Agriculture Reference
In-Depth Information
3. Large mapping populations were necessarily
constructed to decrease the number of can-
didate genes: two for
VRN-1
, three for
VRN-
2
, and only one
for
VRN-3
.
4. An orthologue of any fl owering time gene
(e.g.,
VRN-1
vs.
AP1
,
VRN-3
vs.
FT
) or of
any genes containing a conserved domain of
known fl owering time genes (e.g.,
VRN-2
vs.
Constans
) in Arabidopsis (or other species)
should provide a good candidate for a target
gene.
5. Comparative maps with smaller genome size
[
Oryza sativa
L.,
Sorghum bicolor
(L.)
Moench, and currently
Brachypodium dis-
tachyon
] provided much help in developing
molecular markers in wheat during chromo-
some walking, although evolutionary events
involved in deletion or insertion (in the
VRN-1
region), translocations (in the
VRN-
2
region), and duplication and inversion (in
the
VRN-3
region) among species were fre-
quently observed.
6. Variation in gene expression profi les could
be observed between differing alleles and
between plants before and after vernalization
treatment.
7. Independent natural mutations present in a
large collection of germplasm, and their tight
association with the target phenotype, pro-
vided convincing evidence for identifi cation
of a target gene.
8. Using RNA interference is a powerful tech-
nique to validate candidate genes in trans-
genic wheat.
9. It was fortunate that
VRN-2
was cloned
using the BAC library of DV92 that has a
recessive
vrn-2
allele. The gene is still present
in the library, because DV92 has a point
mutation rather than a complete deletion,
which is common in the diploid wheat
accessions.
PRR proteins are characterized by a pseudore-
ceiver domain and CCT domain of the protein.
Cloning of
PPD-H1
has facilitated isolation of
orthologous
PPD-1
genes in wheat. The
PPD-
D1a
allele that is insensitive to photoperiod and
confers early fl owering in SD or LD plants con-
tains a 2-kb deletion upstream from the coding
region of the wheat
PRR
gene on chromosome
2D; photoperiod insensitivity caused by
PPD-B1
on chromosome 2B is due to a mutation outside
the sequenced region or to a closely linked gene
(Beales et al., 2007).
Three orthologues for Arabidopsis
VIN3
(Vernalization-INsensitive 3)—
TmVIL1
,
TmVIL2
,
and
TmVIL3
—were isolated and mapped in the
centromeric regions of chromosomes 5, 6, and
1, respectively, in
T. monococcum
(Fu et al.,
2007). The
VIN3
(Sung and Amasino 2004) and
VIN3
-Like-1 (
VIL1
) (Sung and Amasino 2006)
genes were up-regulated by low temperature to
repress mitotically stable expression of
FLC
(
Flow-
ering Locus C
), a MADS-box gene repressing fl ow-
ering time (Michaels and Amasino 1999; Sheldon
et al., 1999). The
TmVIL
genes have similar gene
structure and transcription regulation to
VIN3/
VIL
, suggesting that
TmVIL
might have retained
similar function in this gene family (Fu et al.,
2007).
The
TaVRT2
gene, or
T. aestivum
vegetative-
to-reproductive transition gene 2, is a MADS-
box gene that is regulated by vernalization and
photoperiod (Kane et al., 2005). The
TaVRT2
gene exhibits an inverse pattern of expression
relative to
TaVRT1
(i.e.,
VRN-1
or
AP1
) in hexa-
ploid wheat.
Concomitant transcriptional profi les of
fl owering time genes
When
VRN-A
m
2
was cloned
,
increased expres-
sion of
VRN-A
m
1
was found to be concomitant
with decreased expression of
VRN-A
m
2
(Yan
et al., 2004b; Loukoianov et al., 2005; Dubcovsky
et al., 2006). When
VRN-3
was cloned, it was
observed to have a transcription profi le similar to
VRN-1
but opposite to
VRN-2
. These relation-
ships in gene expression profi les confi rmed that
VRN-1
,
VRN-2
, and
VRN-3
act in the same ver-
Orthologues of other known fl owering
time genes
The barley
PPD-H1
gene was recently cloned
and identifi ed as a member of the pseudoresponse
regulator (
PRR
) family (Turner et al., 2005). The
