In Arabidopsis and soybean, for example, 21 and 52 genes encoding HSFs have been identified, respectively, while the presence of fewer than 10 HSF genes is typical in animals ( Nover et al., 2001 Scharf et al., 2012). To cope with HS, sessile plants have evolved transcriptional networks governed by heat shock factors (HSFs), which are often more numerous than those in other higher eukaryotes. HS leads to misfolding and denaturation of proteins, which can severely impair the integrity of the cellular proteome if not curbed by protective mechanisms. 20–28 ☌) poses a critical threat to plants termed heat stress (HS), which often occurs in the range of 37–45 ☌. Appropriately balancing the processes underlying stress memory and the extent of forgetfulness (resetting) is required for the successful development, growth, and reproduction of plants in fluctuating natural environments ( Crisp et al., 2016, 2017 Zhang et al., 2020).Ī rise in environmental temperature above the optimum (e.g. Memory and forgetfulness represent opposing, but interrelated, objectives with specific costs and benefits. Notably, however, some of the priming-triggered changes remain for an extended period of time after disappearance of the stress, thereby manifesting a ‘stress memory’ that helps plants to more effectively combat a later-arriving stress ( Bruce et al., 2007 Hilker et al., 2016 Balazadeh, 2022). Most of the cellular changes induced by a priming stress are rapidly reversed to the pre-stress condition when the stress fades, allowing plants to ‘forget’ stressful situations and relocate available resources towards growth and reproduction in the following ‘stress-free’ period of their existence. A moderate and non-lethal abiotic stress is called a priming stress it elicits a cellular machinery that allows plants to faithfully evade a later-arriving, often more severe, so-called triggering stress ( Hilker et al., 2016). During their evolution, plants have, therefore, established a particular molecular and physiological inventory allowing them to survive and be reproductive under conditions of recursive and multiple stresses ( Bruce et al., 2007 Hilker et al., 2016). However, in nature, abiotic stresses are often repetitive and diverse. An inherent ability to tolerate certain levels of stress, called basal tolerance, enables plants to combat acute stress. Thus, by activating both memory-supporting and memory-resetting genes, HSFA2 acts as a cellular homeostasis factor during thermomemory.Īrabidopsis thaliana, FtsH6, heat stress, HSFA2, HSP21, thermomemory, thermorecovery IntroductionĪbiotic stress impairs the growth, development, and productivity of plants. Importantly, overexpression of HSFA2 improves thermomemory more profoundly in ftsh6 than wild-type plants. Chromatin immunoprecipitation reveals in planta binding of HSFA2 to the FtsH6 promoter. Constitutive and inducible overexpression of HSFA2 increases expression of FtsH6, whereas it is drastically reduced in the hsfa2 knockout mutant. Here, using a yeast one-hybrid screen, we identify HSFA2 as an upstream transactivator of the resetting element FtsH6. The transcription factor heat shock factor A2 (HSFA2) activates downstream genes essential for mounting thermomemory, acting as a positive regulator in the process. A heat-induced metalloprotease, filamentation temperature-sensitive H6 (FtsH6), degrades HSP21 to its pre-stress abundance, thereby resetting memory during the recovery phase. HSP21 functions as a key component of thermomemory, which requires a sustained elevated level of HSP21 during recovery from heat stress. HEAT SHOCK PROTEIN 21 ( HSP21) encodes a small heat shock protein in plastids of Arabidopsis thaliana. To maximize growth after stress, plants ‘reset’ or ‘forget’ memories of stressful situations, which requires an intricate balance between stress memory formation and the degree of forgetfulness. Plants ‘memorize’ stressful events and protect themselves from future, often more severe, stresses.
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