Figure 1. Schematic overview of responses in different warm
temperature ranges.
Plants display a wide array of responses when they experience above
optimal temperatures. At warm ambient temperatures, up to 30°C,
Arabidopsis responds by changes in morphology and development, called
thermomorphogenesis, which could aid in avoidance of future heat stress.
Thermomorphogenesis features the temperature-sensitive function of phyB.
Also in this temperature range, there is thermosensitive regulation of
PIF7 mRNA translation. Warm temperatures lead to the loss of a hairpin
structure of PIF7 mRNA, which allows for its translation. ELF3 undergoes
temperature-dependent phase separation. High temperatures promote the
coalescence of ELF3, and the inhibition of ELF3-DNS binding, At
temperatures up to 38°C, Arabidopsis initiates acclimation responses
that counteract damage to proteins and membranes, and maintain cellular
homeostasis. This process involves the activity of HSFA1 master
transcriptional factors (H. Liu & Charng, 2013). HSPs/sHSP accumulate
to limit misfolding of proteins. and the membrane’s lipid composition is
adjusted so as to prevent disruption of the bilayer structure due to
uncontrolled increases in membrane fluidity. The heat sensors that
activate acclimation are unknown. The accumulation, within the first ±15
min of heat stress, of putative signaling components, such as
Ca2+, H2O2,
PIP2, PA and cAMP suggests their function in heat
perception, closely tied to the sensor. Temperatures above 40°C are
damaging to Arabidopsis, and all responses in this range are devoted to
immediate protection of cellular structures. Mechanisms of clearance and
rescue of unfolded proteins, including the UPR, are important for
survival of severe heat stress. These heat stress responses rely on the
recognition of unfolded proteins in the ER, the cytosol, and diverse
organelles.
Figure 2. Schematic overview of thermomorphogenic pathways in
arabidopsis.1. Under red light, phyB is converted to a Pfr homodimer that is
translocated to the nucleus where it blocks PIF4 and PIF7 activity. High
temperatures promote the reversion of phyB back to its inactive state,
leaving PIF4 and PIF7 free to transcribe thermomorphogenesis promoting
genes. 2. PIF7 mRNA contains a hairpin near its 5’-UTR sequence.
Upon an increase in temperature, this hairpin structure becomes more
relaxed. In the relaxed state, PIF7 mRNA is more easily
translated and PIF7 protein levels are increased. 3. At cooler
temperatures, ELF3 (as part of the evening complex) represses the
expression of PIF4 . As temperatures rise, a prion like domain in
ELF3 promotes its aggregation, thus relieving the transcriptional
repression of PIF4 .