Heat shock factors (HSF) are the transcriptional activators of the heat shock response. The reversible conversion of constitutively expressed HSF to a form that can bind DNA requires the trimerization of the protein, involving leucine zipper interactions as shown for yeast, Drosophila, chicken and human HSFs.

Typically, plant HSF proteins have a conserved patterned structure, including a highly structured DNA-binding domain (DBD) at the N-terminus, and with two hydrophobic 7-peptide repeats (HR-A/B region) oligomerization domain (OD), nuclear localization signal (NLS), nuclear export signal (NES) at the C-terminus, activation structure (AHA motif) and repressor domain (RD). Plant HSF proteins can be divided into three subfamilies of HSFA, HSFB and HSFC based on the length of their basic amino acid sequences between the DBD and HR-A/B regions and the amount of amino acid residues inserted into the HR-A/B regions (Guo et al., 2016).

Like other metazoan HSFs, the endogenous Arabidopsis HSF displays heat shock-inducible DNA-binding activity in gel retardation assays. The heat shock-inducible binding of a recombinant Arabidopsis HSF (ATHSF1) expressed in Arabidopsis plants suggests that ATHSF1 is a major heat shock factor regulating the heat stress response. However, only hsf1(-) /hsf3(-)double mutants were significantly impaired in HS gene expression indicating a redundant role for these two TFs. (Hübel et al., 1995, Lohmann et al., 2004).

In maize 25 HSF genes have been identified which could be divided into three subfamilies (Lin et al., 2011, Jiang et al., 2020). The 25 ZmHsf as well as 22 ZmHsp70 (heat shock protein) genes were confirmed to respond to heat stress treatment, indicating that they have potential effects in heat stress response (Jiang et al., 2020). Another study identified 31 non-redundant ZmHSF genes in the maize reference genome by Single Molecule Real Time (SMRT) sequencing (Zhang et al., 2020). RNA-Seq analysis of anthesis and post-anthesis periods in maize show different expression patterns of ZmHsf family members. Specifically, ZmHsf26 of A2 subclass and ZmHsf23 of A6 subclass were distinctly up-regulated after heat shock (HS) at post-anthesis stage. alternative splicing (AS) events were seen to occur in ZmHsf04 and ZmHsf17 with intron retention events occuring in response to heat shock. The  ZmHsf04-II and ZmHsf17-II, isoforms contain several premature termination codons in their introns which may lead to early termination of translation (Zhang et al., 2020).

Overexpression of ZmHsf08 in maize resulted in enhanced sensitivity to salt and drought stresses, displaying lower survival rates, higher reactive oxygen species (ROS) levels, and increased malondialdehyde (MDA) contents compared with wild-type (WT) plants. Furthermore, RT-qPCR analyses revealed that ZmHsf08 negatively regulates a number of stress/ABA-responsive genes under salt and drought stress conditions. Collectively, these results indicate that ZmHsf08 plays a negative role in response to salt and drought stresses in maize (Wang et al., 2021).

Last updated June 2023 by John Gray

References:

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Lohmann C, Eggers-Schumacher G, Wunderlich M, Schöffl F. Two different heat shock transcription factors regulate immediate early expression of stress genes in Arabidopsis. Mol Genet Genomics. 2004 Feb;271(1):11-21. doi: 10.1007/s00438-003-0954-8. Epub 2003 Dec 4. Erratum in: Mol Genet Genomics. 2004 Apr;271(3):376. PMID: 14655047.

Jiang L, Hu W, Qian Y, Ren Q, Zhang J. Genome-wide identification, classification and expression analysis of the Hsf and Hsp70 gene families in maize. Gene. 2021 Feb 20;770:145348. doi: 10.1016/j.gene.2020.145348. Epub 2020 Dec 15. PMID: 33333230.

Lin YX, Jiang HY, Chu ZX, Tang XL, Zhu SW, Cheng BJ. Genome-wide identification, classification and analysis of heat shock transcription factor family in maize. BMC Genomics. 2011 Jan 27;12:76. doi: 10.1186/1471-2164-12-76. PMID: 21272351; PMCID: PMC3039612

Zhang H, Li G, Fu C, Duan S, Hu D, Guo X. Genome-wide identification, transcriptome analysis and alternative splicing events of Hsf family genes in maize. Sci Rep. 2020 May 15;10(1):8073. doi: 10.1038/s41598-020-65068-z. PMID: 32415117; PMCID: PMC7229205.

Wang J, Chen L, Long Y, Si W, Cheng B, Jiang H. A Novel Heat Shock Transcription Factor (ZmHsf08) Negatively Regulates Salt and Drought Stress Responses in Maize. Int J Mol Sci. 2021 Nov 3;22(21):11922. doi: 10.3390/ijms222111922. PMID: 34769354; PMCID: PMC8584904.