Freezing Tolerance in the Triticeae - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 102

(Galiba et al. 2009; Tondelli et al. 2011). Interest-
ingly, the barley 'Nure' x 'Tremois' genetic sys-
tem developed by Francia and colleagues (2004)
remains the only bi-parental population in the
Triticeae where both FR- QTL are segregating ,
since in wheat FR-A1 (Galiba et al. 1995) and
FR-A2 (Vagujfalvi et al. 2003) were individu-
ally mapped. The best candidate for FR-1 is cur-
rently a MADS box gene coding for a protein
similar to the meristem identity AP1 transcrip-
tion factor in Arabidopsis. The gene is known
as TaAP1 in wheat and HvBM5A in barley, it is
up-regulated (directly or indirectly) by vernal-
ization and by long days (for a review see Galiba
et al. 2009), and was proved to be the deter-
minant for VRN-1, the major locus involved in
the vegetative-to-reproductive transition (Dany-
luk et al. 2003; Trevaskis et al. 2003; Yan et al.
2003; von Zitzewitz et al. 2005). Recent evi-
dence suggests that coincident VRN-1 and FR-1
QTL are the pleiotropic effects of this same gene,
and that VRN-1 allelic variation influences the
expression duration of low temperature-induced
genes (Dhillon et al. 2010). In particular, muta-
tions in the VRN-1 promoter, resulting in high
VRN-1 transcript levels under both long and
short days, dampen the expression of COR genes,
and lowers resistance, especially under long-day
conditions (Galiba et al. 2009; Trevaskis 2010).
Further support for VRN-1 being a determi-
nant of freezing tolerance derives from the work
by Limin and colleagues (2007). These authors
demonstrated in barley cultivars used as parents
in mapping populations that timing of maxi-
mum frost tolerance is usually coincident with
the timing of vernalization saturation. As such,
the hypothesis of pleiotropy would explain the
old general observation of breeders that winter-
type genotypes, in wheat and in barley, carry-
ing a vernalization sensitive (“winter”) allele at
VRN-1 , are more tolerant than spring-type culti-
vars. However, there is evidence that frost resis-
tance is not necessarily a function of vernaliza-
tion, as in the case of facultative varieties that
are frost tolerant but not vernalization sensitive
(see also Limin et al. 2007). Moreover, a direct

interaction of VRN-1/FR-1 with the promoter of
genes involved in the development of tolerance
has not been demonstrated yet. First attempts to
describe the relationships between vernalization-
and photoperiodically-regulated genes/loci and
factors that regulate the development of frost tol-
erance upon cold acclimation have been recently
reviewed (Galiba et al. 2009; Pecchioni et al.
2013), and a possible advance in this direc-
tion has been reported by Eagles and colleagues
(2011) in winter wheats. These authors found
interesting implications for freezing tolerance of
a C-T transition located in exon 4 of the Vrn-A1
gene. The SNP (single nucleotide polymor-
phism) was already predicted to cause a leucine-
to-phenylalanine substitution in the amino acid
sequence of VRN-A1 and to affect a conserved K
domain of the protein (Chen et al. 2009d). Since
K domains typically mediate interaction and/or
dimerization between MADS box proteins, the
C-T allelic variation was hypothesized to alter
vernalization response, freezing tolerance, and
other physiological traits (Eagles et al. 2011).
Although understanding the molecular basis of
the genetic network that interconnect vernaliza-
tion and frost resistance is far from being com-
plete, we propose in Figure 7.2 a hypothetical
model whereby VRN-1/FR-1 and FR-2 interact
with each other and with effector genes, leading
to low-temperature tolerance and reproductive
development.
The frost tolerance conferred by FR-2 ,
appears to be ruled by one or more genes from
the family C-repeat binding factor ( CBF ), also
known as DRE binding protein 1 ( DREB1 ),
members of which occur in clusters of at least 13
elements (Francia et al. 2004, 2007; Skinner et al.
2005; Miller et al. 2006). At present, determina-
tion of whether the FR-2 effect is a result of either
several CBFs acting in an additive manner, or the
action of only one, is still under way. In diploid
einkorn wheat, T. monococcum , a deletion of five
amino acids in the AP2 DNA binding domain of
CBF12 was found to co-segregate with frost tol-
erance between the spring, frost-sensitive acces-
sion DV92, and the winter, frost-tolerant G3116