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mechanisms that protect the plant cell from frost events were mostly focused on the acclimation
process. Species of the Triticeae tribe of the Poaceae, such as wheat and barley, able to acclimate to
and to tolerate frost, are one of the best models for studying freezing tolerance in herbaceous, non-
woody plants. The present chapter reviews in detail the genetic and genomic knowledge accumulated
over the last twenty years in these model species, in terms of genetic loci and sequence variation able
to confer higher tolerance to frost. Lastly, the use of genetic resources, as well as new genomic tools
for producing freezing tolerant varieties, is discussed.
Major Determinants of Frost
Tolerance in the Triticeae
(QTLs and Genes)
1991). Figure 7.1 is an example from barley. In
samples of winter and spring germplasm pools,
there was basically no consistently significant
improvement in frost tolerance during the pro-
gression from ancient local landraces, to old
cultivars, then to modern ones (released after
1980).
This slow or non-existent improvement can
be partly attributed to the fluctuating nature of
winter injuries, which would not allow constant
selection for tolerance across breeding genera-
tions. The minimum temperature that Triticeae
members are able to survive is typically in the
range of -10 to -20
◦
C. However, although
cold acclimation and frost tolerance are still
considered complex polygenic traits, the ever-
expanding accumulation of “omics” data is lead-
ing to an increased understanding of the mech-
anisms that plants have evolved in order to tol-
erate this stress condition (Pecchioni et al. 2012;
Pecchioni et al. 2013).
In temperate cereals the ability of fall-sown vari-
eties to survive winter depends on a complex
trait referred to as winter hardiness. In turn,
this is composed of three different but intercon-
nected components: vernalization requirement,
photoperiod sensitivity, and frost tolerance (Gal-
iba et al. 2009). The latter can be defined as the
ability of plants to survive freezing temperatures,
prevent damage to tissues, and minimize the neg-
ative effects of freezing on eventual yield. During
the last 20 years, several field and growth cham-
ber screening methods have been proposed to
study and assist breeding for frost-tolerant vari-
eties. According to Prasil and colleagues (2007)
such methods can be classified as direct (mon-
itoring plant survival, LT
50
, i.e., at a tempera-
ture lethal for 50% of the plants, and chlorophyll
fluorescence through the
Fv/Fm
ratio) or indi-
rect (selection of molecular markers correlated
with the trait). In barley, T oth and colleagues
(2004) and Rapacz and colleagues (2010) devel-
oped PCR-based markers for assisted selection,
and Rapacz and colleagues (2008) proposed
markers based on
COR
(COld-Regulated) gene
expression. Many direct assays have also been
integrated with molecular markers, to comple-
ment the frost tolerance evaluation and improve
the prediction of winter hardiness (Rizza et al.
2011). On the other hand, conventional breed-
ing strategies based on winter survival possibly
coupled to visual rating of damage in the past
have been rather inefficient in improving frost
resistance in winter cereals (Limin and Fowler
QTL and Genes Responsible for
Vegetative Frost Tolerance
In temperate cereal growing areas, frost is nor-
mally experienced during vegetative develop-
ment. In crops such as wheat and barley, link-
age mapping has been the method of choice
for genetic studies, starting from experimental
bi-parental crosses. In the wake of vertebrate
genetics, association mapping has more recently
begun to be adopted in plants. This involves
searching for genotype-phenotype correlations
in collections of individuals in which relatedness
is unknown. However, as emphasized by Myles
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