Associate Professor Seoul National University, Korea Seoul, SOUTH KOREA
The Gram-positive bacterium Clavibacter michiganensis causes canker on stems, blister-like symptoms on leaves and wilting. It carries two plasmid-borne major virulence genes, celA (cellulase) and pat-1 (putative serine protease), which are required for wilting in tomato. The 88 C. michiganensis field isolates collected between 2011 and 2020 were screened for phenotypic variation, including virulence in host plants. Four isolates lacking plasmids with celA, pat-1, or both failed to cause wilting, and nine isolates, including these four, failed to cause wilting in Nicotiana benthamiana. Whole genome analyses revealed 11 distinct plasmid types, including 9 newly identified, and ten bacterial groups with different plasmid compositions, despite having almost identical chromosomes. Comparative genomic analyses revealed significant genetic diversity among the plasmids, while three plasmids containing the genomic island (GI) α with celA or GIβ with pat-1 and three newly identified plasmids carrying both islands shared large blocks of synteny. In addition, GIα is closely associated with mobile genetic elements, suggesting the genetic rearrangement or transfer at this locus. A paralog of the chromosomally located chpG virulence gene was found in the newly identified plasmid. These results suggest that C. michiganensis harbors a wide variety of virulence and non-virulence plasmids, and that genetic rearrangements occur among plasmids in GI regions that determine bacterial virulence in plants.
