Antioxidant metabolism of grapefruit infected with Xanthomonas axonopodis pv. citri

Grapefruit is one of the most susceptible citrus genotypes to Asiatic Citrus Canker, caused by Xanthomonas axonopodis pv. citri (Xac), that can cause severe losses in citrus yield and quality. Although much is known about citrus response to Xac, little is known of the role of antioxidant metabolism. Grapefruit leaves were artificially injected with a strain of Xac obtained from a commercial grove in Florida and components of oxidative metabolism were measured. Symptoms observed included water soaking (2 dai; days after inoculation), raised and ruptured epidermis (6-8 dai), formation of necrotic lesions (16 dai), and leaf abscission (21 dai). The Xac population increased to a maximum (≈10⁹ CFU/cm²) 8 dai and then declined to ≈10⁷ CFU/cm² by 20 dai. Lipid peroxidation was higher in infected leaves than uninoculated controls from 4 to 21 dai indicating greater oxidative stress. H₂O₂ concentration demonstrated a biphasic pattern with peak concentrations at 4 and 13 dai and minimum concentrations that were lower than the controls at 10 and 20 dai. The H₂O₂ concentration somewhat corresponded with superoxide dismutase (SOD) activity, which generates H₂O₂ via dismutase of superoxide ions. Total SOD activity in Xac-infected leaves increased to a maximum at 4 dai, the day of highest H₂O₂ concentration, and then declined and remained at or below controls. Mn-SOD and Fe-SOD activities both increased to maximum activities at 4 dai. Mn-SOD had four isoforms in Xac-infected leaves but only three in the controls. Fe-SOD had three isoforms in both infected and control plants. Suppression of H₂O₂ in Xac-infected leaves also corresponded to higher activities of the H₂O₂ catabolising enzymes catalase (CAT), ascorbate peroxidase (APOD), and peroxidase (POD). Two additional CAT isoforms were detected in infected leaves and not the controls. Three POD isoforms were detected in both control and infected leaves. Previous research has shown that Xac is sensitive to intraplant H₂O₂ concentration, however, the pattern of Xac in this study did not correspond to H₂O₂ concentration, which initially increased due to enhanced SOD activity, but was later suppressed apparently with the aid of peroxidases. In conclusion, Xac infection altered H₂O₂ metabolism in grapefruit leaves by changes in the activities and isoforms of SODs, CATs, PODs and APOD.