related
modern LB-resistance studies.
Conclusions
The most sustainable strategy for managing
tomato LB is to deploy an integrated system
including cultural practices, fungicide applica-
tion, and the use of cultivars with broad-spectrum
genetic resistance against LB. Unfortunately, tra-
ditional disease resistance breeding approaches
can be slow and may take little advantage of
the emerging knowledge of pathogenicity mech-
anisms. Tomato breeders now have access to
at least a few LB-resistance genes of tomato
origin with associated genetic markers, which
can be introduced into breeding lines and com-
mercial cultivars via marker-assisted breeding.
Accumulating evidence suggests that sustainable
disease resistance constitutes successful pyra-
miding of multiple resistance genes within a
single cultivar. In tomato, the combination of
Ph-2
and
Ph-3
has been shown to confer LB
resistance superior to the resistance conferred
by either of the two genes alone. This superior-
ity has been observed and confirmed by several
tomato researchers. It is noteworthy, that a possi-
ble breach in the pyramided
Ph-2
and
Ph-3
was
reported by R. Gardner (North Carolina State) in
tomato plantings in North Carolina under highly
LB-conducive conditions at the end of the season
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