Abstract
Genome-wide analyses of the effector- and toxin-encoding genes were used to examine the phylogenetics and evolution of pathogenicity amongst diverse strains of Pseudomonas syringae causing bacterial canker of cherry (Prunus avium), including pathovars P.syringae pv morsprunorum (Psm) races 1 and 2, P.syringae pv syringae (Pss) and P.syringae pv avii. Phylogenetic analyses revealed Psm races and P.syringae pv avii clades were distinct and were each monophyletic, whereas cherry-pathogenic strains of Pss were interspersed amongst strains from other host species. A maximum likelihood approach was used to predict effectors associated with pathogenicity on cherry. Pss possesses a smaller repertoire of type III effectors but has more toxin biosynthesis clusters than Psm and P.syringae pv avii. Evolution of cherry pathogenicity was correlated with gain of genes such as hopAR1 and hopBB1 through putative phage transfer and horizontal transfer respectively. By contrast, loss of the avrPto/hopAB redundant effector group was observed in cherry-pathogenic clades. Ectopic expression of hopAB and hopC1 triggered the hypersensitive reaction in cherry leaves, confirming computational predictions. Cherry canker provides a fascinating example of convergent evolution of pathogenicity that is explained by the mix of effector and toxin repertoires acting on a common host.
Highlights
Pseudomonas syringae is a species complex, associated with plants and the water cycle, comprising several divergent clades that frequently recombine (Young, 2010; Parkinson et al, 2011; Berge et al, 2014; Baltrus et al, 2017)
Eighteen P. syringae strains isolated from cherry and plum were phenotyped for pathogenicity and genome sequenced in a previous study (Hulin et al, 2018)
Phylogenetic analysis confirmed that cherry pathogenicity has evolved multiple times within P. syringae
Summary
Pseudomonas syringae is a species complex, associated with plants and the water cycle, comprising several divergent clades that frequently recombine (Young, 2010; Parkinson et al, 2011; Berge et al, 2014; Baltrus et al, 2017). It is a globally important pathogen, causing disease on over 180 different host species. Strains exist that can infect a variety of crop species, indicating that specialization is not always the norm (Monteil et al, 2013; Bartoli et al, 2015a,b) This complexity makes P. syringae an important model to study the evolution of host specificity (O’Brien et al, 2011; Mansfield et al, 2012)
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