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.