Group VII Ethylene Response Factors in Arabidopsis: Regulation and Physiological Roles

Abstract

The role of ERF-VII TFs in higher plants is to coordinate their signature response to oxygen deficiency, but additional layers of modulation of ERF-VII activity enrich their regulatory range.

Figure 1.

Overview of the regulation of group-VII ERF factor stability in Arabidopsis. The stability of plant ERF-VII proteins is controlled by intracellular O₂ and NO levels by means of the Arg-Cys/N-end rule pathway (NERP). The N-terminal Cys, exposed upon Met cleavage by MAP (Met aminopeptidase) enzymes, is susceptible to oxidation. Arginyl transferase (ATE) enzymes conjugate oxidized Cys (*Cys) to Arg, which in turn recruits the Arg-specific N-recognin PRT6 (Garzón et al., 2007), an N-end rule pathway-specialized E3 ubiquitin ligase that labels the substrate for degradation through the 26S proteasome (see Box 1 for additional details of the pathway). Cys oxidation can be promoted by specific thiol oxygenases called plant Cys oxidases (PCOs): In the presence of oxygen, PCOs convert Cys into Cys-sulfinic acid, which acts as an ATE substrate (White et al., 2017). Therefore, PCOs target ERF-VII proteins to the proteasome in an oxygen-dependent fashion. Besides oxygen, nitric oxide (NO) also promotes ERF-VII turnover via the Arg-Cys/NERP, through a still-undetermined Cys-dependent mechanism (Vicente et al., 2017; Gibbs et al., 2014). Finally, although N-terminal Cys reactivity to hydrogen peroxide (H₂O₂) has not been assessed in ERF-VII proteins, *Cys forms generated in relation to H₂O₂ concentration (García-Santamarina et al., 2014) could in principle play a role in the pathway, by either working as an alternative ATE substrate or interfering with Cys-sulfinic acid catalysis. Plasma membrane localization has been observed for RAP2.3 (Abbas et al., 2015) and RAP2.12 (Giuntoli et al., 2017; Kosmacz et al., 2015). As the latter has been found to be associated with peripheral membrane proteins belonging to the ACBP (Acyl-CoA binding protein) family, it has been proposed that this interaction is useful to maintain an inactive pool of RAP2.12 factor at the plasma membrane (Licausi et al., 2011a). ERF-VIIs have a primary role as master activators of the hypoxic metabolism. ERF-VII transcription factors (represented in this figure by the five subfamily members from Arabidopsis, AtRAP2.2/2.3/2.12 and AtHRE1/2) exert direct control on the hypoxia-inducible expression of plant anaerobic genes by binding an HRPE (hypoxia response promoter element) motif present in their promoters (e.g. ADH, PDC, LBD41, HRE1 and HRE2, and HRA1; Gasch et al., 2016). The hypoxia-inducible factors HRE1 and HRE2 are further controlled at the posttranscriptional level through the Arg-Cys/NERP (Gibbs et al., 2011). During hypoxic regulation, HRA1 acts as a feedback repressor of anaerobic gene expression, by interaction with RAP2.12 (Giuntoli et al., 2014). The SINAT pathway, which is an N-end rule pathway-independent proteolysis, is also shown. RAP2.12 can be ubiquitinated by the E3 ligases SINAT1/2 (Papdi et al., 2015). These proteins modulate the autophagy pathway and thereby enhance Arabidopsis tolerance to nutrient starvation (Qi et al., 2017). In fact, autophagy responses are also activated during hypoxia and contribute to plant submergence tolerance (Chen et al., 2015, and 2017b). This evidence suggests that an additional tier of regulation might connect the ERF-VIIs to submergence responses, through the SINAT factors. Solid lines refer to experimentally established reactions or relationships, dashed lines to hypothetical relationships drawn from observed regulation, and dotted lines depict hypothetical reactions.

Figure 2.

Additional roles of ERF-VIIs through interaction with distinct protein partners. A, Resistance to necrotrophic fungi (Zhao et al., 2012). After B. cinerea infection, ethylene accumulation leads to RAP2.2 gene induction downstream of the EIN2-EIN3/EIL ethylene signaling cascade. RAP2.2 activates the resistance genes PDF1.2 and ChiB by interaction with its partner Med25 (Ou et al., 2011) and contributes positively to Arabidopsis resistance to fungal attack. B, RAP2.3 is a positive regulator of apical hook development in Arabidopsis seedlings, and its action is counteracted by interacting DELLA proteins (Marín-de la Rosa et al., 2014). In etiolated seedlings, RAP2.3 gene expression is promoted by dark-induced ethylene production, while low levels of DELLA proteins prevent RAP2.3 functional restriction. Therefore, RAP2.3 participates in the interplay between ethylene and GA, which regulates apical hook formation (Abbas et al., 2013), by hindering premature hook opening under darkness. C, During germination, RAP2.2 is as a negative regulator of ABA responses (Lumba et al., 2014). This function has been associated with RAP2.2 phosphorylation, following its interaction with an SNRK3 kinase complex that mediates ABA insensitivity. D, ERF-VII stabilization enhances plant tolerance to multiple abiotic stresses (Vicente et al., 2017). During salinity, decreased NO biosynthesis due to NR enzyme impairment has been proposed to lead to ERF-VII protein stabilization in the presence of oxygen. The beneficial effects of the ERF-VIIs on plant tolerance to salinity is antagonized by its interacting partner BRM, possibly due to competition for the same cis-element on the target gene promoters (Vicente et al., 2017). EIN2, Ethylene-insensitive2; EIN3, Ethylene-insensitive2; EIL, EIN3-like; Med25, Mediator subunit25; DELLA, GRAS-domain family proteins (GAI, RGA, RGLs); PP2C, Protein phosphatase2C; SnRK3, SNF1-related protein kinase3; NR, nitrate reductases; NO, nitric oxide; BRM, BRAHMA ATPase.