Investigating the role of the homeostasis genes cohLAB in copper resistant strains of Xanthomonas citri subsp. citri

Abstract

Brazil is the largest producer of sweet oranges in the world and citrus is one of the most important economic activities in our country. This activity is threteaned by the Asian citrus canker (ACC) an infectious and incurable plant disease that affects all the commercially important varieties of citrus. The pathogen that causes this disease is the Gram-negative bacterium Xanthomonas citri subsp. citri (X. citri), which is now considered endemic in the state of Sao Paulo. The state was declared a risk mitigation area (SMR), where the control of ACC is done by planting cultivars less susceptible to ACC, installing windbreaks in orchards in order to avoid the dispersion of the bacteria by the combined action of wind and rain, and the use of regular sprays of copper based formulations. Such measures, although effective, are considered palliative. Moreover, copper resistance has already been documented in X. citri. All X. citri strains have copper homeostasis genes, cohLAB, located on their chromosomes, capable of supporting a concentration up to 125 mg/L of the metal. Resistance (growth in copper concentrations greater than 300 mg/L) is associated to the presence of plasmid genes known as copLAB or copABCD. However, tolerant strains X. citri strains have been recently documented growing in intermediate copper concentrations (CuT; growth in concentrations from 150 to 200 mg/L). To better understand copper resistance/tolerance and the contribution of cohLAB genes in this process, we intend to delete the homeostasis genes in X. citri. Such a study is extremely relevant in the process of implementing effective controls using copper to contain ACC.