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.