Revised Phylogeny and Novel Horizontally Acquired Virulence Determinants of the Model Soft Rot Phytopathogen Pectobacterium wasabiae SCC3193

Abstract

Soft rot disease is economically one of the most devastating bacterial diseases affecting plants worldwide. In this study, we present novel insights into the phylogeny and virulence of the soft rot model Pectobacterium sp. SCC3193, which was isolated from a diseased potato stem in Finland in the early 1980s. Genomic approaches, including proteome and genome comparisons of all sequenced soft rot bacteria, revealed that SCC3193, previously included in the species Pectobacterium carotovorum, can now be more accurately classified as Pectobacterium wasabiae. Together with the recently revised phylogeny of a few P. carotovorum strains and an increasing number of studies on P. wasabiae, our work indicates that P. wasabiae has been unnoticed but present in potato fields worldwide. A combination of genomic approaches and in planta experiments identified features that separate SCC3193 and other P. wasabiae strains from the rest of soft rot bacteria, such as the absence of a type III secretion system that contributes to virulence of other soft rot species. Experimentally established virulence determinants include the putative transcriptional regulator SirB, two partially redundant type VI secretion systems and two horizontally acquired clusters (Vic1 and Vic2), which contain predicted virulence genes. Genome comparison also revealed other interesting traits that may be related to life in planta or other specific environmental conditions. These traits include a predicted benzoic acid/salicylic acid carboxyl methyltransferase of eukaryotic origin. The novelties found in this work indicate that soft rot bacteria have a reservoir of unknown traits that may be utilized in the poorly understood latent stage in planta. The genomic approaches and the comparison of the model strain SCC3193 to other sequenced Pectobacterium strains, including the type strain of P. wasabiae, provides a solid basis for further investigation of the virulence, distribution and phylogeny of soft rot bacteria and, potentially, other bacteria as well.

Cardinal temperature differences, determined in vitro, between closely related species and subspecies of pectinolytic bacteria responsible for blackleg and soft rot on potatoes

“…Waleron et al (2013) noted that approximately 50% of their Pw potato isolates were able to grow at 37˚C while the other half failed to do so. Similar results were found by Nykyri et al (2012). This might therefore be an indication of possible variation within the species.…”

The Global Response Regulator ExpA Controls Virulence Gene Expression through RsmA-Mediated and RsmA-Independent Pathways in Pectobacterium wasabiae SCC3193

“…4; also, see Data Sets S1 to S6 in the supplemental material), in accordance with previous RsmA gene expression data for P. wasabiae (23). Inactivation of these secretion systems has been shown to reduce virulence of Pectobacterium species (3,23,28,55,56). This ExpA/RsmA regulation of T6SS-related genes is supported by data from previous studies of Pectobacterium and Pseudomonas species (11,23,46,49).…”

“…The virulence of P. wasabiae strains was tested on tobacco seedlings. Bacterial CFU were counted from inoculated plants at 0, 24, and 48 h postinfection according to a previously utilized protocol for tobacco seedling infections by Nykyri et al (3). One 24-well plate with a tobacco seedling in each well was used per strain during the infection experiments.…”

“…T he virulence of soft rot phytopathogens such as Pectobacterium wasabiae strain SCC3193 depends on the coordinated production and secretion of plant cell wall-degrading enzymes (PCWDEs), the main virulence factors of this pathogen (1)(2)(3)(4)(5)(6). These factors are regulated in part by quorum-sensing signals, two-component regulatory systems (TCS) responding to environmental signals such as those from plants, and posttranscriptional mechanisms (1,(6)(7)(8).…”

The Trk Potassium Transporter Is Required for RsmB-Mediated Activation of Virulence in the Phytopathogen Pectobacterium wasabiae

“…In Pectobacterium wasabiae (previously Erwinia carotovora [2]), production of PCWDEs is regulated mainly through two signal transduction systems. These systems coordinately control the expression and activity of the global posttranscriptional regulator RsmA, which represses the expression of PCWDEs by binding to their mRNAs.…”