Antioxidant metabolism of grapefruit infected with Xanthomonas axonopodis pv. citri

Abstract

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 (2dai; days after inoculation), raised and ruptured epidermis (6–8dai), formation of necrotic lesions (16dai), and leaf abscission (21dai). The Xac population increased to a maximum (≈109CFU/cm2) 8dai and then declined to ≈107CFU/cm2 by 20dai. Lipid peroxidation was higher in infected leaves than uninoculated controls from 4 to 21dai indicating greater oxidative stress. H2O2 concentration demonstrated a biphasic pattern with peak concentrations at 4 and 13dai and minimum concentrations that were lower than the controls at 10 and 20dai. 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 4dai, 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 4dai. 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.