2.4. Temperature-dependent action of the Evening Complex
A large proportion of the genome is regulated by the circadian clock. To
ensure robust rhythmicity, the clock is entrained to daily cycles of
light and temperature. The evening complex (EC) is a group of core clock
components that show peak expression in the early evening. In recent
years it has become apparent that the EC is one of the key points at
which light and temperature signals enter the clock (Ezer et al., 2017).
The EC consists of three components, the scaffold protein EARLY
FLOWERING 3 (ELF3), the transcription factor LUX ARRYTHMO (LUX), and a
protein of unknown function, ELF4. Together they act as a
transcriptional repressor that directly binds DNA (Ezer et al., 2017;
Huang et al., 2016; Nusinow et al., 2011). Besides its function in the
clock, the EC also represses the expression of thermomorphogenesis
promoting genes such as PIF4 , limiting the period of
temperature-induced growth (Box et al., 2015).
Consistent with its importance in conveying temperature information to
the clock, binding of the EC to DNA is temperature dependent. At cool
temperatures, the EC binds to DNA much more strongly than at warm
temperatures (Ezer et al., 2017). Phytochromes play an important role in
regulating the EC (Ezer et al., 2017; Huang et al., 2016) and so
temperature sensitivity of DNA binding could potentially be ascribed to
increased thermal reversion of phyB (Legris et al., 2016). Intriguingly
however, EC DNA binding is also temperature dependent in vitro ,
implying that EC activity is directly modulated by temperature (Silva et
al., 2020).
ELF3 contains a prion-like domain (PrD) with a high proportion of
glutamine residues (Jung et al., 2020). The PrD shows variable length
between species, with Arabidopsis thaliana ELF3 (AtELF3)
containing a much longer PrD than Brachypodium distachyon ELF3
(BdELF3). Replacing the PrD of AtELF3 with the corresponding region from
BdELF3 abolished the temperature-dependent DNA binding of AtELF3 (Jung
et al., 2020). At high temperatures, AtELF3 forms speckles within the
nucleus and this is also dependent on the PrD. Importantly, these
PrD-dependent speckles also form at high temperatures when AtELF3 is
expressed in yeast cells, in the absence of other evening complex
components. Furthermore, the purified PrD from AtELF3 spontaneously and
reversibly forms liquid droplets when in solution, in a
temperature-dependent manner (Jung et al., 2020). Warm
temperature-dependent self-coalescence of ELF3 through its PrD domain
therefore likely constitutes a temperature sensing mechanism in plants
(Fig. 2).
Direct temperature sensing by ELF3 may help to explain a curious
observation about phyB at warm temperatures. Under R light, active phyB
accumulates in several large sub-nuclear foci known as photobodies (Hahm
et al., 2020). In R + FR light, phyB is inactivated and disperses to
numerous smaller foci. Warm temperature also inactivates phyB and so we
might expect it to lead to a similar change in phyB photobodies.
However, in direct contrast to FR light, warm temperature appears to
promote the aggregation of phyB into fewer, larger photobodies (Hahm et
al., 2020). Whether there is a link between phyB aggregation and ELF3
PrD-mediated condensation at warm temperatures remains to be
investigated. However if this is the case, it presents an attractive
mechanism whereby plant cells could distinguish between FR and warm
temperature signals, based on the inactivation of phyB alone or the
inactivation of phyB and ELF3 in conjunction.