2.2. Other photosensors: phototropin, cryptochrome and UVR8
Phytochrome B is not the only plant photosensor with the ability to perceive temperature. The phototropin of liverwort (Marchantia polymorpha ) (MpPHOT ) is also temperature sensitive (Fujii et al., 2017). Phototropins are membrane bound blue light receptors that respond to positional light cues. They regulate phototropism, leaf flattening and chloroplast positioning. In Marchantia , an important phototropin regulated process is the cold avoidance response. At 22°C, blue light induces the movement of chloroplasts to the cell surface, in order to maximise photosynthesis. In contrast, at 5°C blue light induces the movement of chloroplasts to the periclinal cell walls. This process is known as cold avoidance and is thought to protect the photosynthetic machinery in suboptimal temperature conditions (Fujii et al., 2017).
MpPHOT contains two LOV (light, oxygen or voltage) domains that are responsible for light sensing. In darkness, each LOV domain contains a non-covalently bound flavin mononucleotide (FMN) chromophore. Blue light absorption by FMN triggers its covalent attachment to the LOV domain. This in turn causes structural re-arrangement of the phototropin molecule into its active form. Importantly, the covalent bond that links FMN and the LOV domain spontaneously degrades over time, resulting in inactivation of MpPHOT . This degradation rate increases with temperature, meaning that MpPHOT remains more active at cooler temperatures. As a result, MpPHOT promotes the relocation of chloroplasts to the cell periphery when temperatures are too low for efficient photosynthesis (Fujii et al., 2017). The phototropins of Arabidopsis were also recently implicated in temperature signalling (Kostaki et al., 2020). Warm temperatures promote guard cell opening in a cell autonomous manner. Curiously, this process is dependent on blue light and phototropins (Kostaki et al., 2020). This would seem to imply that in contrast to MpPHOT (Fujii et al., 2017), the activity of phototropin in Arabidopsis guard cells is actually enhanced at warm temperatures. Further investigation into the potential temperature sensitivity of Arabidopsis phototropins should help to resolve this point.
Zeitlupes are another class of blue light photoreceptor, which act to accelerate the pace of the circadian clock. Zeitlupes contain a LOV domain with a similar activation mechanism to phototropins (Pudasaini et al., 2017). If the rate of zeitlupe inactivation is increased at warm temperatures, this could potentially reduce the pace of the clock at warm temperatures (a process known as temperature compensation) (Hayes, 2020). Several years ago, zeitlupe was identified as a quantitative trait locus for natural variation in temperature compensation in Arabidopsis (Edwards et al., 2005) and so it would be interesting to experimentally test this hypothesis. Other plant photosensors, such as the blue light sensing cryptochrome (cry) and UV-B sensing UV Resistance Locus 8 (UVR8), could potentially also function as thermosensors (Figure 2). Cryptochromes undergo thermal reversion in a similar manner to phytochromes and phototropins. If cryptochrome thermal reversion is enhanced at warm temperatures, it is feasible that cryptochrome would also exhibit higher activity at cool temperatures (Hayes, 2020). UVR8 exists as a homodimer in the dark, but undergoes monomerization after absorbing UV-B. The active UVR8 monomer then reverts back to the inactive dimer, in a process that is mediated by Repressor of UV-B Photomorphogenesis 1 (RUP1) and RUP2. The RUP-mediated reversion of the UVR8 monomer to the UVR8 dimer seems to be influenced by temperature (Findlay & Jenkins, 2016), but the details of this process are currently unclear. Whether zeitlupe, cryptochrome and UVR8 functions are truly temperature sensitive remains to be investigated.