P-028 - Plasmid-mediated copper resistance in Xanthomonas euvesicatoria pv. perforans: determination of plasmid copy number and stability

The management of bacterial spot disease on tomato is challenged by the widespread occurrence of copper resistance in its causative pathogens. In Xanthomonas euvesicatoria pv. perforans, one of the dominant causal agents, copper resistance is mediated by both chromosomal and plasmid-borne genes. In the USA, chromosomal resistance is predominant, while plasmid-mediated resistance is limited to a few phylogenetic groups but more common globally. Strains carrying plasmid-borne resistance or both plasmid and chromosomal resistance exhibit higher copper tolerance than those with chromosomal resistance alone. This study investigated if increased tolerance is due to plasmid copy number. In addition, we examined plasmid stability in vitro for two phylogenetic groups to better understand the transition to chromosomal copper resistance. Quantitative real-time PCR to determine the copper resistance plasmid copy number indicated at least 2-3 copies per chromosome. Ongoing experiments are comparing the expression levels of copper resistance genes in strains with chromosomal resistance, plasmid-borne resistance, or both. During in vitro growth in the absence of copper, loss of part or the entire plasmid was observed in strains of one phylogenetic group but not the other. Genome sequencing of colonies with reduced plasmid size revealed transposase-mediated rearrangements among plasmids.  These findings will provide valuable insights into the role of mobile genetic elements in bacterial adaptation to copper treatments and their implications for management practices.