Resistance to Late Blight in Potato - Translational Genomics for Crop Breeding: Biotic Stress - page 229

Another late blight resistance gene, Rpi-ver1,
from S. verrucosum, was located on chromosome
VI by NBS-profiling (Jacobs et al. 2010). The
only marker linked to this gene in two rather
small mapping populations (Jacobs et al. 2010),
on the potato consensus map is located 4 cM
away from the Rpi-blb2 gene (Danan et al. 2011).
A finer mapping is needed to resolve the question
of whether the Rpi-ver1 is a member of the Rpi-
blb2 cluster or a separate one.

ancing selection (van der Vossen et al. 2003). A
highly similar gene originating from the same
species, Rpi-bt1 , was also mapped to the potato
chromosome VII and then cloned (Oosumi et al.
2009). The RB is the best characterized Rpi gene.
Its transcription level was shown to correspond
to gene copy number (Bradeen et al. 2009) and
it was correlated with late blight resistance of
the transgenic plants (Kramer et al. 2009). Fac-
tors such as temperature, physiological age of the
plant, and genetic background did not alter the
transcription of the RB gene (Iorizzo et al. 2011).
The Sgt1 gene (suppressor of the G2 allele of
skp1 ) was reported to be essential for the RB-
mediate resistance (Bhaskar et al. 2008). Allele-
specific PCR and RT-PCR markers for the pres-
ence of this gene were developed and applied
in breeding programs (Colton et al. 2006; Millet
and Bradeen 2007). Field tests of transgenic RB
lines indicated their high foliar resistance and
susceptibility of tubers (Halterman et al. 2008).
A proteomics study was also done during the
process of RB-mediated plant defence (Liu and
Halterman 2009).
Several studies were dedicated to the search
for RB/Rpi-blb1 -like sequences in diverse
Solanum species. Several RB orthologs were
found in S. verrucosum and one of them proved
to be functional after transfer into susceptible
potato (Liu and Halterman 2006). Allele min-
ing and effector genomics allowed for discovery
of three more functional homologs of Rpi-blb1 :
Rpi-sto1 from S. stoloniferum , Rpi-pta1 from S.
papita, and Rpi-plt1 from S. polytrichon (Wang
et al. 2008; Vleeshouwers et al. 2008). Screen-
ing of 196 different taxa from Solanum section
Petota resulted in detection of an Rpi-blb1 frag-
ment in S. cardiophyllum subsp. cardiophyllum
and S. stoloniferum , apart from various S. bul-
bocastanum accessions, and showed geograph-
ical confinement of these homologs to Central
America (Lokossou et al. 2010). RB/Rpi-blb1
homologs were also detected in several other
species (Sokolova et al. 2011; Pankin et al. 2011).
Although

Rpi1

The Rpi1 gene encoding resistance against P.
infestans was identified in the S. pinnatisectum
and mapped to potato chromosome VII using an
interspecific cross with S. cardiophyllum (Kuhl
et al. 2001). Later, as a starting point for cloning
this gene, two BAC (bacterial artificial chromo-
some) libraries were constructed from S. pinna-
tisectum, and four markers linked to the Rpi1
gene were hybridized to 14 BAC clones of these
libraries (Chen et al. 2004). So far, the sequence
of this gene has not been published. Within S.
michoacanum , a species that is believed to be
a natural hybrid of S. bulbocastanum and S.
pinnatisectum ,the Rpi-mch1 gene was recently
mapped to a similar location using Diversity
Array Technology (DArT) ( Sliwka et al. 2012a).

RB/Rpi-blb1

The first R gene for P. infestans resistance
located on chromosome VIII was identified and
sequenced independently by two research groups
in S. bulbocastanum . Under the name RB ,itwas
mapped (Naess et al. 2000) and cloned (Song
et al. 2003) using BC2 populations derived from
somatic hybrids (Helgeson et al. 1998). The same
gene, although named Rpi-blb1 , was cloned
using an F1 population of the intraspecific S.
bulbocastanum cross. This CC-NBS-LRR gene
was found in a cluster of four resistance gene
analogs with the same domain composition, and
analyses of their diversity suggested that Rpi-
blb1 could be relatively old and subject to bal-

both RB and Rpi-blb1 were

described as providing broad-spectrum