Abstract
Summary Pantoea agglomerans (Pa), a widespread commensal bacterium, has evolved into a host‐specific gall‐forming pathogen on gypsophila and beet by acquiring a plasmid harbouring a type III secretion system (T3SS) and effectors (T3Es). Pantoea agglomerans pv. gypsophilae (Pag) elicits galls on gypsophila and a hypersensitive response on beet, whereas P. agglomerans pv. betae (Pab) elicits galls on beet and gypsophila. HsvG and HsvB are two paralogous T3Es present in both pathovars and act as host‐specific transcription activators on gypsophila and beet, respectively. PthG and PseB are major T3Es that contribute to gall development of Pag and Pab, respectively. To establish the minimal combinations of T3Es that are sufficient to elicit gall symptoms, strains of the nonpathogenic bacteria Pseudomonas fluorescens 55, Pa 3‐1, Pa 98 and Escherichia coli, transformed with pHIR11 harbouring a T3SS, and the phytopathogenic bacteria Erwinia amylovora, Dickeya solani and Xanthomonas campestris pv. campestris were transformed with the T3Es hsvG, hsvB, pthG and pseB, either individually or in pairs, and used to infect gypsophila and beet. Strikingly, all the tested nonpathogenic and phytopathogenic bacterial strains harbouring hsvG and pthG incited galls on gypsophila, whereas strains harbouring hsvB and pseB, with the exception of E. coli, incited galls on beet.
Highlights
The T3Es HsvG, HsvB, PthG and PseB were selected for this study because they presumably were evolved by pathoadaptive evolution and apparently were instrumental in the emergence of Pag and Pab as new pathogens
The nonpathogenic bacterial strain Pseudomonas fluorescens 55 (Pf) harbouring the cosmid pHIR11 that encodes a functional T3SS from P. syringae (Huang et al, 1988) as well as the nonpathogenic strains of Pantoea agglomerans (Pa), Pa 3-1 and Pantoea agglomerans BRT98 (Pa 98), and two E. coli strains (Table S1) were employed for transformation into gall-forming pathogens
enterohemorrhagic E. coli (EHEC) TUV93-0 triggered hypersensitive response (HR) on beet, whereas no symptoms could be observed with E. coli DH5α (Table 1)
Summary
The T3Es of Pab or Pag could be divided into three groups: a HsvB and HsvG are paralogous T3Es that mimic host-specific transcriptional activators on beet and gypsophila, respectively, and determine pathovar specificity (Nissan et al, 2006, 2012; Valinsky et al, 1998). The nonpathogenic bacteria were provided with the capability to translocate T3Es into plant cells via transformation of pHIR11, a cosmid harbouring a plant-adapted T3SS (Huang et al, 1988), followed by transformation of T3Es from Pag or Pab. The T3Es HsvG, HsvB, PthG and PseB were selected for this study because they presumably were evolved by pathoadaptive evolution and apparently were instrumental in the emergence of Pag and Pab as new pathogens.
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