AvrRxo1 Is a Bifunctional Type III Secreted Effector and Toxin-Antitoxin System Component with Homologs in Diverse Environmental Contexts

Abstract

Toxin-antitoxin (TA) systems are ubiquitous bacterial systems that may function in genome maintenance and metabolic stress management, but are also thought to play a role in virulence by helping pathogens survive stress. We previously demonstrated that the Xanthomonas oryzae pv. oryzicola protein AvrRxo1 is a type III-secreted virulence factor that has structural similarities to the zeta family of TA toxins, and is toxic to plants and bacteria in the absence of its predicted chaperone Arc1. In this work, we confirm that AvrRxo1 and its binding partner Arc1 function as a TA system when expressed in Escherichia coli. Sequences of avrRxo1 homologs were culled from published and newly generated phytopathogen genomes, revealing that avrRxo1:arc1 modules are rare or frequently inactivated in some species and highly conserved in others. Cloning and functional analysis of avrRxo1 from Acidovorax avenae, A. citrulli, Burkholderia andropogonis, Xanthomonas translucens, and Xanthomonas euvesicatoria showed that some AvrRxo1 homologs share the bacteriostatic and Rxo1-mediated cell death triggering activities of AvrRxo1 from X. oryzae. Additional distant putative homologs of avrRxo1 and arc1 were identified in genomic or metagenomic sequence of environmental bacteria with no known pathogenic role. One of these distant homologs was cloned from the filamentous soil bacterium Cystobacter fuscus. avrRxo1 from C. fuscus caused watersoaking and triggered Rxo1-dependent cell collapse in Nicotiana benthamiana, but no growth suppression in E. coli was observed. This work confirms that a type III effector can function as a TA system toxin, and illustrates the potential of microbiome data to reveal new environmental origins or reservoirs of pathogen virulence factors.

Chromosomally Encoded hok-sok Toxin-Antitoxin System in the Fire Blight Pathogen Erwinia amylovora: Identification and Functional Characterization

“…Although TA systems have garnered much attention in the model organism E. coli and other mammalian pathogens, few type II TA systems have been characterized in plant-pathogenic bacteria (25)(26)(27)(28)(29)(30); to our knowledge, no type I TA system has been characterized in a plant pathogen. In this study, we identify a chromosomally encoded hok-sok TA system highly conserved in Erwinia amylovora, a plant pathogen in the family Enterobacteriaceae.…”

Comparative genomics of host adaptive traits in Xanthomonas translucens pv. graminis

“…With view on the recently demonstrated virulence and avirulence function of the type III secreted effector and TA toxin AvrRxoI in Xanthomonas spp. [81], it would be interesting to elucidate the functional role of the identified TA system components in the Xtg – L. multiflorum interaction.…”

Biotrophic Fungi Infection and Plant Defense Mechanism

“…Primary/ Early signaling event those observed during disease defense include reactive oxygen species (ROS), protein phosphorylation and ion fluxes. The regulation of these primary event assisted by different genes expressional changes for example WRKY53 transcriptional network regulates oxidative responses to a wide array of stresses [3]. In tomato and parsley cell suspension cultures, fungal elicitors trigger protein phosphorylation [41].…”

“…For instance AvrRxo1-ORF2 binds AvrRxo1-ORF1, it is structurally different from typical effector-binding chaperones, in that it has a distinct fold containing a novel kinase-binding domain [2]. The gene structure and cis-acting regulatory elements of effector genes are highly conserved between in different pathogens and report several novel effector genes these regulate pathogene responsive genes like WRKY53 [3].…”