Description of copper tolerant Xanthomonas citri subsp. citri and genotypic comparison with sensitive and resistant strains

The copper-based products widely used for control of citrus canker may lead to the development of Xanthomonas citri subsp. citri (X. citri) resistant to copper (Cu (R)). However, the study of copper sensitivity of X. citri strains from Parana state, Brazil, did not reveal the existence of Cu (R), but copper tolerant (Cu-T) strains. The aim of this study was to describe for the first time the existence of Cu-T X. citri and compare the genetic determinants that differentiate the Cu-T strains from the sensitive (Cu-S) and Cu (R) strains. Cu-T strains supported intermediate concentrations of copper in comparison to Cu-S and Cu (R). Cu-T strains lack the gene clusters copLAB or copABCD responsible for copper resistance in Cu (R) strains and the large plasmids (c. >= 200 kb) that normally carry these genes. The nucleotide sequences of chromosomal homologous genes cohLAB, involved in copper homeostasis, were 100% similar in strains of all phenotypes. Cu-T strains differed from Cu-S strains by the higher expression of the homologous chromosomal genes cohA and cohB in the presence of copper. Cu-T X. citri strains are not precursors of Cu (R) strains and do not pose a threat to the efficient use of copper-based bactericides for management of citrus canker in citrus orchards. Copper resistance and tolerance are distinct phenotypes and should not be used as synonyms. The proper characterization of the sensitivity to copper leads to a more confident monitoring of the distribution of copper resistant populations of X. citri and adoption of containment measures only when necessary.