Department of Cell and Developmental Biology, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
* Author for correspondence (e-mail: caroline.dean{at}bbsrc.ac.uk)
SUMMARY
The timing of the floral transition has significant consequences for reproductive success in plants. Plants gauge both environmental and endogenous signals before switching to reproductive development. Many temperate species only flower after they have experienced a prolonged period of cold, a process known as vernalization, which aligns flowering with the favourable conditions of spring. Considerable progress has been made in understanding the molecular basis of vernalization in Arabidopsis. A central player in this process is FLC, which blocks flowering by inhibiting genes required to switch the meristem from vegetative to floral development. Recent data shows that many regulators of FLC alter chromatin structure or are involved in RNA processing.
Introduction
The switch to flowering is a major developmental transition in the plant
life cycle (Simpson and Dean,
2002). Plants initially undergo a period of vegetative
development, characterised by the iterative production of leaves from the
shoot meristem (Poethig,
1990
). Later in development, the meristem undergoes a change in
fate and enters reproductive development, producing flowers and
differentiating the germ line. Plant species exhibit great variability in
flowering-time, and the timing of this floral switch is controlled by multiple
environmental and endogenous cues (Battey,
2000
; Izawa et al.,
2003
; Simpson and Dean,
2002
). This enables plants to align their life history with
favourable environmental conditions.
Genetic analysis of Arabidopsis thaliana has identified numerous
pathways that control the timing of the floral transition
(Fig. 1,
Table 1 and
Table 2). Downstream of many of
the floral pathways are a set of floral pathway integrator genes
(Kardailsky et al., 1999;
Kobayashi et al., 1999
;
Samach et al., 2000
;
Lee et al., 2000
;
Moon et al., 2003a
;
Hepworth et al., 2002
;
Nilsson et al., 1998
; Blazquez
et al., 2000) (see Fig. 1 and
Table 1). It is the activation
of these floral pathway integrator genes that triggers the floral transition.
In turn, the integrators activate a set of genes known as floral meristem
identity (FMI) genes, which encode proteins that promote floral development,
not only by positively regulating genes required for flower development, but
also by repressing AGAMOUS-LIKE 24 (AGL24), a promoter of
inflorescence fate (Yu et al.,
2004
).
|
|
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In contrast to the promotion pathways, enabling pathways determine the activity of repressors of the floral pathway integrators (Fig. 1, Table 2). The expression of the floral repressor FLC is regulated by several independent pathways. FLC is upregulated by a number of genes, including FRIGIDA (FRI), and is downregulated by prolonged periods of cold, a process known as vernalization. In temperate environments, the long period of cold temperature experienced over the winter can promote flowering, aligning reproductive development with spring and summer conditions. Once acquired, the vernalized state is `remembered' by the plant during subsequent growth, suggestive of an epigenetic basis. Several proteins, classified as the autonomous promotion pathway, act in parallel to vernalization and also repress FLC (Table 1). The autonomous pathway was named because of its lack of involvement in either the photoperiodic- or gibberellin-promotive floral pathways. This review focuses on recent work addressing the control of FLC expression in response to the prolonged periods of cold experienced during vernalization. Chromatin regulation and RNA processing have emerged as key mechanisms that modulate expression of the floral repressor FLC. We will speculate on how the different pathways controlling FLC expression may be integrated at the molecular level.
The floral repressor, FLC
FLC is a MADS-box transcriptional repressor, expressed predominantly in
shoot and root apices and vasculature, that quantitatively represses flowering
by repressing the floral pathway integrators
(Michaels and Amasino, 1999;
Sheldon et al., 1999
;
Michaels and Amasino, 2001
).
The mechanism by which it does this is not well understood, although a
MADS-box binding site within the promoter of SOC1 is required
(Hepworth et al., 2002
).
