In plants, the APETALA2/Ethylene-Responsive Factor (AP2/ERF) superfamily is one of the largest families of plant TFs and were thought to be unique to the plant lineage. However, they are distantly related to homing endonucleases genes (HEGs) that reside in group-I self-splicing introns and it has been postulated that they evolved by horizontal transfer from a chloroplast endosymbiont in early plant evolution (Wessler 2005).

The AP2/ERF superfamily proteins contain at least one highly conserved AP2/ERF DNA-binding domain comprising 40–70 amino acid residues. It can be divided into three distinct families according to the number and similarity of AP2/ERF DNA-binding domains, namely the AP2 family, the ERF family (including two subfamilies ERF and DREB) and the RAV (Related to ABI3/VP) family. As transcription factors, AP2/ERFs regulate genes involved in diverse biological processes such as growth, development (floral organ and epidermal cell identity), hormone and stress responses through several mechanisms including transcriptional and post-translational control

In maize, a genomic survey has revealed a total of 214 genes encoding ZmAP2/ERF proteins with at least one complete AP2/ERF domain according to the (RefGen_v4) version of the maize B73 genome (Zhang et al., 2022). Of these 44 were placed in the AP2 family 166 in the ERF family and 4 in the RAV family. The number of the introns in the AP2 family genes was larger than that in the genes of two other families. However the numbers reported included multiple transcripts from individual genes and therefore underestimated the total number of family members..

A more recent genomic survey (that did not reference the Zhang et al., 2022 paper) reported 229 AP2/ERF genes in the latest (B73 RefGen_v5) maize reference genome.  These were categorized into 5 clades, including 27 AP2 (APETALA2), 5 RAV (Related to ABI3/VP), 89 DREB (dehydration responsive element binding), 105 ERF (ethylene responsive factors), and a soloist (Cheng et al., 2023).

ERF Family:  In maize there are 166 members in this family which is further subdivided into two diverse subfamilies, including the DREB and ERF subfamilies with 61 and 105 members, respectively (Zhang et al., 2022). Further, based on phylogenetic analysis, the members of DREB and ERF subfamilies were subdivided into four (Group I-IV) and eight (Group V-XII) groups, respectively. Typically, DREBs recognize Dehydration-Responsive or C-Repeat Element (DRE/CRT) with A/GCCGAC core sequence on stress-responsive genes to confer resistance to drought, cold and heat abiotic stresses. ERFs bind to Ethylene-Response Element (ERE) with AGCCGCC core sequence (also known as GCC-box) to confer resistance to biotic stresses (Xie et al., 2019).

RAV Family: In maize the RAV (Related to ABI3/VP) family is predicted to contain 4 putative genes, that encode proteins with one AP2/ERF domain as well as one B3 domain - a plant-specific DNA-binding domain that is conserved within other transcription factors (see also the ABI3-VP1 family of TFs)(Kagaya et al., 1999). The RAV family genes in maize have no intron except ZmRAV2 containing one intron (Zhang et al., 2022).  Members of the RAV family play a principal role in the plant’s growth and developmental processes, i.e., leaf senescence

Last updated June 2023 by John Gray

References:

Wessler SR. Homing into the origin of the AP2 DNA binding domain. Trends Plant Sci. 2005 Feb;10(2):54-6. doi: 10.1016/j.tplants.2004.12.007. PMID: 15708341.

Kagaya Y, Ohmiya K, Hattori T. RAV1, a novel DNA-binding protein, binds to bipartite recognition sequence through two distinct DNA-binding domains uniquely found in higher plants. Nucleic Acids Res. 1999 Jan 15;27(2):470-8. doi: 10.1093/nar/27.2.470. PMID: 9862967; PMCID: PMC148202.

Xie Z, Nolan TM, Jiang H, Yin Y. AP2/ERF Transcription Factor Regulatory Networks in Hormone and Abiotic Stress Responses in Arabidopsis. Front Plant Sci. 2019 Feb 28;10:228. doi: 10.3389/fpls.2019.00228. PMID: 30873200; PMCID: PMC6403161.

Zhang J, Liao J, Ling Q, Xi Y, Qian Y. Genome-wide identification and expression profiling analysis of maize AP2/ERF superfamily genes reveal essential roles in abiotic stress tolerance. BMC Genomics. 2022 Feb 12;23(1):125. doi: 10.1186/s12864-022-08345-7. PMID: 35151253; PMCID: PMC8841118.

Cheng C, An L, Li F, Ahmad W, Aslam M, Ul Haq MZ, Yan Y, Ahmad RM. Wide-Range Portrayal of AP2/ERF Transcription Factor Family in Maize (Zea mays L.) Development and Stress Responses. Genes (Basel). 2023 Jan 11;14(1):194. doi: 10.3390/genes14010194. PMID: 36672935; PMCID: PMC9859492.