3.4. Heat sensing is mediated by lipid signals
Lipids are the primary structural components of membranes, but also have
signaling and regulatory functions, coupling perception of environmental
cues to cellular responses (Hou et al., 2015). Lipids such as PA and
phosphatidylinositol 4,5-bisphosphate (PIP2), and their
metabolic enzymes, i.e. phospholipases C and D (PLC/PLD), and lipid
kinases, diacylglycerol (DAG) kinase (DGK) and
phosphatidylinositol-4-phosphate 5-kinase (PIP5K) have a wide range of
cellular regulatory functions in environmental stress responses. In
Arabidopsis seedlings, heat stress (40°C) triggers increases in PA and
PIP2 abundance within 2 min (Mishkind et al., 2009).
This extremely rapid response suggests that the synthesis of these
signaling lipids is closely tied to thermosensing, but as of yet it is
unknown how increases in temperature activate these lipid modifying
enzymes (Fig. 4). High-temperature induction of PA is largely dependent
on membrane lipid hydrolysis by PLD (Mishkind et al., 2009; Shiva et
al., 2020) (Fig. 4). The PLD enzyme is localized at the plasma membrane
and associated with microtubules, where it regulates their
membrane-anchorage (Andreeva et al., 2009). In heat-stressed stomatal
cells, apoplastic H2O2 enters the
cytosol through aquaporins. H2O2oxidizes cysteine residues in the C2 domain of PLDδ. The modified
cysteine residues promoted Ca2+ binding to PLDδ, which
resulted in depolymerization of microtubules (Song et al., 2020; S.-S.
Zhang et al., 2017). Blocking microtubule depolymerization by chemical
stabilizers inhibited the upregulation of HSP70 and the induction of
MAPK activity under heat stress (Sangwan et al., 2002; Suri & Dhindsa,
2007), which suggests that this pathway acts to promote thermal
acclimation. Curiously though, mutants lacking PLDδ were more
tolerant to heat stress, which suggests the opposite. The fact that PLDδ
requires Ca2+ and H2O2for its activation hints that, despite its rapid activation,
phospholipid signaling occurs downstream of primary thermosensing.
Glyceraldehyde-3-phosphate dehydrogenase (GAPC) may also play a role in
heat stress signaling. GAPC was shown to translocate to the nucleus
under heat stress, where it activates the transcription factor NF-YC10.
Activated NF-YC10 then promotes the expression of genes that confer
thermotolerance (S.-C. Kim et al., 2020). The mechanism the promotes
GAPC nuclear translocation is as yet unresolved. When in the cytosol,
GAPC can directly bind PLDδ and positively promote its activity (Guo et
al., 2012). It has also been shown to bind PA (McLoughlin et al., 2013)
but it is unclear how or if these attributes contribute to temperature
regulation of GAPC. The involvement of GAPC in PLD/PA signaling raises
the possibility that the heat stress response is coordinated with basal
cell metabolism.
PLC also appears to have a function in the heat stress response, because
PLC9 and PLC3 knock-out seedlings show severely impaired basal and/or
acquired heat tolerance, while overexpression improved this (K. Gao et
al., 2014; Zheng et al., 2012) (Fig. 4). PLC hydrolyzes PIP and
PIP2 to generate DAG and inositolphosphates. The latter
could eventually result in the activation of a
Ca2+channel (Munnik, 2014). PLC9 and PLC3, both
localized at the plasma membrane, were required for the induction of
cytosolic Ca2+ and enhanced expression of sHSPs under
heat stress.
The heat stress-induced accumulation of PIP2 displayed
interesting dynamics. During heat exposure, PIP2accumulated first at the plasma membrane, after which it appeared in
cytoplasmic punctate structures, followed by accumulation at the nuclear
envelope (Mishkind et al., 2009). PIP2 can function in
endocytosis and associates with membrane microdomains (Furt et al.,
2010). Microdomains are considered critical in the regulation of early
stress signaling, as they contain signaling proteins such as RbohD,
HSPs, and CNGCs (Dietrich et al., 2020; Horvath et al., 1998; Niu &
Xiang, 2018). Acting very early in the response pathway, PLC3 and PLC9
may be physically close to the thermosensor. Regulation of PLCs is
complex, involving calcium, G-proteins and post-translational
modifications (Munnik, 2014). Potential protein interactors of PLC3 and
PLC9, including two receptor-like kinases, could provide interesting
clues as to their heat-responsive mode of activation (Pokotylo et al.,
2013).