Natural Arabidopsis accessions vary in their requirement for a
vernalization treatment before flowering. Accessions are isogenic
Arabidopsis backgrounds collected from a single location and
maintained in a seed bank. Rapid cycling accessions, such as the laboratory
strains Columbia and Landsberg erecta, flower early without a
vernalization treatment. By contrast, many wild accessions flower much later,
unless they receive a vernalization treatment; these are termed winter annual
backgrounds (Fig. 2). Allelic
variation at FLC contributes to natural variation in vernalization
requirement, with weak alleles leading to a rapid-cycling habit
(Fig. 2)
(Michaels et al., 2003;
Gazzani et al., 2003
).
Interestingly, the phenotypes of plants with naturally occurring weak
FLC alleles appear to be caused by changes in the regulation of
expression rather than alterations of protein function. The rapid-cycling
Landsberg erecta and Da accessions contain FLC alleles with
independent transposon insertions within the large FLC intron 1
(Michaels et al., 2003
;
Gazzani et al., 2003
).
Sequences important for FLC regulation and expression have been
mapped to this intron, which may account for the effects of these insertions
(Sheldon et al., 2002
;
He et al., 2003
).
|
Activation of FLC
A key activator of FLC expression is FRI
(Fig. 1,
Table 1). Pioneering genetic
analysis performed by Klaus Napp-Zinn (University of Cologne) in the 1950s
identified allelic variation at FRI as the major determinant of
flowering-time variation between rapid-cycling and winter annual accessions.
Active FRI alleles confer late flowering and a vernalization
requirement for early-flowering
(Napp-Zinn, 1955;
Napp-Zinn, 1957
). It is
striking, given that so many genes regulate FLC, that the winter
annual habit can be mapped as a single gene trait to FRI. FRI
represses flowering by upregulating FLC RNA levels
(Michaels and Amasino, 1999
;
Sheldon et al., 2000
) and,
consistent with this, loss of FLC function eliminates the ability of
FRI to delay flowering (Michaels
and Amasino, 2001
) (Fig.
2). Map-based cloning of FRI revealed that it encodes a
novel protein with coiled-coil domains, but gave no indication as to the
mechanism by which it upregulates FLC
(Johanson et al., 2000
).
Analysis of natural Arabidopsis accessions identified at least nine
independent loss-of-function mutations in FRI
(Gazzani et al., 2003
;
Johanson et al., 2000
;
Le Corre et al., 2002
). Hence,
evolution of the rapid-cycling growth habit in some strains of
Arabidopsis may have evolved multiple times through the loss of
FRI. Genetic analysis of natural variation in flowering time has also
identified AERIAL ROSETTE1 (ART1) from the extremely
late-flowering accession Sy-0; ART1 acts synergistically with FRI to
upregulate FLC (Poduska et al.,
2003
).
Recently, an increasing number of FLC activators have been
identified by the analysis of early-flowering mutants. Two FRIGIDA-LIKE (FRL)
genes, FRL1 and FRL2, are required for the upregulation of
FLC expression by FRI (Michaels
et al., 2004). Although FRI, FRL1 and FRL2 are related at the
amino acid sequence level, they appear not to be functionally redundant
(Michaels et al., 2004
). The
VERNALIZATION INDEPENDENCE (VIP) genes are also required for high FLC
expression (Zhang et al.,
2003
; Zhang and van Nocker,
2002
). The VIP4 protein exhibits homology with the yeast Leo1p
protein, a component of the Paf complex, which is required for chromatin
modification and transcriptional activation
(Zhang et al., 2002
;
Porter et al., 2002
). The VIP3
protein encodes WD repeats that typically mediate protein-protein interactions
(Zhang et al., 2003
). Hence,
the VIP proteins may represent a complex that is required for FLC
transcription and chromatin regulation. PHOTOPERIOD-INDEPENDENT EARLY
FLOWERING1 (PIE1) also provides a link to chromatin regulation,
as it encodes a protein that activates FLC and has homology to
ATP-dependent chromatin-remodelling proteins of the ISWI and SWI2/SNF2 family
(Noh and Amasino, 2003
). A
more tenuous link to chromatin regulation may be EARLY IN SHORT DAYS
4 (ESD4), which encodes a nuclear protease that upregulates
FLC and is required for the regulation of SUMOylation (SMALL
UBIQUITIN-RELATED MODIFIER) in Arabidopsis
(Murtas et al., 2003
;
Reeves et al., 2002
).
