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Expression of Peroxidase-like Genes, H₂O₂ Production, and Peroxidase Activity During the Hypersensitive Response to Xanthomonas campestris pv. vesicatoria in Capsicum annuum

Pepper ascorbate peroxidase-like (CAPOA1), thioredoxin peroxidase-like (CAPOT1), and peroxidase-like (CAPO1) clones were isolated from pepper leaves inoculated with avirulent strain Bv5-4a of Xanthomonas campestris pv. vesicatoria. CAPOA1, CAPOT1, and CAPO1 mRNA disappeared 18 to 30 h after the bacterial infection when the hypersensitive response (HR) was visible. In contrast, peroxidase activity reached a peak at 18 h after infection and then declined at 24 and 30 h when H₂O₂ accumulation level was maximal. These results suggest that the striking accumulation of H₂O₂ and strong decrease in peroxidase activity during the programmed cell death may be due to the strong suppression of CAPOA1, CAPOT1, and CAPO1 gene expression. Infection by Phytophthora capsici or Colletotricum gloeosporioides also induced the expression of the three putative peroxidase genes in pepper tissues. CAPOA1 mRNAs were in situ localized in phloem areas of vascular bundles in pepper tissues infected by Colletotricum. coccodes, P. capsici, or C. gloeosporioides. Exogenous treatment with H₂O₂ strongly induced the CAPOA1 and CAPOT1 transcription 1 h after treatment, while the CAPO1 transcripts accumulated 12 h after H₂O₂ treatment. We suggest that pepper ascorbate peroxidase and thioredoxin peroxidase genes may function as regulators of H₂O₂ level and total peroxidase activity in the oxidative burst during the HR to incompatible pathogen interaction in pepper plant.