Characterization of the pathogenicity of strains of Pseudomonas syringae towards cherry and plum

Abstract

Bacterial canker is a major disease of Prunus avium (cherry), Prunus domestica (plum) and other stone fruits. It is caused by pathovars within the Pseudomonas syringae species complex including P. syringae pv. morsprunorum (Psm) race 1 (R1), Psm race 2 (R2) and P. syringae pv. syringae (Pss). Psm R1 and Psm R2 were originally designated as the same pathovar; however, phylogenetic analysis revealed them to be distantly related, falling into phylogroups 3 and 1, respectively. This study characterized the pathogenicity of 18 newly genome‐sequenced P. syringae strains on cherry and plum, in the field and laboratory. The field experiment confirmed that the cherry cultivar Merton Glory exhibited a broad resistance to all clades. Psm R1 contained strains with differential specificity on cherry and plum. The ability of tractable laboratory‐based assays to reproduce assessments on whole trees was examined. Good correlations were achieved with assays using cut shoots or leaves, although only the cut shoot assay was able to reliably discriminate cultivar differences seen in the field. Measuring bacterial multiplication in detached leaves differentiated pathogens from nonpathogens and was therefore suitable for routine testing. In cherry leaves, symptom appearance discriminated Psm races from nonpathogens, which triggered a hypersensitive reaction. Pathogenic strains of Pss rapidly induced disease lesions in all tissues and exhibited a more necrotrophic lifestyle than hemibiotrophic Psm. This in‐depth study of pathogenic interactions, identification of host resistance and optimization of laboratory assays provides a framework for future genetic dissection of host–pathogen interactions in the canker disease.

Comparative genomics of Pseudomonas syringae reveals convergent gene gain and loss associated with specialization onto cherry (Prunus avium)

“…Methods used for bacterial culture and sources of plants were as described in Hulin et al (2018) and are detailed in Supporting Information Methods S1. Plant species utilized included P. avium L. and Nicotiana tabacum L. Pseudomonas strains are listed in Table 1.…”

“…Pathogenicity tests, performed on detached cherry leaves and analysed as in Hulin et al (2018) are described in Methods S1.…”

“…Bioinformatics commands for analyses performed in this paper are available on Github (https://github.com/harrisonlab/pseud omonas/). Genome sequencing using Illumina and genome assembly were performed as in Hulin et al (2018). For long-read sequencing, PacBio (Pacific Biosystems, Menlo Park, CA, USA) and MinION (Oxford Nanopore, Oxford, UK) were used.…”

“…Long-read sequenced genomes are highlighted with shading. Strains are ordered, first with those sequenced in this study, followed by other Pseudomonas strains from cherry and plum used in plasmid profiling analysis and previously pathogenicity tested in Hulin et al (2018). Next, further strains isolated from cherry and plum sequenced elsewhere are listed.…”

“…The cherrypathogenic clades of P. syringae are reported to exhibit differences in virulence, host range and lifestyle (Crosse & Garrett, 1966;Scortichini, 2010), making the P. syringae-cherry interaction a good pathosystem to study convergent gain of pathogenicity. The genomic analysis has been coupled with robust pathogenicity testing (Hulin et al, 2018) and functional analysis of potential avirulence (avr) genes. This study provides a proof of concept that genomics-based methods can be used to identify candidate genes involved in disease and will likely become the major tool in disease monitoring, diagnostics and host range prediction.…”