SUMOylation is a recently discovered modification of histones and may be a
part of the `histone code' (see Box
1) (Shiio and Eisenman,
2003
). Many other proteins, however, are also SUMOylated, so ESD4
may function to regulate the levels, activity or compartmentalization of an
FLC regulator. Interestingly, mutations in VIP3, PIE1 and
ESD4 suppress the high FLC expression caused by either
dominant FRI alleles or mutations in the autonomous pathway
(Zhang et al., 2003
). By
contrast, FRL1 and FRL2 are specifically required for the
activation of FLC via FRI
(Michaels et al., 2004
).
Repression of FLC by vernalization
FLC mRNA levels are downregulated by vernalization. In nature,
winter provides the necessary cold and results in the alignment of flowering
with the favourable conditions of spring. At the molecular level, FLC
regulation by the cold shows similarities with many of the physiological
properties of vernalization (Chouard,
1960; Lang, 1965
).
The vernalization response is strongly quantitative, with increasing durations
of cold leading to progressively accelerated flowering once the plants return
to ambient temperatures (Chouard,
1960
; Lang, 1965
).
The downregulation of FLC RNA is also a quantitative process, with
longer periods of cold exposure leading to progressively lower FLC
mRNA expression (Michaels and Amasino,
1999
; Sheldon et al.,
2000
). For annual plants (those that germinate and flower within
one year), the vernalization response is saturated after several weeks of cold
and, once established, the vernalized state is stable though subsequent growth
at ambient temperatures, although it is reset after meiosis
(Chouard, 1960
;
Lang, 1965
). Similarly,
repression of FLC levels is achieved after several weeks of cold and
is then maintained at low levels throughout subsequent development, whilst
being reset in the next generation
(Sheldon et al., 2000
).
FLC repression must therefore be `remembered' through mitotic
proliferation until flowering occurs. Furthermore, grafting experiments reveal
that the site of cold perception during vernalization is the shoot apex and
this is a region of FLC expression
(Wellensiek, 1962
;
Wellensiek, 1964
; Sung et al.,
2004).
The maintenance of FLC repression following vernalization
indicates that this gene is epigenetically silenced. Epigenetic silencing of
genes is mediated by numerous covalent modifications of both the DNA and
histones (Box 1)
(Fischle et al., 2003;
Bird, 2002
). Early work on the
control of vernalization focused on the role of DNA cytosine methylation
(Finnegan et al., 1998
).
However, recent data has demonstrated a more important role for histone
modifications at FLC chromatin during vernalization
(Sung and Amasino, 2004
;
Bastow et al., 2004
). Specific
residues of histone H3 tails are modified by acetylation and methylation, and
changes in these modifications serve as part of a `histone-code' specifying
active or repressed gene activity states
(Fischle et al., 2003
).
Vernalization increases histone H3 deacetylation in the 5'-region of
FLC very early after exposure to the cold, a modification typically
associated with gene repression (Sung and
Amasino, 2004
). Vernalization also induces increased methylation
of histone H3 lysine residues 9 and 27, modifications associated with
repressed gene states (Sung and Amasino,
2004
; Bastow et al.,
2004
). In animal systems, deacetylation is typically a prelude to
acquisition of histone methylation
(Fischle et al., 2003
).
Furthermore, histone methylation marks can act as signals to recruit further
mediators of gene silencing (Orlando,
2003
). Interestingly, the histone marks observed at the
FLC locus appear to be localised to specific regions of the gene
(Bastow et al., 2004
), at the
5' end of the gene and within intron 1, co-localising with sequences
already known to be involved in the regulation of FLC by
vernalization (Sheldon et al.,
2002
; He et al.,
2003
).
