The Flowering Pathway in Wheat - Wheat Science and Trade - page 61

nalization pathway, consistent with their epistatic
interactions observed in previous genetic experi-
ments (Takahashi and Yasuda 1971; Tranquilli
and Dubcovsky 2000).
In each of the Triple Dirk spring isogenic lines
carrying a single dominant Vrn-1 allele at one of
the Vrn-A1 , Vrn-B1 , or Vrn-D1 loci, plus two
recessive vrn-1 alleles at the other two loci, only
the dominant Vrn-1 allele was transcribed in the
seedling stage; however, a few weeks later, tran-
scripts from the recessive vrn-1 alleles were also
detected (Loukoianov et al., 2005). The recessive
alleles in these spring-type plants carrying both
dominant Vrn-1 and recessive vrn-1 alleles were
preceded in transcription compared with the
recessive vrn-1 alleles in winter-type plants. This
phenomenon also occurred in diploid spring
plants with VRN-1 in the heterozygous condi-
tion. These observations lead to a hypothesis
that a positive feedback regulatory loop may
coordinate transcription of recessive vrn-1 genes
to enhance VRN-1 transcriptional levels in
wheat.
The VRN-1 gene is up-regulated not only
by vernalization but also by LD in NILs either
with or without a photoperiod-insensitive gene
(Danyluk et al., 2003; Murai et al., 2003). Integra-
tion of signals from vernalization and photoperiod
to regulate fl owering time is consistent with
their interactions observed in genetic experiments
(Fowler et al., 2001).
The VRN-A m 2 gene is down-regulated not
only by vernalization but also by SD in winter
lines of T. monococcum (Dubcovsky et al., 2006).
However, no VRN-A m 1 transcripts were observed
in the SD-treated plants until transferred to LD,
whereas VRN-A m 1 transcripts were observed
during the vernalization process. This compara-
tive study suggested that two different repressors
inhibited VRN-1 expression. A more recent study
showed that one of them is TaVRT2 protein,
which binds to the VRN-1 promoter (Kane et al.,
2007), supporting the hypothesis that the impaired
CArG -box located in the VRN-1 promoter in
natural mutants is the most likely regulatory site
(Yan et al., 2003; Dubcovsky et al., 2006). Except
TaVRT2 , VRN-2 is the only gene that has been
found to repress fl owering in wheat, but no

evidence shows how VRN-2 directly represses
VRN-1 .

COMPARATIVE STUDIES ON
FLOWERING PATHWAYS IN PLANTS

Flowering pathways in model species
The fl owering pathway in Arabidopsis has been
extensively studied, and the rapid progress in
this model plant species has been discussed else-
where (Amasino 2004, 2005; Baurle and Dean
2006; Jaeger et al., 2006; Sung and Amasino
2006). It was indeed fortunate that the vernaliza-
tion phenomenon also exists in Arabidopsis, in
which rapid-fl owering accessions do not require
vernalization as in spring wheat and vernaliza-
tion-requiring accessions behave as winter
annuals such as winter wheat (Amasino 2005).
A major vernalization gene in Arabidopsis is
FLC (Michaels and Amasino 1999; Sheldon et al.,
1999), which is a central repressor of fl owering and
is positively regulated by Frigida ( FRI ) (Johanson
et al., 2000; Gazzani et al., 2003) but negatively
regulated by VIN3 that is induced by vernaliza-
tion (Sung and Amasino 2004) or by genes in the
autonomous pathway (Marquardt et al., 2006).
The FLC gene delays fl owering by repressing
expression of FT in the leaf and Suppressor of
Overexpression of CONSTANS 1 ( SOC1 ) in meri-
stem tissue, preventing up-regulation of a bZIP
transcription factor FD, a partner of FT in the
induction of fl owering (Abe et al., 2005; Wigge
et al., 2005; Searle et al., 2006). The SOC1 and
FT genes activate AP1 and Leafy ( LFY ), a fl ower-
ing signal integrator, by inducing fl oral meristem
identity. CO , a major gene controlling fl owering
by photoperiod in Arabidopsis (a LD plant like
wheat) (Putterill et al., 1995), plays a central role
in the regulation of this pathway by inducing tran-
scription of FT (reviewed in Thomas 2006).
The fl owering pathway in rice, a SD plant, is
mainly regulated by photoperiod. Rice has no ver-
nalization requirement, or more precisely, it has
not been tested how orthologues of vernalization
genes, if any, are regulated by low temperature in
rice. Analyses of natural variation showed that