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.