The Flowering Pathway in Wheat - Wheat Science and Trade - page 58

fl ower development. Only two candidates present
in the fi nal version of the wheat physical contigs
fi t the hypothesis that alternatively either AP1 or
AGLG1 is VRN-A m 1 .
The AP1 gene was identifi ed as being VRN-
A m 1 based on allelic variation and gene expression
profi les (Yan et al., 2003). The presence of a 20-
bp deletion adjacent to a putative CArG -box for
the MADS-box protein binding site in the pro-
moter region of the dominant allele Vrn-A m 1
raised the possibility that this deletion might have
impaired the recognition site, so that a repressor
cannot bind to the dominant allele but can bind
to the recessive allele. Validating this identity
between AP1 and VRN-A m 1 was the discovery
that different lengths of deletions existed in a
similar region in several independent spring
wheat accessions (Yan et al., 2003), with subse-
quent confi rmation of their dominant effects
in controlling spring growth habit (Dubcovsky
et al., 2006).
In hexaploid wheat, a dominant Vrn-A1a allele
in most spring accessions has an insertion of a
foldback element in the promoter region fl anked
by 9-bp host-direct duplication; a dominant Vrn-
A1b allele has two mutations in the same 9-bp
host-direct duplication and a 20-bp deletion in
the 5′ untranslated region; also, a dominant Vrn-
A1c allele has no mutation in the promoter region
but has a deletion in the fi rst intron , which is
similar to the dominant Vrn-B1 and Vrn-D1 genes
in polyploid wheat (Yan et al., 2004a; Fu et al.,
2005). The combined Vrn-A1 , Vrn-B1 , and Vrn-
D1 mutations just described explained the spring
growth habit of all spring wheat cultivars tested
in the previous studies, whereas no mutations
were observed in the vrn-1 genes in the A, B, or
D genome of any winter wheat cultivars tested in
the same studies. The Vrn-A1a allele has the
largest contribution to early fl owering time. It has
been incorporated into spring wheat cultivars in
Canadian breeding programs to provide frost
avoidance in short-season environments (Iqbal
et al., 2007).
Expression of a dominant Vrn-A m 1 allele was
observed in nonvernalized plants of spring wheat,
but the recessive vrn-A m 1 allele in winter wheat
was not expressed until such plants were vernal-

ized. The vrn-A m 1 transcriptional levels were pro-
gressively increased during vernalization (Yan
et al., 2003).
Regulation of vrn-1 expression by vernalization
was similarly observed in different genotypes and
near-isogenic lines of hexaploid wheat. Without
vernalization, Vrn-1 was strongly expressed in
spring wheat but not in winter wheat. Vernaliza-
tion strongly induced vrn-1 expression and thus
accelerated fl owering time in winter wheat
(Danyluk et al., 2003; Murai et al., 2003;
Trevaskis et al., 2003; Loukoianov et al., 2005).
Acceleration of fl owering time by Vrn-1 in
spring wheat, and its replacement by vernaliza-
tion in winter wheat, was validated in subsequent
transgenic experiments. A reduction in VRN-1
transcript levels by RNA interference (RNAi)
delayed fl owering time for 2 to 3 weeks in trans-
genic plants of the hexaploid spring wheat culti-
var Bobwhite (Loukoianov et al., 2005). Ectopic
expression of the wheat VRN-1 in Arabidopsis
not only promoted fl owering but also altered
development of fl oral organs, demonstrating
pleiotropic effects of VRN-1 in plants (Adam
et al., 2007). An artifi cial mutant having a deletion
of a region including the promoter and MADS
box of the VRN-1 gene prevented the shift from
vegetative to reproductive phase (Shitsukawa
et al., 2007).

VRN-A m 2 , a CCT-domain-containing gene,
represses fl owering
Using 5,698 gametes from the DV92 × G3116 F 2
population in T. monococcum , VRN-A m 2 was
delimited within a complete physical contig of
438,828 kb sequenced from BAC clones of DV92
(Yan et al., 2004b). Three genes, including two
Zinc fi nger-CCT domain transcription factors
ZCCT1 and ZCCT2 , were left in the candidate
interval. The proteins ZCCT1 and ZCCT2 show
similarities to Arabidopsis Constans (CO) and
CO-like proteins that regulate fl owering time
(Putterill et al., 1995), but the similarities are
restricted to the conserved CCT (CONSTANS,
CONSTANS-LIKE, TOC1) domains, indicat-
ing that ZCCT1 and ZCCT2 are unique genes in
wheat.