Genetic screens for mutants compromised in vernalization have identified
trans-factors that mediate repression of FLC in response to the cold
(Chandler et al., 1996;
Sung and Amasino, 2004
). The
earliest acting gene is VERNALIZATION INSENSITIVE 3 (VIN3),
which encodes a protein with a plant homeodomain (PHD) and fibronectrin type
III repeats (Sung and Amasino,
2004
). PHD domains have been found in proteins associated with
chromatin-remodelling complexes and can bind phosphoinositides, whereas
fibronectin repeats are often involved in protein-protein interactions
(Sung and Amasino, 2004
). In
vin3 mutants, the vernalization-mediated decrease in histone
acetylation and increase in H3 K9 and K27 methylation does not occur, and thus
FLC is not repressed by vernalization
(Sung and Amasino, 2004
).
Intriguingly, VIN3 expression increases with cold, and only
significantly accumulates after a period of cold sufficient to trigger
vernalization (Sung and Amasino,
2004
). The VIN3 expression domain also overlaps with that
of FLC (Sung and Amasino,
2004
). Hence, upregulation of VIN3 expression is an early
step during the vernalization-signalling pathway. Understanding how prolonged
cold induces expression of VIN3 is a key question for future
research.
A second class of gene involved in the vernalization response is
represented by the genes VERNALIZATION1 (VRN1) and
VERNALIZATION2 (VRN2)
(Chandler et al., 1996;
Gendall et al., 2001
;
Levy et al., 2002
). The
vrn1 and vrn2 mutants are distinct from vin3 in
that initial repression of FLC expression by the cold still occurs
(Gendall et al., 2001
;
Levy et al., 2002
). However,
when vrn1 and vrn2 mutants return to ambient temperatures,
FLC repression is not maintained and FLC RNA levels
progressively increase (Chandler et al.,
1996
; Gendall et al.,
2001
; Levy et al.,
2002
). Unlike VIN3, expression of VRN1 and
VRN2 is not upregulated by cold, and hence VIN3 may provide a
cold-induced activity that recruits them to FLC
(Sung and Amasino, 2004
;
Gendall et al., 2001
;
Levy et al., 2002
).
Furthermore, VRN1 and VRN2 are not required for the VIN3-mediated FLC
deacetylation early in vernalization (Sung
and Amasino, 2004
). The VRN2 protein shows homology to the
Drosophila Polycomb protein Suppressor of Zeste 12 (Su(z)12)
(Gendall et al., 2001
). The
Polycomb-Group (PcG) proteins function to maintain epigenetic gene activity
states throughout Drosophila embryogenesis and cell proliferation
(Orlando, 2003
). Su(z)12 acts
in a PcG complex, PRC2, with histone methyltransferase activity directed
against histone H3 lysines 27 and 9
(Kuzmichev et al., 2002
;
Muller et al., 2002
). Hence,
VRN2 is likely to mediate stable repression of FLC activity by a
PcG-like mechanism. Indeed, in vrn2 mutants, increased histone H3
methylation of lysines 27 and 9 does not occur at FLC during
vernalization (Sung and Amasino,
2004
; Bastow et al.,
2004
). This indicates that elements of the `histone-code' involved
in developmental gene regulation are highly conserved between plants and
animals. By contrast, the VRN1 protein is plant-specific and carries two B3
domains, which mediate non-sequence specific DNA binding in vitro
(Levy et al., 2002
). Unlike
VRN2, VRN1 is required only for increases in histone H3 lysine 9 methylation,
and not for methylation of lysine 27 (Sung
and Amasino, 2004
; Bastow et
al., 2004
). This suggests that VRN1 may function either downstream
or independently of VRN2 during FLC repression. Overexpression of
VRN1 revealed a vernalization-independent function for VRN1, mediated
predominantly through the floral pathway integrator FT, and
demonstrated that VRN1 requires vernalization-specific factors to target
FLC (Levy et al.,
2002
).
