The RAV family is a subgroup of the APETALA2/Ethylene-Responsive Factor (AP2/ERF) superfamily which is one of the largest families of plant TFs.  These 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. 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).

RAV Family: In maize the RAV (Related to ABI3/VP) family is predicted to contain 5 putative genes, that encode proteins with one AP2/ERF domain as well as one B3 domain (basic domain 3 from maize VIVIPAROUS1 gene - 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

Based on binding sites selection assays (BSSAs) and deletion analyses using recombinant GST fused RAV1 protein, it was found that the AP2 and B3- like domains bind autonomously to CAACA and CACCTG motifs respectively which constitute a two bipartite recognition sequence and the binding flexibility to these sequences is contributed by charged amino acid rich region, glycine rich region and poor hydrophobic amino acid that exists between AP2 and B3-like domains (Kagaya et al., 1999).

The diverse plant processes regulated by RAV proteins include response to abiotic stress, pathogen infection, bud outgrowth, leaf senescence, flowering time, response to light and hormone signaling (Matías-Hernández et al., 2014). Arabidopsis RAV1 is a brassinosteroids (BR) response gene whose expression is down regulated in response to epiBrassinolide (epiBL). RAV1 act as a negative regulator of Arabidopsis growth and development since its overexpression resulted in retarded lateral root and rosette leaf development whereas its underexpression caused earlier flowering phenotype along with cytokinin mediated transcriptional repression (Hu et al., 2004). AtRAV1 act as a positive regulator of leaf senescence with maximum expression level at early stage of the leaf senescence and lowering at later stage. Arabidopsis, RAV1 homologs TEM1 and RAV3 exhibited similar expression patterns implying functional redundancy among RAV proteins. RAV1 possibly plays a interconnecting role between age dependent and environmental induced leaf senescence due to induction of the RAV transcripts in response to senescence accelerating hormone and darkness or phytohormone exposure (Woo et al., 2010). TEMPRANILLO- TEM1 (At1g25560) and TEM2 (At1g68840; RAV2) are direct repressors of flowering locus T (FT) genes and a quantitative balance between activator CONSTANS (CO) and repressor TEM is needed to achieve FT threshold which ultimately triggers flowering (Castillejo and Pelaz., 2008). Cotton GhRAV1 play a role in abiotic stress response and ABA signaling due to its induction in response to NaCl, ABA and PEG. Transgenic Arabidopsis overexpressing GhRAV1 exhibited reduced chlorophyll content and earlier etiolate event thus indicating sensitivity to drought and salt stress as compared to the wild type. Moreover, due to alteration of stress related genes in response to drought and salt stress in transgenic lines, GhRAV1 might play a role in regulating expressions of these stress related genes during cotton development (Li et al., 2015)

There are 13, 13, 15 and 5 RAV genes in Arabidopsis, Soybean, rice, and maize respectively (Swaminathan and Peterson., 2008, Zhao et al., 2017, Cheng et al., 2023). Soybean RAV proteins play role in the nucleus due to the nuclear localization of GFP fused RAV proteins and these genes are involved in response to ABA or salt, drought stresses due to upregulation of all RAVs in response to ABA stress and down regulation of most of the RAVs in response to salt, drought stress except GmRAV-03 which was upregulated in response to salt stress, thus implying diverse roles of RAV proteins. However, transgenic lines overexpressing GmRAV-03 showed higher resistance to high salt, drought stress resulting in exogenous ABA insensitive Arabidopsis transgenic lines (Zhao et al., 2017). However, GmRAV1 is induced by cytokinin, and it is associated with key factors involved in cytokinin signaling pathways and act as a positive regulator for shoot and root regeneration (Zhang et al., 2018). Fifteen rice RAV genes are involved in various hormonal signaling pathway and virus infection due to their either up regulation or down regulation in response to hormone treatments and down regulation of most of the RAV genes in response to rice stripe virus (RSV) and upregulation of most of the RAV genes in response to Southern rice black streaked dwarf virus (SRBSDV). For instance, OsRAV9, OsRAV11, OsRAV12 were significantly upregulated in response to RSV and SRBSDV infection and OsRAV3 and OsRAV15 were down regulated by both viruses (Chen et al., 2021)

Last updated June 2023 by Ankita Abnave and John Gray

References:

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