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In-Depth Information
the 11 isolates tested; only isolate EC1 from
Ecuador was able to overcome RPI-VNT1.1 and
cause disease on the inoculated plants (Foster
et al. 2009).
Because potato and tomato are closely related
and share high genome similarity, the LB-
resistance genes and QTLs identified in potato
may have relevance in tomato or correspond to
tomato LB-resistance genes and QTLs (Foolad
2007; Vleeshouwers et al. 2011). By analyz-
ing the tomato genome regions to which LB R-
genes have been mapped and taking into account
that all such potato genes thus far cloned pos-
sess the NBS-LRR structure, and also consider-
ing the conservation of LB signaling pathways
between potato and tomato, candidate genes for
LB resistance can be rather easily traced down.
For example, the
the 1950s and managed relatively well through
the use of fungicides and semi-resistant cul-
tivars (Fry 2008; Vleeshouwers et al. 2011),
P. infestans
remains one of the most devastating
plant pathogens of all time. In potato, the iden-
tification, characterization, and introgression of
new LB-resistance genes and QTLs within the
past two decades have significantly contributed
to the effective control of the disease. Some-
what similar but to a lesser extent, efforts have
been made in tomato and some progress has
been made. For example, pyramiding of the
known major resistance genes
Ph-2
and
Ph-
3
and recent identification of novel resistance
resources and genes has increased the prospect
for development of tomatoes with strong resis-
tance against LB. Nevertheless, because of the
risk of the pathogen breaching the resistance con-
ferred by the available resistance genes, future
efforts are imperative to identify new and more
desirable sources of resistance and new resis-
tance genes and QTLs. With the availability of
genome sequencing data and the advent of mod-
ern high-throughput genomic methods exempli-
fied by potato LB effectoromics, identification,
cloning, characterization, and deployment of yet
undiscovered tomato LB-resistance genes will
not be unexpected.
1.5 Mbp region of chro-
mosome 10, to which Ph-2 has been mapped,
contains only two of the 155 genes annotated
as 'similar to resistance-like' (Mueller et al.
2005) or containing the aforementioned NBS-
LRR domains (Moreau et al. 1998; Vleeshouw-
ers et al. 2011) (Figure 13.2A). On the other
hand, ambiguous reports exist regarding the
Ph-
3
in tomato: Within the originally mapped
Ph-
3
region of
∼
2.5 Mbp, two NBS-LRR 'hot-
spots' exist, and different groups have announced
discovering the
Ph-3
-related molecular markers
(Chunwongse et al. 2002; Zhu et al. 2006; Park
et al. 2010; Foolad and Panthee 2012) (Figure
13.2B). Comprehensive comparison of potato
and tomato genetic maps containing information
on the location of LB-resistance traits may guide
future searches for the tomato
R
-genes, previ-
ously mapped to corresponding chromosomal
regions (Foolad 2007; Nowicki et al. 2012). Fur-
ther information on potato LB-resistance genes
is reported in Chapter 12.
∼
Genomic Resources in Tomato
The
recently
sequenced
potato
genome
of
∼
844
Mbp
(
∼
39,000
genes,
mostly
located
within the
570 Mbp of euchromatin) (Mueller
et al. 2009; Xu et al. 2011) and the tomato
genome of
∼
∼
950 Mbp (
∼
40,000 genes,
>
90%
of which lie within the
220 Mbp of euchro-
matin) (Mueller et al. 2005; Wang et al. 2006;
lycopersicum/genome)
significantly contribute
to a larger research initiative known as the Inter-
national Solanaceae Genome Project (SOL):
Systems Approach to Diversity and Adapta-
tion (
http://www.sgn.cornell.edu/sol
anaceae-
∼
Future Prospects
More than 165 years ago, LB made its mark as
a destructive disease of potato. Although con-
trol of the disease was eventually attained in
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