Given their sessile nature, it is critical for the survival of plants to adapt to their environment. Accordingly, plants have evolved the ability to sense seasonal changes to govern developmental fates such as the floral transition. Temperature and day length are among the seasonal cues that plants sense. We recently reported that
VIN3-LIKE 1 (
VIL1) is involved in mediating the flowering response to both cold and day length via regulation of two related genes,
FLOWERING LOCUS C (
FLC) and
FLOWERING LOCUS M (
FLM), respectively.Key Words:
flowering, vernalization, photoperiod, chromatin, histone, gene expressionVernalization renders plants competent to flower after exposure to the prolonged cold of winter.
1,2
Arabidopsis exhibits facultative responses to both vernalization and photoperiod to initiate the floral transition. The facultative nature of the responses makes
Arabidopsis a tractable genetic system to study these aspects of flowering time control.In
Arabidopsis, vernalization creates competence to flower via silencing of the potent floral repressor,
FLC, in a mitotically stable manner.
3,4 Thus, the vernalization response is an environmentally induced epigenetic switch in that exposure to cold permanently affects the plants'' developmental program. This epigenetic switch is associated with increased levels of
FLC chromatin methylation on Histone H3 Lys 9 and Lys 27.
5,6
VERNALIZATION INSENSITIVE 3 (
VIN3) plays an essential role in this switch since no modifications to
FLC chromatin occur in
vin3 mutants.
5 Furthermore, the levels of expression of
VIN3 mRNA are tightly correlated with the degree of the vernalization response.
5
VIN3 encodes Plant HomeoDomain (PHD) finger-containing protein. PHD finger-containing proteins are often associated with protein complexes that are involved in chromatin remodeling.
7We performed a yeast two-hybrid screen to identify potential protein partners of VIN3.
VIN3-LIKE 1 (
VIL1) was identified by this screen.
8
VIL1 encodes a PHD finger-containing protein that is related to
VIN3. As expected for proteins that are associated with VIN3, plants containing loss-of-function alleles of
VIL1 do not respond to vernalization. Furthermore, no vernalization-mediated histone modifications occur at
FLC in
vil1 mutants similar to the situation in
vin3 mutants. Thus, by yeast two hybrid and genetic analysis, VIL1 is a bona fide VIN3 partner that is required for vernalization-mediated histone modifications at
FLC chromatin. Unlike
VIN3, the expression of
VIL1 does not change over the course of cold exposure. Rather,
VIL1 mRNA levels are affected by photoperiod.
VIL1 expression is significantly increased in non-inductive photoperiods (short days; SD). Consistent with this expression pattern,
vil1 mutants in the Columbia accession exhibit a SD-specific late-flowering phenotype. Furthermore,
VIL1 is required for attenuating expression of
FLOWERING LOCUS M, a
FLC-related gene, in a SD-specific manner. It is possible that the attenuation of
FLM by
VIL1 has a role in creating the facultative nature of photoperiod response in
Arabidopsis since
vil1 mutants tend towards an obligate photoperiod response (i.e.,
vil1 mutants often fail to flower in SD).In
Arabidopsis, there are four
VIN3-related genes, which we named as
VIL1 ∼
VIL4,
8 and which have also been called
VRN5 and
VEL1 ∼
VEL3.
9 The C-terminal domain is highly conserved among these genes and was named the VIN3-Interacting Domain (VID) since it is required for protein-protein interaction between VIN3 and VIL1. The effect of cold on the expression patterns of
VIN3-related genes varies. For example, VIL2 and VIL3 are induced specifically by vernalizing cold exposures whereas others such as VIL1 are, for the most part, constitutively expressed. It will be interesting to determine the functions of the remaining
VIL genes.
FLC is the main target for vernalization in
Arabidopsis. Interestingly,
FLC orthologs have not been found in vernalization-responsive varieties of cereals. However, in wheat, VRN2 appears to have a role equivalent to that of
FLC in
Arabidopsis.
10
VRN2 encodes a ZCCT type zinc-finger protein that does not have a homolog in the
Arabidopsis genome. In diploid wheat, down regulation of
VRN2 is correlated with the vernalization response.
11 Interestingly, wheat contains three
VIN3-LIKE (
VIL) genes,
TmVIL1, TmVIL2 and
TmVIL3.
12 Furthermore,
TmVIL1 is up-regulated by vernalization.
12 However, whether
TmVIL1 has a direct role in the vernalization-mediated repression of
VRN2 in wheat has not yet been addressed. Similar to
VIL1, TmVIL3 shows elevated level of expression in SD. Furthermore,
VRN2 is downregulated in SD;
13,14 thus there is a correlation between the induction of
TmVIL genes and the downregulation of the floral repressor
VRN2 similar to the
VIN3/FLC and
VIL1/FLM relationships (). Perhaps
VIN3-related genes have similar roles both in
Arabidopsis and in temperate wheat, but act on different target genes, possibly as a result of convergent evolution. Interestingly, the wheat gene
TmVRN3 is homologous to
FLOWERING LOCUS T (
FT) of
Arabidopsis, and
TmVRN3 is repressed by
TmVRN2 as
FT is repressed by
FLC,
15 suggesting another similarity in the regulation of flowering time between
Arabidopsis and temperate wheat ().
Open in a separate windowProposed relationship of
VIN3 family genes to the regulatory network controlling flowering time in response to environmental cues in
Arabidopsis and diploid wheat (adapted from ref.
16).Although the PHD finger can be found in various eukaryotes, the VID is unique to plants. It is also noteworthy that
VIN3-related genes can be found in various plant species, including rice, which does not have a vernalization response. It will be interesting to address whether the
VIN3-related genes from various plant species are more broadly involved in relaying environmental signals to developmental programs.
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