Mediates both basal and acquired thermotolerance via HSFA1s-directed pathways (e.g. HSFA1A, HSFA1B, and HSFA1D). Triggers the expression of HSFA1A and HSFA1B. Interacts with the heat shock proteins HSP70-14 and At2g33735/HSP40, and with NFYC2 in both cytoplasm and nucleus. Expressed ubiquitously, including in seedlings, leaves, stems, inflorescences and siliques. Target of TAS1 (trans-acting siRNA precursor 1)-derived small interfering RNAs in response to temperature variations, thus reducing both basal and acquired thermotolerance (PubMed:24728648). Repressed by trans-acting small interfering RNA (ta-siRNAs) siR480(+)/siRNA255-mediated transcript cleavage (PubMed:16061187, PubMed:18753245). Highly up-regulated in seedlings exposed to heat shock, with higher levels of isoform 1 than isoform 2. Induced by HSFA1s-mediated (e.g. HSFA1A, HSFA1B, and HSFA1D) promoter activation (PubMed:24728648). Belongs to the heat induced plant HTT protein family. Extended N-terminus. Extended N-terminus.