The Cys-Arg/N-End Rule Pathway Is a General Sensor of Abiotic Stress in Flowering Plants

Abstract

SummaryAbiotic stresses impact negatively on plant growth, profoundly affecting yield and quality of crops. Although much is known about plant responses, very little is understood at the molecular level about the initial sensing of environmental stress. In plants, hypoxia (low oxygen, which occurs during flooding) is directly sensed by the Cys-Arg/N-end rule pathway of ubiquitin-mediated proteolysis, through oxygen-dependent degradation of group VII Ethylene Response Factor transcription factors (ERFVIIs) via amino-terminal (Nt-) cysteine [1, 2]. Using Arabidopsis (Arabidopsis thaliana) and barley (Hordeum vulgare), we show that the pathway regulates plant responses to multiple abiotic stresses. In Arabidopsis, genetic analyses revealed that response to these stresses is controlled by N-end rule regulation of ERFVII function. Oxygen sensing via the Cys-Arg/N-end rule in higher eukaryotes is linked through a single mechanism to nitric oxide (NO) sensing [3, 4]. In plants, the major mechanism of NO synthesis is via NITRATE REDUCTASE (NR), an enzyme of nitrogen assimilation [5]. Here, we identify a negative relationship between NR activity and NO levels and stabilization of an artificial Nt-Cys substrate and ERFVII function in response to environmental changes. Furthermore, we show that ERFVIIs enhance abiotic stress responses via physical and genetic interactions with the chromatin-remodeling ATPase BRAHMA. We propose that plants sense multiple abiotic stresses through the Cys-Arg/N-end rule pathway either directly (via oxygen sensing) or indirectly (via NO sensing downstream of NR activity). This single mechanism can therefore integrate environment and response to enhance plant survival.

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

“…without previous S-nitrosylation (White et al, 2017), a role for NO in the degradation of the ERF-VIIs has been observed in vivo. In fact, NO has been shown to promote an N-end rule pathway-dependent proteolysis of full-length ERF-VII factors, as well as artificial ERF-VIIderived Arg-Cys/NERP substrates (MC-GUS, UBI-C-GUS) in Arabidopsis and barley Vicente et al, 2017). However, the mechanism connecting NO to ERF-VII stability still needs to be revealed ; Fig.…”

“…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 Arg-Cys/NERP has recently been proven to promote Arabidopsis and barley tolerance to salinity, drought, and heat (multiple abiotic stresses sharing an oxidative stress component) through ERF-VIIs. It has been proposed that, during salinity, ERF-VII proteins become stabilized following a decline in nitrate reductase activity and subsequent decrease in NO levels (Vicente et al, 2017).…”

“…Constitutive ERF-VII gene expression relates to more sustained activation of ROS scavenging reactions, and, conversely, mutations inside the gene family lead to elevated ROS under stress. Therefore, prompt and sustained ROS scavenging is believed to account for the positive impact of ERF-VII overexpression in those conditions entailing oxidative stress (Ogawa et al, 2005;Tang et al, 2005;Park et al, 2011;Yao et al, 2017;Vicente 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 (H 2 O 2 ) has not been assessed in ERF-VII proteins, *Cys forms generated in relation to H 2 O 2 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.…”