Diversity of plasmids and plasmid-encoded phenotypic traits in Xanthomonas campestris pv. Vesicatoria

Abstract

Plasmid profiles were determined in a world collection of 522 strains of Xanthomonas campestris pv. Vesicatoria (Doidge) Dye (Xcv) causal agent of bacterial spot of tomato ( Lvcopersicon esculentum Mill.) and pepper ( Capsicum annuum L.). Plasmids were grouped into 13 size classes. Thesizes ranged from 2 to 300 kbp, but most plasmids were larger than 30 kbp. A few strains had no plasmid. Seventyone different plasmid profiles were observed with a maximum of six plasmid size classes in each profile. Diversity was evident in strains from a culture collection as well as from freshly isolated strains. Some profiles were more dominant than others. Thirty-eight percent of the strains examined were resistant to streptomycin. Seventy percent of the strains were resistant to copper. A DNA fragment encoding copper resistance hybridized to large plasmid DNA of resistant strains only. Comparison of those plasmids by restriction analysis showed polymorphism and many of the plasmids were self-transmissible. Most strains from Argentina hydrolyzed starch and were pectolytic, whereas strains from USA and Taiwan were negative for those traits. The plasmid-borne avirulence genes that determine races 1 and 2 of the pepper group of strains and the tomato group of strains were not randomly distributed. Race 2 strains were detected in strains from the USA but only rarely in strains from other parts of the world. Race 1 strains were found in all areas. The tomato group of strains were also found in all areas, but were isolated only from tomato fields. Strains avirulent to tomato but virulent to pepper were distinguished with carborundum-ammended inoculum. The hypersensitive reaction in tomato caused by those strains was confirmed by electrolyte leakage analysis and multiplication of the bacterium in tomato leaves. The avirulence gene avrBsP was cloned from strain 87-7 and it converted strains virulent in tomato to avirulent. A 1.7 kbp subclone hybridized to plasmid bands in avirulent strains only. The avrBsP gene was linked in most strains to the avirulence gene, avrBs3, that determines race 1 on the pepper group of strains.