Biology Reference
In-Depth Information
Intriguingly, mitotic activity is necessary to achieve spreading of
H3K27me3 over the
FLC
locus (
Finnegan & Dennis, 2007
). A possible
explanation for this requirement is that the epigenetic switch relies on the
replication-dependent deposition of a H3.1 variant, usually found over tran-
scriptionally silent chromatin (
Stroud et al., 2012; Wollmann et al., 2012
)
and preferentially enriched in H3K27me3 in comparison with H3.3
(
Johnson et al., 2004
). The mechanism by which epigenetic silencing is
maintained at
FLC
through DNA replication is unknown. The observation
that some PcG proteins such as CLF or LHP1 appear to be bound to chro-
matin during most of the cell cycle, though not on metaphasic chromosomes
(
Libault et al., 2005; Schubert et al., 2006
) and interact with replication-
associated proteins (
Barrero, Gonzalez-Bayon, del Pozo, Ponce, & Micol,
2007
) might indicate that it is the propagation of PRC2 complexes, rather
than the H3K27me3 mark itself, that maintains the repressive chromatin
state, which is in keeping with recent models proposed in metazoans
(
Hansen et al., 2008; Petruk et al., 2012
).
Importantly, the vernalized state is not transmitted to the progeny, which
ensures that each generation requires proper cold exposure to flower.
FLC
silencing is relieved during gametogenesis upon the removal of H3K27me3,
and
FLC
expression is reactivated during early embryo development (
Choi
et al., 2009; Sheldon et al., 2008
). This resetting could result either from a
genome-wide reprogramming associated with histone turnover as shown in
the zygote (see
Section 4
) or from the local recruitment of H3K27me3
demethylases or Trithorax-associated ATP-dependent chromatin remodelers.
3.3. Time to bloom
The vernalization process can be viewed as the conversion of digital epige-
netic information stored at the cellular level into a quantitative response of
the whole organism. Plants mainly sense environmental conditions through
organs such as leaves, but the developmental decision to flower occurs in the
stem cell niche at the apex. This implies that the epigenetic silencing of
FLC
is relayed by a signal promoting flowering at the level of the organism and
over a distance.
FT
is an immediate downstream target of FLC repression
and encodes a small globular protein that translocates from the leaves to
the shoot apex, thus representing a key component of this mobile flowering
signal (
Corbesier et al., 2007; reviewed in Wigge, 2011
). The decision of
when to flower largely depends on the level of
FT
expression, which is
strongly influenced by seasonal cues such as the length of the photoperiod
and warm temperatures
(Reviewed in
Andres & Coupland, 2012
).
