HSFA1-driven heat stress response in Arabidopsis

In Arabidopsis thaliana, the transcriptional response to high temperature is a cascade headed by the four class A1 heat stress transcription factors (HSFA1a, HSFA1b, HSFA1d and HSFA1e). These are constitutively expressed master regulators. In unstressed cells they are held inactive by the cytosolic chaperones HSP90 and HSC70/HSP70. Heat titrates the chaperones away, and the HSFA1s then accumulate in the nucleus, bind heat shock elements (HSEs) and switch on most heat-responsive genes. Their targets include genes for secondary transcription factors that amplify and prolong the response. DREB2A, an AP2/ERF factor also used in drought signalling, has an HSE in its promoter and is induced by HSFA1. DREB2A in turn induces HSFA3, the only HSF known to be controlled by DREB2A. HSFA2 is the most strongly heat-induced HSF and sustains chaperone expression during recovery and heat acclimation memory. The cascade's output is the cytosolic chaperone network. The AAA+ disaggregase HSP101 (ClpB1) is essential for acquired thermotolerance. With HSP70 it pulls proteins out of heat-induced aggregates, including those held by the ATP-independent small HSP (sHSP) holdases. HSP90 and HSP70 refold and stabilise clients and also feed back to repress HSFA1. Loss of HSFA1a/b/d globally abolishes heat-induced gene expression and greatly reduces heat tolerance. Loss of HSP101 abolishes acquired thermotolerance.

Relation to MODULE:heat_shock_response. That module covers the eukaryotic HSF-to-chaperone logic with opisthokont representatives only (human, yeast and worm), and its small HSP node is placed at Eumetazoa. This plant module was built as a separate concrete instance rather than an extension of it. It adds the plant-specific HSFA1 multi-gene master tier and the secondary transcription factor cascade (DREB2A -> HSFA3, HSFA2), which have no counterpart in the generic module. It also adds the HSP101/ClpB disaggregase, which is absent from metazoans. Hsp70 J-domain and nucleotide-exchange cofactors are not re-modelled here. A later option is to add an Arabidopsis HSFA1 variant to the generic module's activation-mode variant set. Boundary. Upstream heat sensing (membrane fluidity, Ca2+/calmodulin and CBK3 signalling, ROS, and the HSFA1 kinases and phosphatases), DRIP-mediated turnover of DREB2A, DREB2A's drought and osmotic programme, HSFA1-PIF4 thermomorphogenesis, and the HSFA2-REF6 transgenerational memory loop are outside this module. Other HSFs (HSFB1/B2b repressors, HSFA7a/b), HSA32 and other output chaperones (HSP70 isoforms, other sHSP classes, ROF1, HSBP) are not modelled. They are referred to in prose only where they explain a reviewed member. HSFA1 MF terms follow each gene review's core function: GO:0003700 for HSFA1a, HSFA1b and HSFA1e and GO:0001228 for HSFA1d. Both sHSPs have no GO MF id, because their reviews record holdase chaperone activity as a proposed new term. PMID:11576425 (in vitro chaperone activity of "At-HSP17.6A") is attributed to the class II protein HSP17.7 (At5g12030, O81822), not to the class I HSP17.6A (At1g59860, Q9XIE3). UniProt disagrees: the Q9XIE3 entry cites PMID:11576425 for its FUNCTION line ("Possesses chaperone activity"). The module follows the paper's own statement that the protein is the cytosolic class II smHSP, and the TAIR cross-reference that gives At5g12030 the HSP17.6A symbol. This is a nomenclature collision, and the Q9XIE3 attribution should be raised with UniProt rather than taken as settled either way. No production GO-CAMs in gocams/index.tsv contain these Arabidopsis proteins.