The Transcriptional Cascade in the Heat Stress Response of Arabidopsis Is Strictly Regulated at the Level of Transcription Factor Expression

Abstract

Group A1 heat shock transcription factors (HsfA1s) are the master regulators of the heat stress response (HSR) in plants. Upon heat shock, HsfA1s trigger a transcriptional cascade that is composed of many transcription factors. Despite the importance of HsfA1s and their downstream transcriptional cascade in the acquisition of thermotolerance in plants, the molecular basis of their activation remains poorly understood. Here, domain analysis of HsfA1d, one of several HsfA1s in Arabidopsis thaliana, demonstrated that the central region of HsfA1d is a key regulatory domain that represses HsfA1d transactivation activity through interaction with HEAT SHOCK PROTEIN70 (HSP70) and HSP90. We designated this region as the temperature-dependent repression (TDR) domain. We found that HSP70 dissociates from HsfA1d in response to heat shock and that the dissociation is likely regulated by an as yet unknown activation mechanism, such as HsfA1d phosphorylation. Overexpression of constitutively active HsfA1d that lacked the TDR domain induced expression of heat shock proteins in the absence of heat stress, thereby conferring potent thermotolerance on the overexpressors. However, transcriptome analysis of the overexpressors demonstrated that the constitutively active HsfA1d could not trigger the complete transcriptional cascade under normal conditions, thereby indicating that other factors are necessary to fully induce the HSR. These complex regulatory mechanisms related to the transcriptional cascade may enable plants to respond resiliently to various heat stress conditions.

Dynamic control of Hsf1 during heat shock by a chaperone switch and phosphorylation

“…While our results are consistent with studies reporting biochemical and genetic interactions between Hsf1 and Hsp70 (Abravaya et al, 1992; Baler et al, 1992, 1996; Brandman et al, 2012; Guisbert et al, 2013; Ohama et al, 2016; Shi et al, 1998), they are inconsistent with other reports that implicate Hsp90 as a major repressor of Hsf1 activity (Brandman et al, 2012; Duina et al, 1998; Guo et al, 2001; Zou et al, 1998). Biochemically, our inability to detect Hsp90 binding to Hsf1 could be due to the serial affinity purification strategy that we employed: if Hsp90 weakly associates with Hsf1, we would likely lose the interaction during the two-step purification.…”

“…Once proteostasis is restored, client-free chaperones again bind to Hsf1 and deactivate it. There is biochemical, pharmacological and genetic evidence to support roles for the Hsp70 and Hsp90 chaperones, their co-chaperones and the TRiC/CCT chaperonin complex in regulating Hsf1 (Abravaya et al, 1992; Baler et al, 1992, 1996; Duina et al, 1998; Guo et al, 2001; Neef et al, 2014; Ohama et al, 2016; Shi et al, 1998; Zou et al, 1998). However, direct, unequivocal evidence for this model – i.e., a complete cycle of Hsf1 ‘switching’ by dynamic dissociation and re-association with specific chaperone(s) during heat shock – is lacking.…”

EGRINs (Environmental Gene Regulatory Influence Networks) in Rice That Function in the Response to Water Deficit, High Temperature, and Agricultural Environments

“…The simplest large-scale EGRINs are based on transcriptome data, measured by high-throughput sequencing or array-based technology, without regard for other posttranscriptional and translational regulatory events that are known to influence transcriptional regulation (Koryachko et al, 2015). These methods assume that the expression of genes across environmental conditions, perturbations, and genotypes can be used to predict regulatory relationships; however, many TF proteins exist in an inactive form in the cytosol or nucleus until they are activated by environmental or developmental signals (Fu et al, 2011;Ohama et al, 2016). It is not feasible to measure all of the complex and varied factors contributing to the regulation of gene expression; as such, network inference algorithms have been developed to predict regulatory interactions in the absence of complete data by incorporating additional complementary data types or prior knowledge of the network structure to estimate the effects of unmeasured regulatory layers (Arrieta-Ortiz et al, 2015;Bonneau, 2008;Fu et al, 2011;Greenfield et al, 2013;Misra and Sriram, 2013;Roy et al, 2013).…”

Reactive electrophilic oxylipins trigger a heat stress-like response through HSFA1 transcription factors

“…Rather, it suggests that the heat-shock response is triggered by unfolded proteins that are the result of a variety of stresses, including RES, oxidative stress (ROS), heavy metals, ethanol or other toxic substances (Richter et al , 2010). In Arabidopsis, it has been shown that HSP70 and HSP90 indeed interact with HSFA1 and repress the activation of the heat-shock response in the absence of heat (Yamada et al , 2007; Ohama et al , 2016). …”

“…However, it has been questioned if the level of unfolded proteins regulates the dissociation of the inhibitory complex and alternative heat sensing mechanisms have been suggested (Mittler et al , 2012; Ohama et al , 2016). With respect to the activation of the heat-shock response by RES, it has been proposed that RES bind to and covalently modify HSP70 and/or HSP90 causing a destabilization of the inhibitory complex and the release of HSF (Scroggins and Neckers, 2007; Jacobs and Marnett, 2010).…”