Plants are able to sense and remember heat stress. An initial priming heat stress enables plants to acclimate so that they are able to survive a subsequent higher temperature. The heat shock transcription factors (HSFs) play a crucial role in this process, but the mechanisms by which plants sense heat stress are not well understood. By comprehensively analyzing the binding targets of all the HSFs, we found that HSFs act in a network, with upstream sensory HSFs acting in a transcriptional cascade to activate downstream HSFs and protective proteins. The upstream sensory HSFs are activated by heat at the protein level via a modular prion-like domain (PrD) structure. PrD1 enables HSF sequestration via chaperone binding, allowing release under heat shock. Activated HSFs are recruited into transcriptionally active foci via PrD2, enabling the formation of DNA loops between heat-responsive promoters and enhancer motifs, boosting gene expression days after a priming heat stress. The ability of HSFs to respond rapidly to heat via a protein phase-change response is likely a conserved mechanism in eukaryotes.