Genetic diversity of non-pathogenic Clavibacter strains isolated from tomato seeds

Clavibacter michiganensis subsp. michiganensis (Cmm) is a seed-transmitted, quarantine pathogen which causes bacterial wilt and canker of tomato. Despite efforts to prevent seed contamination, new introductions are regularly detected, associated with new regions of tomato seed production. It seems as if the expanding diversity of Cmm also challenges the limited host range. Clavibacter-like isolates from tomato seed are phenotypically similar to Cmm in the common diagnostic semi-selective media and are identified as Cmm in the customary tests but are not pathogenic to tomato. In our first study four representatives formed a separate cluster in gyrB sequence analysis and in MALDI-TOF MS. Their presence on seed prevents clear judgment on the health status of tomato seeds. As their nature and function are unclear we aimed to investigate and compare them to Cmm. Twenty strains described as Clavibacter-like isolated from tomato seed and not pathogenic to tomato plantlets were selected. Leaf spots, wilting or cankers were not induced after local or systemic inoculation. Tomato stems were not colonized nor was there evidence of survival in tomato stems. Total DNA-DNA hybridization and sequence analysis of gyrB and dnaA proved that they belong to the Cm species but can be unambiguously separated from Cmm. Some of the genes encoding virulence determinants in Cmm strains were also detected in some of the non-pathogenic isolates. Moreover, Cmm strains formed a coherent group, while non-pathogenic Cm strains were heterogenic. The latter was confirmed by BOX-PCR. We speculate that tomato seeds likely represent a larger reservoir of unexplored Clavibacter diversity.