Repression of FLC by autonomous pathway genes
The autonomous pathway acts in parallel to vernalization to repress
FLC expression (Koornneef et al.,
1991; Simpson and Dean,
2002
). In the absence of FRI, this pathway is the major
regulator of FLC levels and therefore confers a vernalization
requirement (Koornneef et al.,
1991
). Mutants in the autonomous pathway are late-flowering
because of elevated levels of FLC mRNA, and this late-flowering is
vernalization responsive (Koornneef et
al., 1991
; Sheldon et al.,
2000
; Michaels and Amasino,
2001
) (Fig. 1 and
Table 1). Although all members
of this pathway act to limit FLC expression, genetic analysis has
revealed that they have distinct functions. Two epistasis groups
FCA, FY and FPA, FVE have been found using double
mutants, although the significance of this is not yet fully understood
(Koornneef et al., 1998
). The
ld and fld mutations are strongly suppressed by the
FLC allele in Ler, the background in which the other
mutations were isolated, so epistasis analysis of these genes has not yet been
performed (Lee et al., 1994b
;
Sanda and Amasino, 1996
).
HDACs in the flowering response
FLD encodes a protein with homology to a human protein that
functions in the histone deacetylase 1,2 (HDAC1/2) co-repressor complex
(He et al., 2003). Histone
deacetylation mediated by this complex is commonly associated with gene
repression (He et al., 2003
).
The FLD protein carries an N-terminal SWIRM domain, such as that found in
chromatin remodelling enzymes, in addition to a polyamine oxidase domain
(He et al., 2003
). In
fld mutants, the 5'-end of FLC displays
hyperacetylation of histone H4 (He et al.,
2003
), indicating that FLD is required to deacetylate FLC
chromatin and thereby repress its expression. Intriguingly, removal of a
295-base pair region of FLC intron 1 prevents this regulation and
results in high FLC expression, independent of FLD activity
(He et al., 2003
). Thus, this
FLC intronic region may contain cis sequences required for
recruitment of a HDAC complex. Currently, the identity of the HDAC that
functions with FLD is unknown.
The Arabidopsis genome encodes four HDAC1/2 homologs but
late-flowering mutations in these genes have yet to be identified
(Pandey et al., 2002).
However, an antisense construct designed to target multiple HDACs does result
in delayed flowering, which may be due to a failure to repress FLC
(Tian and Chen, 2001
).
Analysis of histone H4 acetylation status in the other autonomous mutants
revealed a similar hyperacetylation phenotype only in fve
(He et al., 2003
).
FVE encodes the nuclear WD-repeat protein, MSI4
(Ausin et al., 2004
). There are
five MSI-related proteins in Arabidopsis, which display homology to
the mammalian Retinoblastoma Associated Protein46 (RbAp46) and RbAp48 proteins
(Ausin et al., 2004
). MSI-like
proteins are typically found in complexes involved in chromatin assembly and
histone modification, and FVE was demonstrated to co-immunoprecipitate with
plant Rb (Retinoblastoma protein) (Ausin et
al., 2004
). In other systems, Rb functions in histone deacetylase
complexes, which again is consistent with the histone hyperacetylation of
FLC observed in fve and fld mutants
(Ausin et al., 2004
;
He et al., 2003
). In addition
to a histone H4 hyperacetylation phenotype, analysis in fve mutants
also revealed hyperacetylation of histone H3, indicating that both histones
are deacetylated by this pathway (Ausin et
al., 2004
). Hence, FVE and FLD are likely to act together in a
HDAC complex to repress FLC expression
(Ausin et al., 2004
;
He et al., 2003
). It will be
important to determine if this HDAC complex is specifically targeted to
FLC or whether it performs broader functions that are covered by
redundancy. How this deacetylase activity integrates with the epigenetic
modifications directed by vernalization is also an interesting question.
