Copper-induced stress mechanisms in <i>Erwinia amylovora</i> : a comparative phenotypic and transcriptomic study using copper-sensitive and -tolerant strains

<i>Erwinia amylovora</i> causes fire blight of apple and pear. Among the management strategies, copper-based treatments are widely used to control the disease. However, the impact of copper on the pathogen is not uniform, and different strains show different tolerance levels to the heavy metal. Upon identifying <i>E. amylovora</i> strains with unusually high (EaR2 and Ea17) and intermediate (Ea19) copper sensitivity, we conducted phenotypic and transcriptomic analyses to understand the basis of these differences. The highly copper-sensitive strains EaR2 and Ea17 grew slower, showed increased sensitivity to paraquat and cadmium, and developed a characteristic copper-dependent overproduction of amylovoran and levan, with patterns not observed in strain Ea273, with regular copper tolerance. Copper sensitivity was also associated with higher copper-shock death rates after copper pre-exposure during growth. Transcriptomic analysis via RNA-Seq revealed similar responses to copper shock in EaR2 and Ea273 but very different transcriptomic responses during copper adaptation (prolonged growth with copper). EaR2 responded to copper adaptation with earlier activation of stress responses, exopolysaccharide biosynthesis pathways, and protein quality control systems, while reducing the expression of genes linked to iron uptake. Ea273 mostly displayed an activation of copper homeostasis-related genes, with a characteristic downregulation of histidine catabolism.IMPORTANCEThis study identified and characterized, for the first time, copper hypersensitive <i>E. amylovora</i> strains, providing critical insights into the mechanisms of copper homeostasis and detoxification in the fire blight pathogen. Our study contributes to a broader understanding of bacterial adaptation to copper as well as the connection between phenotypic traits and transcriptomic responses to copper. Furthermore, our findings set the basis for future optimization of copper-based treatments and future development of more effective disease control methods.