Agriculture Reference
In-Depth Information
Vernalization response, photoperiod sensitiv-
ity, and other developmental processes provide
the necessary adaptive and protective mechanisms
to ensure successful reproduction in diverse envi-
ronments, and ultimately they allow wheat to be
the most widely grown crop worldwide. Genetic
studies on wheat fl owering time have been exten-
sively reviewed (Laurie et al., 1995; Law and
Worland 1997; Hay and Ellis 1998; Snape et al.,
2001a). Recent advances in molecular under-
standing of wheat fl owering time genes have been
reviewed by scientists working directly with cereal
crops (Cockram et al., 2007; Trevaskis et al.,
2007) and those working with the model plant,
Arabidopsis (Henderson et al., 2003; Searle
and Coupland 2004; Sung and Amasino 2004;
Roux et al., 2006). The following sections explore
the current state of our knowledge of wheat fl ow-
ering time genes, with an emphasis on how these
genes were technically cloned from large and
complex genomes and how they function in
various genetic backgrounds, with the outlook
that this knowledge may be applied to wheat
improvement.
successful cloning of three major vernalization
genes in wheat, VRN-1 , VRN-2 , and VRN-3
(Yan et al., 2003, 2004b, 2006).
Response to photoperiod places wheat cultivars
into sensitive and insensitive types. Wheat is
usually classifi ed as a long-day (LD, >14 hours of
light) plant, because it typically fl owers earlier
when exposed to longer days. Photoperiod insen-
sitivity (or photoperiod neutrality), which arose
by mutation from this LD sensitivity, enables
wheat to fl ower earlier without LD treatment
(Laurie et al., 1994; Law and Worland 1997;
Snape et al., 2001a). Genes which confer photo-
period response in wheat have not been cloned,
but the orthologous photoperiod gene PPD-H1
was cloned in barley ( Hordeum vulgare L.) that
provides intuitive information on the mechanism
of the LD response in wheat (Turner et al., 2005;
Beales et al., 2007). Wheat, as a member of
the winter grass subfamily Festucoideae , was ini-
tially considered a short-day-long-day (SD-LD)
dual-induction plant, that is, SD (<10 hours of
light) and/or low temperature treatment primar-
ily induces the developmental transition and LD
secondarily induces growth to fl ower (Heide
1994). It has been rarely noted that a SD period
can accelerate the developmental transition of
some winter cultivars, thus resulting in earlier
fl owering without vernalization (Evans 1987).
The replacement of vernalization by SD was not
found or characterized in numerous modern
wheat cultivars, leading to the general classifi ca-
tion of wheat as a LD plant (Dubcovsky et al.,
2006).
Besides vernalization and photoperiod, plant
development provides an additional internal
mechanism to regulate fl owering time in wheat
(Laurie 1995; Snape et al., 2001a). Without treat-
ment with vernalization or SD, winter wheat may
still eventually fl ower due to consequences of
plant development or interchangeability between
plant development age and vernalization (Wang
et al., 1995a,b). A group of earliness per se
( EPS ) genes are responsible for fi ne regulation of
fl owering time in both spring and winter wheat
cultivars (Snape et al., 2001a; Valárik et al., 2006).
However, no EPS genes have been cloned in
wheat.
GENETIC LOCATIONS OF FLOWERING
TIME GENES
Genetic loci regulating vernalization
response
Vernalization is believed to be the most important
adaptive mechanism allowing winter wheat to
synchronize plant development with changes in
seasonal climate (Flood and Halloran 1986;
Rawson et al., 1998; Kirby et al., 1999; Griffi ths
et al., 2003). Major genetic loci responsible for
vernalization effects were located in certain
genomic regions using molecular markers to map
the growth habit in populations generated from
crosses between spring and winter wheat or
barley.
VRN-1 on the long arm of homoeologous
chromosomes 5
The VRN-1 gene is the fi rst vernalization gene
that was found to be dominant for spring growth
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