Freezing Tolerance in the Triticeae - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 115

relatively simple. The QTLs found generally act
in an additive way, without significant epista-
sis. Moreover, the major loci acting in the dif-
ferent species appear to be located in syntenic
chromosome locations (Galiba et al. 2009; Pec-
chioni et al. 2013). It is possible that new popula-
tions obtained by crossing among winter, faculta-
tive, and spring types could identify other QTLs
for the trait; other contributions could come
from multi-parent-derived populations, and from
genome-wide association (GWA) studies. How-
ever, until present, breeding for freezing toler-
ance could be simply effective as marker-assisted
selection (MAS) at a few selected loci. Accord-
ing to Kumar and colleagues (2012), another
form of MAS should be named LD-MAS (i.e.,
MASusingmarkersinLDwithaQTL),anda
modern way to pursue LD-MAS would be the
substitution of linked markers with candidate
gene (CG) “perfect” markers (Varshney et al.
2005).
On the other hand, in a short/medium-term
perspective, at least three facts suggest genomic
selection (GS) could be possible and beneficial
in selecting for freezing tolerance. The first of
these is the progress made by international ini-
tiatives in sequencing wheat and barley (Feuillet
et al. 2011). Secondly, field conditions most often
impose combined stresses on a plant through-
out its life cycle. Mittler (2006) reviewed evi-
dence that plants have responses to a combina-
tion of two different abiotic stresses that cannot
be deduced by applying each stress individu-
ally. Lastly, the traditional “reductionist science”
approach is evolving into a “systems biology”
approach. Such a systemic approach could tell
us about interactions between loci and move
marker-assisted breeding from the differential
view of genetics to a comprehensive integra-
tive approach. Interactions between loci, cod-
ing and non-coding, and between these and
other factors such as cellular structures could
significantly contribute to the phenotype. As a
demonstration of such an integrative hypothe-
sis, Joosen and colleagues (2009) reported a
metabolome study where more than one third

of the compounds present in RILs of Ara-
bidopsis were not detected in either parent,
but were the most likely result of the recom-
bination of loci contributing to biosynthesis

pathways.

LD-MAS for Freezing Tolerance

It seems clear that for freezing tolerance only
a small battery of master regulators (i.e., FR-1
and FR-2 ) should be included as priority tar-
gets in LD-MAS efforts aimed at improving the
trait in temperate cereal crops. A strikingly suc-
cessful example of how the selection of a sin-
gle gene variant can consistently improve toler-
ance to an abiotic stress has been reported by
Neeraja and colleagues (2007). They showed in
rice how the simple introgression by marker-
assisted backcrossing of the cloned Sub1 QTL
of submergence tolerance (a regulator of the
MYB family), successfully improved the trait
in a rice cultivar widely grown in flood-prone
Asiatic regions. In the case of FR-1 , the possi-
bility of co-selecting growth habit together with
frost tolerance is evident. Akar and colleagues
(2009) reported the VRN-H1 gene as the best pre-
dictor for marker-assisted selection for frost tol-
erance within Turkish highly frost-tolerant bar-
ley accessions and other winter, facultative, and
spring barley germplasm. Similar indications
came from the work of Rapacz and colleagues
(2010), where allelic variation in the promoter
region of VRN-H1 ( HvBM5a ) was reported to
be significantly related to freezing tolerance of
plants that had been partially de-acclimated in
the field. Moreover, simple diagnostic markers
for VRN genes could be deployed in breeding
programs to increase genetic diversity by uti-
lizing winter

spring crosses (Cockram et al.
2009). For example, programs for developing
freezing-tolerant facultative barley germplasm
could also be pursued by a very simple strat-
egy: fix winter alleles at VRN-H1/FR-H1 and
FR-H2 on chromosome 5H, with the large dele-
tion on chromosome 4H where VRN-H2 resides.
Photoperiod responses could then be targeted to

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