2.1. Phytochrome B (phyB) as temperature sensor
In recent years, there has been a growing realization that certain molecules originally characterized as photosensors moonlight as thermosensors, fine tuning growth and differentiation in response to moderate temperature changes. Here, the photo/thermo sensor acts as a receptor for a change in temperature, and in doing so initiates downstream signaling processes.
There is extensive crosstalk between light and temperature signalling in plants (Hayes, 2020). Plants perceive light conditions with at least five types of photoreceptors, i.e. (1) phytochromes, (2) cryptochromes, (3) phototropins, (4) zeitlupes and (5) UVR8 (Voitsekhovskaja, 2019). Of these photoreceptors, the temperature sensitive activity of phytochromes is the most well characterised, in particular phytochrome B (phyB). Phytochromes control many aspects of thermomorphogenesis, especially architectural changes, accelerated flowering and senescence (Jung et al., 2016; C. Kim, 2020; Quint et al., 2016).
Phytochromes are red/far red (R/FR) sensitive photoreceptors that absorb light through a phytochromobilin chromophore. The absorption of light by phytochromobilin induces its isomerisation, and this translates to conformational changes in phytochrome structure. R light promotes a shift to the active form of phytochrome (Pfr) whereas FR light promotes reversion to the inactive form (Pr) (Hayes, 2020). Importantly, Pfr can also spontaneously revert to Pr, and this process is temperature dependent. The rate of thermal reversion from Pfr to Pr is accelerated at warm temperatures (Legris et al., 2016), resulting in a reduced pool of active phytochrome (Fig. 2).
When activated by R light and cool temperatures, phytochromes promote the degradation of a family of bHLH transcription factors known as PHYTOCHROME INTERACTING FACTORS (PIFs).When phytochrome function is reduced by FR light or warm temperatures, PIFs accumulate and promote hypocotyl elongation through the enhanced expression of auxin biosynthesis genes (Jung et al., 2016; Koini et al., 2009; Legris et al., 2016). Hypocotyl elongation at warm temperatures is largely driven by PIF4 , PIF7 and to some extent PIF5 (Chung et al., 2020; Fiorucci et al., 2020; Koini et al., 2009). PIF-mediated elongation and hyponasty at warm temperatures results in an open architecture and enhances leaf cooling (Crawford et al., 2012; Park & Park, 2019).