RNA processing
Mutations in the autonomous pathway gene FCA display no effect on
FLC acetylation status (He et
al., 2003). Indeed, FCA appears to be genetically
distinct from FVE (Koornneef et
al., 1998
). FCA encodes a plant-specific, nuclear
RNA-binding protein (Macknight et al.,
1997
). In addition to two RNA recognition motif (RRM) domains, FCA
possesses a C-terminal WW protein interaction domain
(Macknight et al., 1997
;
Sudol and Hunter, 2000
). This
domain mediates interaction with another component of the autonomous pathway,
FY (Simpson et al., 2003
). In
contrast to FCA, FY is highly conserved throughout eukaryotes and displays
homology to the yeast polyadenylation factor, Pfs2p
(Ohnacker et al., 2000
;
Simpson et al., 2003
). Pfs2p
carries seven WD repeats and acts as a scaffold protein within the large CPF
(cleavage and polyadenylation factor) complex
(Ohnacker et al., 2000
). The
CPF complex is required for 3'-cleavage and polyadenylation of pre-mRNA
transcripts, and strong mutations in polyadenylation factors, including
PFS2, are lethal because of a failure to correctly express RNA
polymerase II transcripts (Ohnacker et
al., 2000
). In addition to these WD repeats, FY possesses a novel
C-terminal domain with which FCA interacts. FY may perform a generic function
in RNA processing, while also functioning in regulated polyadenylation through
interaction with FCA. FPA encodes a second plant-specific RRM domain
protein within the autonomous pathway
(Schomburg et al., 2001
).
Although FPA is required for the regulation of FLC, the level at
which it functions is unknown. Finally, FLK is the most recently
identified member of the autonomous pathway and encodes a nuclear KH-type
RNA-binding protein (Lim et al.,
2004
). Hence, multiple RNA-binding proteins are required for
repression of FLC expression by the autonomous pathway. Determining
whether this reflects a cascade of post-transcriptional regulators or a
complex of RNA-binding factors will require further analysis of proteins of
the autonomous pathway.
Currently there is no evidence that FCA/FY, FPA or FLK directly regulates
FLC mRNA processing. However, FCA expression itself is
complex and exhibits an autoregulatory mechanism involving polyadenylation
site choice (Macknight et al.,
1997; Macknight et al.,
2002
; Quesada et al.,
2003
). There are four FCA transcripts, and intron 3 is a
major site of alternative processing. Premature cleavage and polyadenylation
within this intron generates the truncated, non-functional FCA-ß
transcript (Macknight et al.,
1997
; Macknight et al.,
2002
). FCA negatively autoregulates its own expression by
promoting intron 3 polyadenylation
(Quesada et al., 2003
). This
regulation also requires the functional interaction between FCA and FY,
demonstrating that these proteins mediate alternative 3'-end processing
(Macknight et al., 1997
;
Macknight et al., 2002
;
Quesada et al., 2003
). Hence,
FCA may function as a novel trans-regulator of polyadenylation site choice via
interaction with the core 3'-processing factor FY. An intriguing aspect
of FCA autoregulation is its tissue specificity. Premature
polyadenylation is inhibited in meristematic regions relative to
non-meristematic regions (Macknight et
al., 2002
; Quesada et al.,
2003
). The mechanism by which this occurs is currently unknown but
might also have a consequence for the regulation of FLC. FPA and
FLK appear not to be required for FCA intron 3 regulation
(Lim et al., 2004
;
Quesada et al., 2003
). Hence,
the proteins of the autonomous pathway appear to have partially redundant
activities that repress FLC by distinct mechanisms. It is not known
whether the chromatin regulation and RNA processing activities of the
autonomous pathway are integrated during the control of FLC
expression, although chromatin modification and 3'-processing interact
functionally in yeast (Alen et al.,
2002
).
Integration of the pathways regulating FLC expression
Plants need to monitor their environmental conditions during growth and
development, and acquire sufficient resources to complete reproductive
development. The FLC activators are considered to function early in
development to ensure high levels of FLC and floral repression at germination,
thus avoiding precocious flowering before resources have accumulated. The
repressors of FLC expression may be downregulated early in
development for the same reason. This appears to be the case for FCA,
as production of the active FCA transcript via a change in
polyadenylation site usage increases significantly in meristems 4-5 days after
germination (Macknight et al.,
2002). Indeed, bypassing this control on FCA overrides
FRI repression of flowering
(Quesada et al., 2003
).
