Biology Reference
In-Depth Information
favorable environmental conditions. Multiple pathways control flowering
time in
Arabidopsis
, including the autonomous pathway, the photoperiod
pathway, and the vernalization pathway, which form a regulatory network
that converges on two main regulators, FLOWERING LOCUS C (FLC)
and FLOWERING LOCUS T (FT) (Reviewed in
Andres & Coupland,
2012
). The regulation of
FLC
has become a paradigm to understand the
general roles of chromatin in the transcription process in plants and has illu-
minated the paramount contribution of chromatin-based mechanisms in
sensing and monitoring the environment. In addition, it is one of the very
few examples in which initiation, maintenance, and resetting of epigenetic
control can be dissected in a developmental context.
3.2. License to flower
FLC
encodes a MADS-box transcription factor that acts, in a dosage-
dependent manner, as a potent repressor of the floral transition (
Michaels
& Amasino, 1999
). In winter-annual
Arabidopsis
,
FLC
expression is gradu-
ally turned off and epigenetically silenced by vernalization, which refers to
the acquisition of the ability to flower (in spring) after prolonged exposure to
cold temperatures in winter (
Chouard, 1960
). The vernalization process can
be subdivided into three main phases that correspond to three distinct chro-
matin landscapes at the
FLC
locus: (1) high expression level before exposure
to cold, (2) cold-induced silencing, and (3) epigenetic maintenance of the
repressed state after return to warm conditions (
Fig. 6.1
).
The winter-annual growth habit results from high expression level of
FLC
in young seedlings that depends on the FRIGIDA (FRI) coiled-coiled protein
and multiple components of conserved chromatin-modifying complexes
(Reviewed in
Crevillen &Dean, 2011
). Recruitment of the ATPase chroma-
tin remodeling SWR1 complex by the FRI complex leads to the incorpora-
tion of nucleosomes containing H2A.Z around the transcription start site, thus
rendering
FLC
chromatin permissive for transcription. The activity of differ-
ent WDR5a-containing COMPASS complexes then triggers the deposition
of H3K4me3/2 and H3K36me3/2 (
Jiang, Gu, &He, 2009
). In addition, high
FLC
expression relies on the cyclic monoubiquitylation of histone H2B by
the ubiquitin ligases UBC1-3 and HUB1-2 as well as the ubiquitin protease
UBP26 (
Cao, Dai, Cui, & Ma, 2008; Gu, Jiang, Wang, Bachmair, & He,
2009; Schmitz, Tamada, Doyle, Zhang, & Amasino, 2009
). This leads to
the establishment of an active chromatin state that is tightly associated with
high transcription of
FLC
.
