Agriculture Reference
In-Depth Information
The
ZCCT1
gene was identifi ed as
VRN-A
m
2
based on allelic variation (Yan et al., 2004b). In
spring wheat DV92 ZCCT1
has a point mutation
in the CCT domain that results in loss of its func-
tion. This mutation is also present in 22 indepen-
dent spring wheat accessions but absent in all
winter accessions tested. In addition, 17 indepen-
dent spring accessions were found to have a com-
plete deletion of this gene.
Transcripts of
Vrn-A
m
2
in leaves of winter
wheat were down-regulated progressively
during
vernalization (Yan et al., 2004b). Down-regulation
of
VRN-2
expression by vernalization was also
confi rmed in the hexaploid winter wheat cultivar
Jagger (Yan et al., 2004b) and Triple Dirk lines
(Loukoianov et al., 2005).
The fl owering repres-
sion by
VRN-2
was confi rmed by RNAi in trans-
formed Jagger. Reduction of the RNA level of
VRN-2
by RNAi accelerated the fl owering time
of transgenic plants by more than one month (Yan
et al., 2004b). No allelic variation in
VRN-2
has
been reported yet in tetraploid (
T. turgidum
ssp.
durum
L.) or hexaploid forms, but the presence of
VRN-2
gene structure and function in polyploid
wheat has been demonstrated by its expression in
normal and transgenic plants described previ-
ously and by sequencing the orthologous
VRN-2
gene from BAC clones of the tetraploid
T. durum
wheat cultivar Langdon (Dubcovsky and Dvorak
2007).
date gene
Hd3a
, an
FT
orthologue in rice (Kojima
et al., 2002).
In spring wheat, the dominant
Vrn-B3
allele
was associated with the insertion of a retroele-
ment in its promoter region in the cultivar Hope,
whereas in barley, mutations in the dominant
Vrn-H3
allele occurred in the fi rst intron of this
gene (Yan et al., 2006). Variation in the noncod-
ing intronic region in
VRN-A3
(=
FT-A
) and
VRN-D3
(=
FT-D
) was also tightly associated
with heading date in a large collection of diverse
germplasm (Bonnin et al., 2008). The
vrn-3
gene
in winter types of wheat and barley was up-
regulated by vernalization and long days. Winter
wheat plants transformed with the dominant
Vrn-
B3
allele carrying the promoter retroelement
insertion fl owered signifi cantly earlier than non-
transgenic plants (Yan et al., 2006).
It is particularly noteworthy that the genetic
effect of the dominant
Vrn-H3
allele was detected
in two barley populations (BG213 ×
H. sponta-
neum
and BG213 × 'Igri'), which featured a reces-
sive
vrn-H1
(winter) genetic background; the
genetic effect of the dominant
Vrn-B3
allele was
detected in a wheat CS × CS(Hope7B) population
with a dominant spring allele
Vrn-D1
in its back-
ground (Pugsley 1971, 1972).
Successes in positional cloning of
vernalization genes
Several technical points may be gleaned from
the successful cloning of
VRN-1
,
VRN-2
,
and
VRN-3
from large and complex genomes
of wheat.
VRN-B3
, an orthologue of
FT
,
promotes fl owering
Both
VRN-B3
in wheat and
VRN-H3
in barley
were cloned in parallel experiments (Yan et al.,
2006) The
VRN-3
gene is an orthologue of Ara-
bidopsis
Flowering Locus T
(
FT
) (Samach et al.,
2000; Hayama et al., 2003). The product of the
FT
gene was believed to be “fl origen,” a hypo-
thetical fl owering hormone (Chailakhyan 1936)
that has been pursued for years. Considerable
progress has been made in this research area
according to recent studies (reviewed in
Corbesier and Coupland 2006 and Zeevaart 2006).
The rapid cloning of
VRN-3
in two temperate
species greatly benefi ts from a complete sequence
of the rice colinear region, including the heading
1. Cloning of the three vernalization genes
greatly benefi ted from Mendelian segrega-
tion of the vernalization requirement accord-
ing to a single gene. The phenotype was
precisely and consistently identifi ed when
plants were grown under controlled green-
house conditions without vernalization.
2.
Diploid wheat or barley was used as a model
species for cloning of the genes present in
hexaploid wheat. Once the target gene was
cloned, orthologous genes in hexaploid wheat
could be readily isolated by PCR.
