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The differential role of plasmids in Clavibacter virulence on tomato

Shree Thapa: University of California

<div><em>Clavibacter michiganensis</em> subsp. <em>michiganensis </em>(<em>Cmm</em>) is a Gram positive bacterium that proliferates in the xylem vessels of tomato, causing bacterial wilt and canker. The reference strain NCPPB382 has one chromosome (3.2 Mb) and two conjugative plasmids pCM1 (27.5 kb) and pCM2 (72 kb). Whole genome sequencing of <em>Cmm</em> strains collected in North America and Europe was performed with Illumina and PacBio platforms. Genomes were <em>de novo</em> assembled with the SPAdes and Hierarchical Genome Assembly Process 3. The sequenced strains have undergone significant alterations in plasmid number, size and gene composition. Most of the sequenced strains possess pCM1 and pCM2-like plasmids with sizes ranging from 27-100 kb. Curing of either plasmid resulted in a significant decrease in virulence. The β-1,4-endocellulase <em>CelA</em> in pCM1 and serine protease <em>Pat-1</em> in pCM2 are highly conserved in most <em>Cmm</em> strains and were previously demonstrated to be important for virulence. Interestingly, we have identified virulent <em>Cmm </em>strains lacking either pCM1 (NZ2541) or pCM2 (CASJ001). The absence of pCM1 or pCM2 in particular strains indicates that these plasmids, which are required for virulence in some <em>Cmm</em> strains, are not universally required for pathogenicity. Rearrangement, gene gain and/or loss in these conjugative plasmids have shaped <em>Cmm</em> diversity and pathogenicity.</div>