Biology Reference
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P. infestans
have been identified that overcome
all 11
S. demissum
-derived R
-
genes in potato
(Chen and Halterman 2011). Nonetheless, pyra-
miding various race-specific resistance genes
often results in stronger and more durable resis-
tance, as has been observed in many plant
species, including potato and tomato (Tan et al.
2010; Kim et al. 2012; Luo et al. 2012).
Pyramiding of LB-resistance genes using
conventional breeding approaches may not
always be possible or it may be laborious
and time-consuming. An alternative approach
to introducing single or multiple LB-resistance
genes is genetic transformation. This approach,
however, requires prior knowledge of the
gene(s), including identification, cloning, and
characterization. In this respect, a recent strong
body of evidence that tomato and potato share
the conserved pathway for LB signaling is
very beneficial for LB-resistant tomato breed-
ing (Jia et al. 2009; Faino et al. 2010; Jia
et al. 2010). The conserved LB-signaling path-
way phenomenon has been demonstrated by sev-
eral research groups (see below), which heterol-
ogously over-expressed the cloned potato LB
R
-genes in tomato plants and showed increased
LB resistance in the transgenic plants.
An attempt with the transgenic approach
using potato
R1
gene heterologously expressed
in tomato indicated the activation of the trans-
gene in the isolate-specific,
R1
-incompatible
reaction during
P. infestans
attack (Faino et al.
2010). Increased expression of the transgene, but
also of the native tomato
PR-1
gene, has been
recorded as early as three hours post infection
(hpi); additionally, expression of
PR-1
increased
by one-fold between three and six hpi in the
transgenic plants (incompatible reaction), while
in the native tomato plants remained at the
three hpi level (Faino et al. 2010). In another
study,
Agrobacterium
-mediated transformation
and heterologous expression in tomato of the
coding sequences of the
R3a
,
R1
(
S. demissum
),
or
RB
(
S. bulbocastanum
) genes was employed to
obtain transgenic tomato plants (Jia et al. 2009).
When challenged with
P. infestans
isolates, the
transgenic plants displayed a strong resistance
response. The results demonstrated that
R3a
and
R1
conferred resistance against some tomato-
virulent isolates, while
RB
granted resistance
against all five isolates examined (Jia et al.
2009). Follow-up studies proved that the trans-
genic tomatoes carrying the potato
R3a
devel-
oped HR specific to
P. infestans
strains with
the corresponding avirulence gene,
Avr3a
(Jia
et al. 2010). This seems to indicate that the sig-
naling pathway from the
R3a-Avr3a
recognition
to HR is conserved between potato and tomato.
The transgenic tomatoes carrying both
R3a
and
Avr3a
genes, with the latter under the control of
a glucocorticoid-inducible promoter, exhibited
a localized HR under dexamethasone-induced
R3a-Avr3a
interaction (Jia et al. 2010).
A similar transgenic approach employing
novel LB
R-
genes from potato wild species
S. okadae
(
Rpi-oka1
) and
S. mochiquense
(
Rpi-
mcq1.1
,
Rpi-cmq1.2
) was employed to enhance
LB resistance in potato and tomato cultivars
(Jones et al. 2010). In this study, eight of the nine
recovered transgenic potato plants of susceptible
cultivar Desiree exhibited resistance against the
tested
P. infestans
isolates. Furthermore, when
cloned under their native promoter and termina-
tor sequences and stably transformed into potato
or tomato cultivars, expression of all three genes
induced resistance against a wide spectrum of
P. infestans
isolates tested. Interestingly, the
spectrum of LB resistance exhibited for each
construct was in agreement with that exhibited
by the original wild potato accessions harboring
these genes. In particular,
Rpi-oka1
-derived LB
resistance was retained in the transgenic plants,
and the resistance phenotype was not a result of
constitutive activation of defense pathways by
the transgene (Jones et al. 2010). Similarly, LB
susceptible potato and tomato cultivars exhib-
ited resistance when transformed with resistance
gene
Rpi-vnt1.1
from
S. venturii
(Foster et al.
2009). In this study, the transgenic potato and
tomato plants exhibited resistance against 10 of
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