3.1. The heat stress response is partly based on unfolded
protein sensing
Heat stress triggers adaptive responses which protect macromolecular
structures, restore cellular homeostasis and prevent damage. The
composite response likely involves the action of multiple parallel
sensors that activate signaling pathways in different cellular
compartments with different dynamics. Cellular defense mechanisms are
activated by monitoring protein, DNA and membrane damage (Balogh et al.,
2013; Ding et al., 2020; Niu & Xiang, 2018). The expression of Heat
Shock Proteins (HSPs) is induced by elevated temperatures in a large
variety of eukaryotes. HSPs act as molecular chaperones to promote the
correct folding of proteins in the cell and are crucial for tolerance to
high temperatures in plants. HSPs can also contribute to the
thermomorphogenic response. For example, HSP90 promotes the stability of
the auxin receptor TIR1 and in doing so, promotes root and shoot
elongation at warm temperatures (R. Wang et al., 2016).
Studies in yeast have suggested a mechanism by which HSPs are induced at
warm temperatures. HSPs bind to a class of transcription factors known
as Heat Shock Factors (HSF), which are thus kept in an inactive state.
At warm temperatures, the binding of HSPs to HSFs is disrupted. HSFs are
then free to travel to the nucleus to induce the expression of HSPs and
other heat stress genes. This increased abundance of HSPs then
sequesters the free HSFs, thus reaching another equilibrium.
Currently it is unclear whether the HSP:HSF module functions in the same
manner in plants, but similar mechanisms are known to activate the
unfolded protein response (UPR). When a plant experiences heat stress,
unfolded and misfolded proteins can accumulate to such levels that they
overload the protein quality control system, leading to ER stress. When
unfolded proteins accumulate in the ER, they are bound by binding
protein (BiP), an HSP70 chaperone. In the absence of unfolded proteins,
BiP binds to the ER membrane-tethered transcription factors (MTTFs)
bZIP17 and bZIP28. When unfolded proteins sequester BiP, bZIP17 and
bZIP28 are activated by translocation and proteolytic cleavage of their
membrane anchors at the Golgi. These transcription factors then travel
to the nucleus to promote the expression of chaperones and foldases to
assist in protein folding, a process known as the unfolded protein
response (UPR). Unfolded proteins also interact with the lumenal domain
of the transmembrane sensor IRE1b (inositol-requiring enzyme1), inducing
unconventional splicing of bZIP60, which then activates the ER stress
response genes. IRE1b splicing activity is induced by heat (Deng et al.,
2011). Moreover, IRE1 was found to be inducible by lipid bilayer stress
in yeast (Ernst et al., 2018; Halbleib et al., 2017), and to regulate
the degradation of specific mRNA’s, shaping the stress transcriptome.
Interestingly, the transcription factor ELONGATED HYPOCOTYL 5 (HY5) was
recently found to compete with bZIP17 and bZIP28 to repress the UPR
(Nawkar et al., 2017). HY5 abundance is reduced at warm temperatures
(Park et al., 2017), potentially through the photo/thermo sensing
mechanism described above. Reduced HY5 abundance may help to promote the
UPR under heat stress.
High temperature and light conditions can also activate UPR responses
through a signal from the chloroplast, methyl-D-erythritol
2,4-cyclodiphosphate (MEcPP, Fig. 3), a retrograde signaling metabolite
(Rivasseau et al., 2009; Walley et al., 2015). This intermediate of the
methylerythritol-4-phosphate (MEP) pathway of isoprenoid synthesis
accumulates as a direct consequence of a heat stress-induced metabolic
bottleneck, and induces CAMTA3-dependent transcription of IRE1a and
bZIP60 and other genes with a rapid stress response element (Benn
et al., 2016). Hence, the MEP pathway is considered an important stress
sensor (Xiao et al., 2012). The functions of retrograde signals in heat
stress have been reviewed elsewhere (Sun & Guo, 2016). Clearly,
different sensory modules feed into the UPR response.
Under heat stress, unfolded proteins also accumulate in chloroplasts.
The small heat shock protein (sHSP) of Chlamydomonas, HSP22E/F, forms
high-molecular weight complexes with them to prevent proteotoxicity
(Rütgers, Muranaka, Mühlhaus, et al., 2017). Heat-inactivation of
specific thermolabile proteins in the chloroplast, and their
sequestration into complexes with sHSP, is considered an adaptive
mechanism to directly regulate metabolism and signaling processes in
response to heat.
AtNTL4 and OsNTL3, which are MTTFs of the NTL (NAC transmembrane
transcription factor-like) group, also appear critical in regulating
heat stress responses. The basis for their thermosensitive proteolytic
activation remains however, unresolved (Lee et al., 2014; X. Liu et al.,
2020). Evidence of their ER localization (Liang et al., 2015) and
partial convergence with ER-stress responses (Yang et al., 2014),
suggests that they could signal heat-induced protein and lipid
perturbations at the ER.