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 .