Fluctuation in some enzymes related to antioxidant defense system in common bean against Xanthomonas axonopodis pv. Phaseoli

Antioxidant enzymes play an important role in plant defense against pathogenic agents. Following the identification of the pathogen, plants produce active oxygen species (ROS) as one of their first defense responses. To maintain the balance of ROS levels and prevent their harmful effects, plants produce antioxidant peroxidase (POX), catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) enzymes. In the present study, the resistance of bean plants cultivars, namely Sadri, Paak, Darakhshan and Dorsa, to Xanthomonas axonopodis pv. phaseoli (Xap) were studied in greenhouse conditions. The catalase, peroxidase and ascorbate peroxidase enzyme activities were studied in healthy and Xap-infected bean cultivars Sadri and Derakhshan at 0, 24, 48, 72 hours and 20 days post inoculation by a completely randomized design with 5 treatments and 4 replications. The result showed that disease symptoms appeared in all tested cultivars. Derakhshan and Sadri cultivars, with 58.33 and 80.56 percentages of infected plants 20 days after inoculation, showed the least and highest infection rates, respectively. The highest catalase and peroxidase activities were recorded 24 and 48h post inoculation. These records reduced 48 and 72 hours post inoculation, respectively. The activities of these two enzymes in the susceptible cultivar were less than those in the semi-resistant one. The chlorophyll a and chlorophyll b contents of Xap-infected plants reduced significantly. The total chlorophyll content of uninfected Sadri and Darakhshan cultivars were 2.93 and 3.23 µg/g, respectively, which reduced to 1.96 and 2.14 µg/g of leaf tissue in infected plants, respectively. Based on these results, it is suggested that the Derakhshan cultivar should be planted in disease-susceptible regions as the semi-resistant cultivar.