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).