Agriculture Reference
In-Depth Information
(McIntosh et al., 1998). Based on the same
genetic effects dominant for spring growth habit
and similar chromosomal locations between the
wheat
VRN-B4
gene and the barley
VRN-H3
gene, the wheat
VRN-B4
gene was recently
renamed once again as
VRN-B3
(Yan et al.,
2006).
Genetic loci regulating
photoperiod sensitivity
Flowering is expected to occur either for spring
wheat without a vernalization requirement or for
winter wheat in which any requirement for ver-
nalization has been satisfi ed. However, fl owering
time will be signifi cantly altered when these plants
are placed under various environmental stimuli
such as photoperiod.
The only example of precise location of
PPD
genes in wheat is a series of homoeoallelic
PPD-1
genes on chromosome group 2
.
A
PPD-1
locus was fi rst reported by detailed studies
of photoperiod insensitivity in near-isogenic
lines generated from cultivar Mari and the
photoperiod-sensitive cultivar Cappelle-Desprez;
this locus was originally named
PPD-D1
on chro-
mosome 2D (Worland and Law 1986). Homoeol-
ogous loci were
PPD-2
on chromosome 2B
(Scarth and Law 1983; Mohler et al., 2004) and
PPD-3
on chromosome 2A (Law et al., 1978;
Scarth and Law
1984
). Following the nomencla-
ture recommended by McIntosh et al. (1998),
these homoeoallelic genes were renamed
PPD-A1
for
PPD-3
,
PPD-B1
for
PPD-2
, and
PPD-D1
for
PPD-1
.
In barley the orthologue to the
PPD-1
genes is
PPD-H1
, a major determinant of photoperiod
response (Laurie et al., 1995; Decousset et al.,
2000). The later-fl owering response is controlled
by the recessive
ppd-H1
allele, which resulted
from a mutation that impairs gene function
(Turner et al., 2005). Cloning of
VRN-H1
has
facilitated isolation of the wheat
PPD-1
genes
(Beales et al., 2007).
Other vernalization genes in wheat
Near-isogenic lines (NILs) of 'Triple Dirk' were
developed to evaluate roles of each
VRN-1
gene
in vernalization. Three independent lines pos-
sessed only one dominant spring allele,
Vrn-A1
in
the Triple Dirk D line,
Vrn-B1
in Triple Dirk B
line, or
Vrn-D1
in Triple Dirk E line; each line
had a recessive winter
vrn-1
allele on the other
two homoeologous chromosomes (Pugsley 1971,
1972). However, an additional line 'Triple Dirk
F' (TDF) was found to carry an allele for spring
growth habit that differed from any of these three
dominant
Vrn-1
alleles. This TDF line was thus
believed to carry a dominant
Vrn-4
allele for
spring growth habit, compared with the winter
Triple Dirk C line. The gene symbol
VRN-4
was
proposed to keep as its original name for future
use, since the same name for the gene on chromo-
some 7B had been replaced by
VRN-B3
as previ-
ously described. The presence of the dominant
Vrn-4
gene was confi rmed by Goncharov (2003),
who transferred
Vrn-4
from the near-isogenic
TDF line to 'Gabo-2'.
A dominant gene
Vrn-8
for spring growth
habit was introgressed from
T. sphaerococcum
into hexaploid wheat by Stelmakh and Avsenin
(1996). It was found to be allelic to
Vrn-4
but
not to
any of the three homoeologous
VRN-1
genes (Goncharov 2003). Goncharov also demon-
strated that
VRN-6
sc
and
VRN-7
sc
that were
introgressed from rye into hexaploid wheat
(Stelmakh and Avsenin 1996) were not allelic to
Vrn-4
or any
VRN-1
genes. Therefore, in addi-
tion to
VRN-1
,
VRN-2
, and
VRN-3
that have
been mapped in precise chromosomal locations,
at least three more loci (
VRN-4, VRN-6
sc
, and
VRN-7
sc
)
may condition vernalization responses
in wheat.
Genetic loci regulating plant
development processes
The
EPS
genes affect fl owering time in a subtle
way, which generally results in a difference of
only a few days, independently of vernalization
and photoperiod (Snape et al., 2001a; Bullrich
et al., 2002; Valárik et al., 2006). The minor
effects of
EPS
genes on fl owering time can be
masked in segregating populations generated
