Biology Reference
In-Depth Information
Latest advances in genomic tools have
allowed fast and cost-effective mining of
genome-wide single nucleotide polymorphisms
in crops, the so-called GWA (genome-wide asso-
ciation) studies, with the aim of finding asso-
ciations between SNP alleles and phenotypes
of interest. Currently only three association-
mapping approaches for studying frost tolerance
have been published on the Triticeae: two in bar-
ley and one in rye, the most frost-tolerant species
of the tribe. In a first attempt, not genome-
wide, Fricano and colleagues (2009) investi-
gated the allelic variation of only four barley
CBF
genes out of 13 in a panel of 216 acces-
sions composed of European cultivars, landraces
and
H. spontaneum
genotypes. Two nucleotide
variants of
HvCBF14
and one nucleotide vari-
ant of
HvBM5A
were identified as statistically
associated with frost tolerance (Fricano et al.
2009). Von Zitzewitz and colleagues (2011)
performed a genome-wide association-mapping
study of winter hardiness traits in 148 bar-
ley accessions consisting of advanced breed-
ing lines and cultivars, assembled as part of the
USDA barley Coordinated Agricultural Project
(
www.barleyCAP.org)
. Scoring freezing toler-
ance as percent winter survival, all the signifi-
cant associations were on chromosome 5H, and
only a few associations with markers on 3H, 4H,
and 6H approached the threshold. In the
FR-
H2
region, two SNPs were significantly asso-
ciated with the trait, one in the
HvCBF9
gene,
and the other located in a gene encoding a heat-
shock transcription factor (HSF). At the
FR-
H1
locus a specific
HvBM5A
intron 1 amplicon
showed the most significant association, whereas
a third significant SNP, about 8 cM proximal to
HvBM5A
,wasina
Glu-tRNA aminotransferase
subunit C
, a gene with no obvious relationship
to frost tolerance (von Zitzewitz et al. 2011).
Finally, Li et al. (2011) studied eleven candi-
dates involved in frost response (including sev-
eral
ScCBFs
,
ScDREB2
,
ScICE2
,
and ScVRN1
)
in a panel of 201 lines selected from Eastern and
Middle European rye populations. High levels
of nucleotide diversity and a fast intragenic LD
(linkage disequilibrium) decay (within approxi-
mately 520 bp on average) were revealed, indi-
cating that the low linkage disequilibrium in rye
compared to self-pollinating species promises
a high resolution in genome-wide association
mapping.
Significant associations between freezing tol-
erance and SNPs/haplotypes of candidate genes
were identified. In particular, two SNPs in
ScCBF15
and one in
ScCBF12
, all leading to
amino acid changes, were related to frost tol-
erance. Although no significant SNP-frost tol-
erance association was observed for
ScVRN1
,
this same gene had significant interaction effects
with six other candidates (four
ScCBF
splus
ScDREB2
and
ScDHN3
), underlining the impor-
tant role of
ScVRN1
in the frost responsive net-
work (Li et al. 2011). Taken as a whole, the
association-mapping studies conducted so far in
the Triticeae support the findings of bi-parental
linkage mapping and expression studies, that is,
that the
FR-2
and
FR-1
loci play crucial roles in
frost resistance, with only a minor contribution
of a few, still unknown other genes, residing at
loci on other chromosome groups.
QTL and Genes Responsible for
Reproductive Frost Tolerance
The degree of tolerance to freezing shown by
temperate cereals depends largely on the stage
of development at which the stress occurs. This
is particularly true for winter cultivars at the
vegetative stage, which become virtually dor-
mant under the influence of steadily reduced
temperature and day length. In later growth
stages (i.e., from pre-heading to flowering) the
plants become more susceptible to low temper-
atures (Fuller et al. 2009). In areas experienc-
ing subtropical/Mediterranean climates, such as
the Australian cereal belt, West Asia, and North
Africa, wheat and barley are grown to reach
maturity during winter, when conditions are most
favorable for growth but occasional nighttime
frost events in the order of -1 to -5
◦
C occur
(Reinheimer et al. 2004; Chen et al. 2009a). In
Search WWH ::
Custom Search