Biology Reference
In-Depth Information
Modulation of the chromatin state at the
FT
locus is another illustration of
how chromatin contributes to precise transcriptional regulation by sensing
environmental signals. In unfavorable conditions,
FT
expression is kept to a
basal level via the H3K4me2/3 demethylase activity of the plant-specific,
Jumonji C-domain JMJ14, which might act in conjunction with PcG-
mediated repression (
Adrian et al., 2010; Jeong et al., 2009; Jiang, Wang,
& He, 2008; Lu, Cui, Zhang, Liu, & Cao, 2010; Yang, Jiang, Jiang, &
He, 2010
). This likely prevents precocious flowering and sets up the ground
for a gradual response to warm temperatures that relies in part on H2A.Z
dynamics, as described above (
Kumar & Wigge, 2010
). Thus, a rise from
12 to 27
C triggers the gradual eviction of H2A.Z-containing nucleosomes
at the regulatory regions of
FT
, which facilitates the binding of the PIF4
transcriptional activator (
Kumar et al., 2012
). H2A.Z occupancy appears
therefore to provide a mechanism for sensing a graded increase in ambient
temperature by modulating chromatin accessibility to PIF4 at the
FT
locus,
thereby inducing
FT
expression and flowering initiation.
3.4. Emerging regulatory principles coming in from the cold
and more
How chromatin-modifying complexes are recruited to their specific
targets remains a key question. Polycomb Response Elements (PRE) have
been described originally in
Drosophila
as DNA sequences bound by PcG
proteins that create a domain for H3K27 trimethylation and convey tran-
scriptional repression (Reviewed in
Schuettengruber et al., 2007; Simon
& Kingston, 2009
). Despite the thousands of PRC2 targets detected in
Ara-
bidopsis
, only two genomic regions have been identified that meet most of
the criteria expected for PRE-like sequences. One coincides with the nucle-
ation region described above for
FLC
(
Buzas, Tamada, & Kurata, 2012;
Sheldon et al., 2002; Sung et al., 2006
) and the other one corresponds to
a 50-bp sequence that is required for PRC2-mediated repression of
LEC2
during late embryogenesis and vegetative development (
Berger,
Dubreucq, Roudier, Dubos, & Lepiniec, 2011
). Identification of additional
PRE-like sequences and of the proteins that specifically recognize them is
obviously of considerable interest.
Analysis of the vernalization pathway also led to the emergence of new
regulatory principles in plants, such as the complex interplay between
lncRNA transcription and the recruitment of specific chromatin-modifying
complexes. The COOLAIR antisense lncRNA was named with reference
to the HOTAIR lncRNA that is required for the silencing of
HOX
loci
through the targeted enrichment of PRC2 in mammals (
Rinn et al., 2007
).
