Impairment of photosynthetic capacity and hydrogen peroxide removal in castor bean affected by bacterial leaf spot. (August 2020) - Möller, Priscilla Aguiar
Abstract: Bacterial leaf spot caused by Xanthomonas axonopodis pv. ricini ( Xar ) is the only important disease of bacterial etiology reported to threaten castor bean production. No cultivars expressing resistance to the pathogen are known. Hence, knowledge of the physiological and biochemical alterations caused by the bacterium to castor bean plants could shed light into alternative approaches to obtain plant resistance. Leaf gas exchange parameters were measured with a portable infrared gas analyzer; Chlorophyll a fluorescence parameters were determined with an Imaging-PAM chlorophyll fluorometer; photosynthetic pigments, malondialdehyde (MDA) and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX) and glutathione reductase (GR) were quantified using standard biochemical assays; and production of hydrogen peroxide (H2 O2 ) and superoxide anion (O2 - ) was detected by histochemistry. Inoculated leaves exhibited significant reduction in photosynthetic capacity, accumulation of H2 O2, absence of O2 - accumulation, higher concentrations of MDA and higher activities of SOD, APX and POX compared to non-inoculated plants. Bacterial infection of castor bean triggered severe alterations, mainly related to stomatal closure that consequently compromised the CO2 influx into infected tissue. Small increases in SOD activity seemed to be sufficient to remove O2 -, but detoxification of the H2 O2 generated during the O2 - dismutation process appeared to be inefficient. Higher concentrations of MDA in inoculated compared to non-inoculated plants indicated membrane damage due to lipid peroxidation. Some of the physiological and biochemical alterations in castor bean plants resulting from Xar infection have not previously been reported for other plants infected by bacteria. Highlights: The physiology and biochemistry of castor bean affected by Bacterial Leaf Blight was studied. Processes, enzymes and metabolites related to photosynthesis and oxidative stress were measured. Inoculated leaves showed reduction in photosynthesis, mainly related to stomatal closure. Infected tissue seemed to be inefficient to remove hydrogen peroxide. Infected tissue exhibited membrane damage due to lipid peroxidation.