Origin of hydrogen peroxide during the cowpea- Xanthomonas interaction

Abstract

To assay hydrogen peroxide (H 2 O 2 ) production by phytopathogenic bacteria in vivo, cowpea seedlings were inoculated using wild-type Xanthomonas campestris pv. phaseoli and its mutant strain on ahpC (with attenuated H 2 O 2 production). Antibiotic rifampicin (Rif) was used to kill off the Xanthomonas campestris pv. phaseoli in cowpea leaves. The optimal concentration of Rif to eradicate the bacteria was 500 μg mL -1 , and duration of the Rif treatment to eradicate the bacteria in the cowpea tissues was 48 h. This level of Rif did not significantly affect chloroplast and mitochondrial function in cowpea leaves. The concentration of H 2 O 2 in cowpea inoculated with wild-type or mutant strain of Xanthomonas campestris pv. phaseoli following Rif treatment was measured. Increased H 2 O 2 accumulation was observed in cowpea leaves respectively inoculated with wildtype and mutant strain. The accumulation of excess H 2 O 2 was significantly decreased by the Rif treatment on cowpea wild-type bacteria systems. However, the level of H 2 O 2 in cowpea leaves inoculated with mutant strains following Rif treatment was at the same level as in plant leaves without Rif treatment. The results demonstrate that the change in H 2 O 2 concentration observed in vivo was caused by wild-type bacteria. Keywords Cowpea . Hydrogen peroxide . Plant-pathogen interaction . Rifampicin . Xanthomonas campestris pv. phaseoli Abbreviations Δψ m mitochondrial membrane potential Gs stomatal conductance H 2 O 2 hydrogen peroxide Pn net photosynthetic rate Rif rifampicin ROS reactive oxygen species SHAM salicylhydroxamic acid Tr transpiration rate V alt capacity of alternative respiratory pathway V t capacity of total respiration pathway X. Xanthomonas XpahpC alkyl hydroperoxide reductase subunit C (ahpC) gene mutant of X. campestris pv. phaseoli Xpw wild type of X. campestris pv. phaseoli.