Copper resistance and hormetic-like response in Xanthomonas euvesicatoria pv. perforans

Abstract Bacterial spot (BS) is a worldwide important tomato disease caused by Xanthomonas species. Xanthomonas euvesicatoria pv. perforans ( Xep ) is the most prevalent causal agent of the disease in Central Brazil. Copper resistant Xep isolates have been detected, which might have been driven by the intensive use of copper-based products to manage the disease. The copLAB gene cluster is one of the genetic determinants of copper resistance in Xanthomonas . In this study, a collection of 45 Brazilian Xep isolates was characterized for the presence of the copA gene and their in vitro sensitivity to copper. The copA gene was detected by PCR in 28.8% of the isolates which showed higher minimum inhibitory concentration (MIC) values than the copA - isolates. Two isolates (EH 2016-08 and EH 2017-69) copA + required preconditioning to express the resistant phenotype, and the same procedure resulted in an increase in copper MIC of EH 2020-12 ( copA -). Growth stimulation at copper concentrations below the MIC was observed for R (EH 2017-27) and S (EH 2020-12) isolates in a hormetic-like effect, a dose response phenomenon characterized by low-dose stimulation and high-dose inhibition. This effect seemed to be independent of copA presence. Increase in biofilm production was observed for R and S isolates by subinhibitory doses of copper, but there was no stimulatory effect on virulence of R or S isolates on tomato plants. To our knowledge this is the first report of a hormetic-like effect in copper-sensitive and resistant isolates of Xep .