Freezing Tolerance in the Triticeae - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 104

(Knox et al. 2008). In barley the data suggest
that an increase in the copy-numbers of CBF2
and CBF4 might be the underlying genetic basis
for FR-H2 tolerance alleles (Stockinger et al.
2007; Knox et al. 2010). Further reinforcing the
existence of the prominent role played by the
CBFs on frost tolerance in cultivated germplasm,
Tondelli and colleagues (2006) mapped to two
loosely linked positions on chromosome 7H, bar-
ley orthologs of the AtFRY1 and AtICE1 genes,
putative upstream regulators of CBF genes in
Arabidopsis (Thomashow 2010). No cold toler-
ance QTL have been mapped on chromosome 7H
suggesting that, at least in barley, allelic variation
at these two CBF regulators are not important for
the trait. In the studies of Francia and colleagues
(2004) and von Zitzewitz and colleagues (2011),
the proportion of phenotypic variance accounted
for by FR-H1 ranged from 15% to 37%, whereas
for FR-H2 it ranged from 9% to 22%. Similar
major effects were also observed at orthologous
chromosome 5A loci in wheat (for a review see,
e.g., Galiba et al. 2009).
In the presence of such effects, which are
of similar weight at 5A loci, that explained a
large part although not all the phenotypic vari-
ance for the trait, studies that attempt to iden-
tify other QTLs contributing to vegetative frost
tolerance in Triticeae would deserve attention.
These studies, on one hand allowed the investi-
gation of contributions of other wheat chromo-
some 5 homoeologous to frost tolerance. Snape
and colleagues (1997) identified a FR-D1 locus
on chromosome 5D, with a maximum likelihood
peak about 6 cM proximal to VRN-D1 (Snape
et al. 2001). FR-D1 might therefore be homoe-
ologous to FR-A2 . A similar situation exists for
the FR-B1 locus (Toth et al. 2003), for which a
recombinant substitution-line population derived
from the cross 'Chinese Spring' x 'Chinese
Spring'('Cheyenne' 5B) located FR-B1, close to
but separate and proximal to VRN-B1 . This was
redesignated as FR-B2 in the 2004 supplement
of the Catalogue of Gene Symbols for Wheat
(McIntosh et al. 2004). However, it has still not
been verified experimentally whether this locus

contains any CBF cluster like its Fr-A2 homoe-
olog. Effects from 5B chromosome seemed to be
less important for the trait than 5A and 5D loci,
at least from results in Chinese Spring/Cheyenne
(susceptible/resistant) chromosome substitution
lines (Sutka 2001). In the same study, when
comparing frost testing in phytotron and in the
field, it was found that chromosome substitu-
tions of the 5 th homoeologous group and the 2B
and 4B ones played an important role in both
environments (Sutka 2001). A few other QTLs
were mapped to other chromosomes, mainly in
studies where at least VRN-1 / FR-1 was not seg-
regating. A pioneering study by Tuberosa and
colleagues (1997) found a slightly more com-
plex situation in a “winter x winter” type barley
cross 'Arda' x 'Opale.' Nine freezing-tolerance
QTLs were mapped on chromosomes 2H, 3H,
6H, and 5H, by screening in a controlled envi-
ronment, with the one on 5H roughly coinciding
with FR-1 (Tuberosa et al. 1997). It is also possi-
ble that this represented FR-H2 instead, although
in this case it couldn't be tested by comparative
mapping (Cattivelli et al. 2002). In the wheat
'Chinese Spring' x 'Synthetic' population,
Borner and colleagues (2002) found a region on
chromosome arm 6AS that could be orthologous
to the barley 6H-Bin6 segment which is associ-
ated with low-temperature tolerance (Tuberosa
et al. 1997) and salt tolerance (Cattivelli et al.
2002). Another attempt to find additional loci
in hexaploid wheat was made by Baga and
colleagues (2007), by developing two doubled
haploid populations with no allelic variation at
VRN-1/FR-1 . A major frost tolerance locus was
again coincident with the position of FR-A2 in
the “winter x winter” population 'Norstar' x
'Winter Manitou' (which carries the recessive
alleles vrn-A1 , vrn-B1 , vrn-D1 from 'Norstar').
Two CBF genes (i.e., CBF14 and CBF15 )were
located under the QTL peak, with the favorable
allele contributed by the tolerant parent 'Norstar.'
Interestingly, besides FR-A2 , a weaker QTL was
highlighted in a large region (about 20 cM) on
chromosome 1D, with the favorable allele from
'Norstar.'