A horizontally acquired expansin gene increases virulence of the emerging plant pathogen Erwinia tracheiphila

Jorge Rocha,Roberto Kolter,Lori R Shapiro

doi:10.1038/s41598-020-78157-w

Abstract

Erwinia tracheiphila is a bacterial plant pathogen that causes a fatal wilt infection in some cucurbit crop plants. Wilt symptoms are thought to be caused by systemic bacterial colonization through xylem that impedes sap flow. However, the genetic determinants of within-plant movement are unknown for this pathogen species. Here, we find that E. tracheiphila has horizontally acquired an operon with a microbial expansin (exlx) gene adjacent to a glycoside hydrolase family 5 (gh5) gene. Plant inoculation experiments with deletion mutants in the individual genes (Δexlx and Δgh5) and the full operon (Δexlx–gh5) resulted in decreased severity of wilt symptoms, decreased mortality rate, and impaired systemic colonization compared to the Wt strain. Co-inoculation experiments with Wt and Δexlx–gh5 rescued the movement defect of the mutant strain, suggesting that expansin and GH5 function extracellularly. Together, these results show that expansin–GH5 contributes to systemic movement through xylem, leading to rapid wilt symptom development and higher rates of plant death. The presence of expansin genes in diverse species of bacterial and fungal wilt-inducing pathogens suggests that microbial expansin proteins may be an under-appreciated virulence factor for many pathogen species.

Highlights

The surfaces of all land plants are colonized by complex microbial communities
These results suggest that the Et–exlx–gh[5] locus is a non-canonical yet potent virulence factor, and horizontal acquisition of this locus was a key event driving the recent emergence of E. tracheiphila[32,35] as a virulent plant pathogen that causes high mortality by inducing wilt symptoms and plant death by blocking the flow of xylem sap (Fig. 1)
We identified a locus with two open reading frames (ORFs) flanked by mobile DNA elements during manual curation of ab initio gene predictions in the E. tracheiphila reference strains (Fig. 2A)[29,30]

Summary

Introduction

The surfaces of all land plants are colonized by complex microbial communities. For a microbe, the ability to colonize a plant increases access to the nutritional resources produced by that p lant[1,2]. Co-inoculation experiments with Wt and deletion mutants suggest that these proteins likely function extracellularly, probably as an assembled EXLX–GH5 complex Together, these results suggest that the Et–exlx–gh[5] locus is a non-canonical yet potent virulence factor, and horizontal acquisition of this locus was a key event driving the recent emergence of E. tracheiphila[32,35] as a virulent plant pathogen that causes high mortality by inducing wilt symptoms and plant death by blocking the flow of xylem sap (Fig. 1). These findings highlight the continued risk of horizontal gene transfer driving an increase in pathogen virulence, and the continuing vulnerability of agricultural populations to invasion by virulent new pathogens

Methods

Results

Conclusion