## 4 Transcriptional Regulation during Salinity and Drought Stresses
### Ap2/Erebp  
The APETALA2/ethylene-responsive element binding protein (AP2/EREBP) family is a family of transcription factors thought to have a significant role in regulating gene expression at the transcription level [23; 60; 61]. These TFs have a highly conserved DNA-binding domain known as AP2/ERF DNA-binding domain. This AP2/ERF (APETALA2/ethylene-responsive element binding factor) is a domain consisting of 50-70 amino acids and is found in many proteins in the plant kingdom, such as _A. thaliana_ ERF1, tobacco EREBPs (homolog of the ERF), _A. thaliana_ AP2, _A. thaliana_ C-repeat/dehydration-responsive element (DRE) binding factor 1 (CBF1 orDREB1) and DREB2 and _Arabidopsis thaliana_ and maize abscisic acid (ABA)-insensitive 4 (ABI4) proteins [54; 61].  
Upon stresses, the AP2/EREBP superfamily are induced through _cis_-acting elements present at their promoter region. These regulatory elements include GCC box, ABRE, EBS,HSE, LRT and many other unknown binding sites that respond to a variety of abiotic stress stimuli such as AREBs, EIN3 and other transcription factors [61; 62]. Thus, the transcriptional regulation pathways of this superfamily of transcription factor proteins are both ABA-dependent and -independent. For instance, promoter analysis studies on the DREB subfamily determined that it contains the most diverse _cis_-acting elements that are related to abiotic stresses, including ABRE, MeJA response, TCA, ERE, MBS, HSE, TC-rich and LTR motifs. This finding suggests that those transcription factor-encoding genes are the most frequently expressed and involved in the signal transduction pathways related to abiotic stresses [51; 54]. It also suggests that this subfamily is regulated through ABA-dependent pathways due to the presence of the ABRE _cis_-acting element and through the ABA-independent pathway due to the availability of the other _cis_-regulatory elements.  
The DREB 2 subfamily of the AP2/EREBP superfamily can regulate the expression of multiple _responsive to dehydration_ (RD) genes by interacting with a GCC-like box called the DRE/CRT _cis_-acting element found at the promoter region of many stress-inducible genes in a variety of plant species (e.g., _RD29A_, _RD17_, _ERD10_) [63; 64; 65]. On the other hand, the ethylene-responsive factor (ERF) subfamily, which also belongs to the AP2/EREBP superfamily, activates an array of stress-responsive genes through the interaction with _cis_-regulatory sequences that are found upstream of the coding region of many stress-inducible genes such as the ethylene-response element (ERE) or GCC box with the core sequence of AGCCGC, jasmonic acid- and elicitor-responsive element (JERE), coupling element 1 (CE1) and CT-rich element [6; 66; 55]. For instance, it was found that a certain ERF encoded by the _ERF74_ binds to the RbohD promoter region and triggers its expression [67; 54] and hence participates in the defense mechanism against salinity and drought by generating ROS that act as signaling molecules. There are many other pieces of evidence suggesting the solid regulatory effect of the AP2/ERF family of TFs in the plant response to abiotic stresses [68].