However, the precise temporal expression of many FLC activators, and
when their functions are required in flowering control, remains to be
determined.
The interaction between FLC activators and repressors effectively
determines whether a plant adopts a winter annual or rapid-cycling habit. It
is possible that this interaction is determined by the antagonistic effects of
the different pathways on FLC chromatin. PIE1 and VIP proteins are
FLC upregulators that may act to promote active chromatin, whereas
FVE and FLD act to deacetylate histones, thus promoting a silent chromatin
state. The roles of the multiple RNA-binding proteins (FCA, FPA, FLK), and the
polyadenylation factor FY, in repressing FLC raises some interesting
possibilities. They may function to repress FLC directly or by
regulating components of the activation pathway. Alternatively, the recent
demonstrations of non-coding RNA acting in chromatin regulation means that
they may play a role in generating RNA intermediates that feed back to
regulate FLC chromatin (Volpe et
al., 2002; Zilberman et al.,
2003
).
The onset of winter perturbs the steady-state FLC expression by
the induction of VIN3 after several weeks of cold, potentially
initiating a chain of epigenetic modifications at the FLC locus. An
early step in this sequence appears to be histone deacetylation
(Fig. 3), and the stable
maintenance of FLC repression involves the activities of VRN1, VRN2
and histone methylation. In animals, histone methylation recruits further
proteins required to maintain gene repression
(Orlando, 2003), although the
identity of such factors in plants and during vernalization remains unknown.
FLC expression then remains low during subsequent development and
flowering, but at some stage during meiosis, gametogenesis or early
embryogenesis, FLC expression is reset. The epigenetic modifications
at FLC established during vernalization, or by the activity of the
autonomous pathway, are erased, allowing high FLC expression in the
young seedlings and determining a requirement for vernalization in each
generation. This molecular sequence accounts for flowering in annual plants.
Many plants, however, are perennials, that is they live for many years with
only a proportion of the apical meristems undergoing the transition to
flowering each year. Whether similar mechanisms are involved in controlling
flowering in perennials remains to be established.
|
Multiple mechanisms have evolved to ensure the fine control of FLC levels and thus the timing of the transition to flowering. Considerable progress has been made towards elucidating the molecular mechanisms involved, but several important questions remain. Is VIN3 expression really the cold-induced trigger that initiates the chromatin changes at FLC? How do these changes overcome the function of activators such as FRI, and how do genes of the autonomous pathway fit into the molecular picture? Understanding the mechanisms involved in the resetting of FLC expression may provide insights into fundamental aspects of epigenetic reprogramming in plants and animals. The power of forward genetics, together with the exploitation of natural variation, will undoubtedly be key to unravelling many of these questions, and will provide answers as to how the different Arabidopsis reproductive strategies have been selected.
Recent progress in wheat has also identified key regulators determining the
vernalization requirement in cereals (Yan
et al., 2003; Trevaskis et
al., 2003
; Yan et al.,
2004
). The genes identified are so far distinct from those
identified in Arabidopsis. Wheat VRN1 functions as a floral promoter
and is a MADS-box protein with homology to APETALA1
(Yan et al., 2003
). Wheat VRN2
contains a CCT domain (a 43-amino acid region with homology to
Arabidopsis proteins CO, CO-LIKE and TOC1), and it functions to
repress directly or indirectly the expression of wheat VRN1
(Yan et al., 2004
).
Vernalization progressively reduces levels of wheat VRN2 RNA,
preventing repression of VRN1 and promoting flowering. The
involvement of distinct proteins in cereals and Arabidopsis implies
that different pathways have evolved to regulate the vernalization
requirement. However, it will be interesting to determine whether chromatin
regulation of these targets also mediates the epigenetic memory of winter in
wheat. Together, work in cereals and Arabidopsis should allow the
manipulation of vernalization, a key agricultural trait.
ACKNOWLEDGMENTS
Thanks to all members of the Dean group, past and present, for helpful discussions, and especially Gordon Simpson for his comments on the manuscript.
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