Promoter and domain swap analysis delineates heat stress memory-specific determinants of heat shock factor HSFA2.

Exposure to moderate heat stress (HS) primes plants to better withstand future exposure to more severe HS conditions. In Arabidopsis (Arabidopsis thaliana), the primed state is maintained for several days, referred to as HS memory. HEAT SHOCK TRANSCRIPTION FACTOR A2 (HSFA2) and HSFA3 promote this HS memory by jointly mediating transcriptional memory in a subset of HS-inducible genes. Why only 2 of the 21 HSFs in Arabidopsis function specifically in HS memory is unknown. Here, we investigated this question through a promoter and domain swap analysis between HSFA2 and HSFA1D, a regulator of the acute HS response (HSR), aiming to uncover the requirements for HS memory HSFs. We determined that conferring the expression pattern of HSFA2 to HSFA1D is not sufficient for restoring HS memory. In chimeric proteins, the C-terminal regions and the DNA-binding domains (DBDs) of the 2 HSFs are interchangeable, while the presence of the HSFA1D repression domain abrogates HS memory. Interestingly, the oligomerization domain (OD) of HSFA1D cannot replace its HSFA2 counterpart in mediating physiological HS memory, suggesting a role for distinct interacting HSF complexes. The OD-exchanged chimeric HSF hyper-induced memory genes after a single HS, suggesting an altered response of the protein to HS. In summary, our study provides insights into the roles of individual HSF domains in specifying functions in HS memory or the acute HSR, thus providing avenues for tailoring HSFs to the demands of a changing climate.