Freezing Tolerance in the Triticeae - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 106

these environments, the choice of sowing date
is constrained both by the higher probability of
frost early in the season and by the hot and
dry conditions that typically limit growth late
in the season. In temperate regions, mid- to late-
spring freezes may also cause spike damage to
the winter wheat crops that flower during spring
and summer (Whaley et al. 2004). These frost
events can cause floret and spike abortion, as
well as damage to the developing grain (Rein-
heimer et al. 2004; Frederiks et al. 2011). Efforts
over the last few decades to improve tolerance of
Australian cereal varieties to low temperatures in
reproductive tissues (LTR tolerance) have been
largely unsuccessful, owing to various difficul-
ties of working with the trait (Fuller et al. 2009).
Variation in heading time represents one such
hindrance, because spikes at different growth
stages differ in their frost susceptibility and flow-
ering time can enable escape. However, this fac-
tor, which can potentially confound the measure-
ment of genuine LTR tolerance, can be accounted
for to some extent by scoring heads that were at a
specific developmental stage at the time of frost-
ing (Reinheimer et al. 2004). By adopting this
strategy, Reinheimer et al. (2004) identified sev-
eral lines with potential tolerance in the field,
including the Japanese barley cultivars 'Amagi
Nijo' and 'Haruna Nijo.' In two mapping popu-
lations derived from them (i.e., 'Amagi Nijo' x
'WI2585' and 'Haruna Nijo' x 'Galleon'), QTLs
implicated in the genetic control of reproductive
frost tolerance were found on chromosomes 2HL
and 5HL, with tolerance alleles deriving from
the Japanese parents (Reinheimer et al. 2004;
Chen et al. 2009a). The frost resistance locus
on chromosome 2HL was linked to Flt-2L ,a
locus controlling flowering time, spike density,
and plant height (Chen et al. 2009b), whereas
the QTL on chromosome 5HL was in the same
general area (indeed co-segregation was not
tested) of VRN-H1/FR-H1 and was responsible
for resistance to frost-induced floret sterility and
frost-induced

ing potential LTR tolerance mechanisms, the
'Amagi Nijo' x 'WI2585' and 'Haruna Nijo' x
'Galleon'populations were examined for flower-
ing time and spike morphology traits associated
with the 2HL and 5HL LTR tolerance loci (Chen
et al. 2009c). In spring-type progeny of both
crosses, winter alleles at VRN-H1/FR-H1 were
linked in coupling with LTR tolerance and were
associated with earlier flowering. On the other
hand, tolerance on 2HL was coupled with late
flowering alleles at Flt-2L . Both chromosome
regions influenced chasmogamy/cleistogamy
(open/closed florets), although tolerance was
associated with cleistogamy at the 2HL locus
and chasmogamy at the 5HL locus. LTR toler-
ance controlled by both loci was accompanied by
shorter spikes, which were due to fewer florets
per spike on 5HL, but shorter rachis internodes
on 2HL (Chen et al. 2009c). Taken together, these
results suggest that mechanisms of LTR toler-
ance differ from those proposed to control freez-
ing tolerance at the vegetative stage, involving
an extended vegetative growth phase. However,
it remains to demonstrate, whether also in the
case of LTR tolerance, the CBF-response path-
way is activated and at which extent.

Copy-Number Variation and Winter
Hardiness Co-selection

Copy-number variation is the term used to
describe where allelic diversity occurs in the
form of variable unit numbers of a given genomic
segment. Typically the segment length varies
in size from about 1 kb to 1 Mb (Scherer
et al. 2007). Greatest insight into this phe-
nomenon comes primarily from animal systems,
in which there are whole genome reference
sequences and genomes of other individuals have
been resequenced. One of the main means by
which copy-number variation has been revealed
is array comparative genomic hybridization
(CGH), in which reference and test genomic
DNAs are hybridized to arrays representing the
reference genome (Alkan et al. 2011). Regions
common

grain damage in all the stud-

ied populations (Reinheimer et al. 2004).

In another experiment aimed at identify- to both genomes but differing in