Physiological and antioxidant insights into common bean resistance to common bacterial blight

Abstract

Common bacterial blight (CBB), caused by Xanthomonas axonopodis pv. phaseoli (Xap) (known also as X. campestris pv. phaseoli and its variant Xap var. fuscans (Xapf)), is the most important bacterial disease of common bean worldwide. Information regarding Xap-common bean interaction and the mechanisms of host resistance involved needs to be elucidated. In the present study, we investigated physiological and biochemical responses in a susceptible (Ouro Negro) and a resistant (Diamante Negro) cultivar of common bean to CBB. To this end, analyses of gas exchange and chlorophyll (Chl) a fluorescence, activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POX), and ascorbate peroxidase (APX)) as well as concentrations of photosynthetic pigments, hydrogen peroxide (H2O2), and malondialdehyde (MDA) were performed in cotyledonary leaves that were either infiltrated with a saline solution (control) or with a Xapf suspension. The CBB severity and lesion area were lower in plants from the resistant cultivar than in plants from the susceptible cultivar. The Xapf infection reduced gas exchange parameters and concentration of photosynthetic pigments but such decreases were significantly higher for the susceptible cultivar than for the resistant cultivar. Higher SOD, POX, and APX activities were recorded in the Xapf-infiltrated cotyledonary leaves than in control but greater activities occurred for the resistant cultivar than for the susceptible cultivar. The Xapf-infiltrated cotyledonary leaves, particularly those of the susceptible cultivar, displayed higher concentrations of H2O2 and MDA than plants from the control treatment. Images of Chl a fluorescence showed that changes stemmed from Xapf infection were evident as early as 6 h after inoculation and became more expressive as CBB progressed, mainly for the susceptible cultivar. Area under the curve of Chl a fluorescence parameters was lower for inoculated plants than for plants from the control treatment but it was higher for the susceptible cultivar than for the resistant cultivar. Results of the present study suggest that the antioxidant system played a major role in common bean resistance to CBB limiting Xapf-triggered generation of reactive oxygen species, cellular damage as well as photosynthetic and photochemical dysfunctions.