Agriculture Reference
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In this model, winter growth habit and photo-
period sensitivity would be considered ancestral
traits, whereas spring growth habit and photope-
riod insensitivity are mutated traits in domesti-
cated wheat cultivars (Yan et al., 2003; Beales
et al., 2007). A mutation in any one of the domi-
nant Vrn-2 or recessive vrn-1 or vrn-3 alleles in
winter wheat produces a spring wheat genotype
(Takahashi and Yasuda 1971; Pugsley 1972;
Dubcovsky et al., 1998; Tranquilli and
Dubcovsky 2000; Snape et al., 2001a ) . Vernaliza-
tion can promote vrn-3 to induce vrn-1 (Yan et
al., 2006) or bypass vrn-3 and directly promote
vrn-1 (Danyluk et al., 2003; Trevaskis et al., 2003;
Yan et al., 2003; Loukoianov et al., 2005) by
releasing the repressor TaVRT2 on vrn-1 (Kane
et al., 2005, 2007). The Vrn-2 gene can be
repressed by vernalization (Yan et al., 2004b) or
through release of vrn-1 during vernalization
(Trevaskis et al., 2007), or it can be repressed by
short days alone, which replace the role of vernal-
ization (Dubcovsky et al., 2006). After primary
induction of fl owering by short days and/or low
temperature, secondary induction of growth to
fl owering occurs via long days and moderately
high temperature (Heide 1994). The PPD-1 gene
can promote fl owering of wheat cultivars sensitive
to LD photoperiod (Beales et al., 2007), but over-
exposure of winter wheat to low temperature or
short days will delay fl owering.
winter type requiring 4 to 8 weeks' cold exposure
(Berry et al., 1980; Crofts 1989; Baloch et al.,
2003).
The phenomenon of quantitative vernalization
requirement in winter type Arabidopsis can be
explained by two alternative hypotheses. One is
that gradual expression of VIN3 induced by ver-
nalization results in a quantitative reduction in
FLC mRNA levels that negatively correlate with
fl owering time (Sung and Amasino 2004). The
other is that multiple inputs from a network of
pathways in response to vernalization, photope-
riod, and light quality are integrated, since FLC
is not the only target of the vernalization process
(Reeves and Coupland 2001; Simpson and
Dean 2002). Neither of these mechanisms applies
to wheat since no FLC homologue exists in
wheat.
Winter wheat is planted on approximately 17.6
million hectares each year in the US. This
amounts to 75% of all wheat grown in the US and
is approximately 11% of the world wheat supply
and nearly 35% of world wheat exports. Advances
in understanding the genetic basis and molecular
mechanisms of winter wheat growth and develop-
ment are critically important to maintaining the
productivity of winter wheat wherever it is grown.
To reveal the mechanism of how vernalization
requirement duration is biologically “measured”
to permit fl owering would be particularly valu-
able for improving adaptation of winter wheat.
Manipulation of genes for vernalization require-
ment duration will allow the development of
winter wheat cultivars capable of adapting to
anticipated global climate shifts. Scientists predict
an increase of approximately 3 ºC worldwide
toward the end of this century (Kerr 2007).
Increased temperatures can lead to insuffi cient
vernalization or failed vernalization.
Another key area for future research efforts lies
in the regulation of developmental phases to
produce wheat for different purposes in world
agriculture. As an easily observable trait, fl ower-
ing time and heading date have been used to rep-
resent plant developmental transition in extensive
studies on the effects of genetic loci and environ-
mental stimuli. A recent study also showed a posi-
tive, linear relationship between stem-elongation
FUTURE PERSPECTIVES
Successful cloning of the three vernalization
genes greatly benefi ted from clear segregation of
a qualitative trait controlled by singular vernaliza-
tion genes in three independent mapping popula-
tions. However, the genetic basis of quantitative
differences in vernalization requirement in winter
wheat has not been adequately addressed. Rela-
tively little is known about why various durations
of vernalization are required among winter wheat
cultivars, which are generally classifi ed into three
types: a weak winter type that is stimulated to
fl ower by brief exposure to low temperature, a
moderate winter type that requires 2 to 4 weeks'
cold exposure to induce fl owering, and a strong
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