Interference In Oxidative Metabolism In Citrus by Xanthomonas citri pv citri

Aspects of oxidative metabolism were compared between canker-resistant kumquat and canker-susceptible grapefruit using a compatible strain of Xanthomonas citri pv citri (Xcc).  Xcc directly injected into leaves demonstrated substantially lower bacterial populations and disease symptoms in kumquat compared to grapefruit.   H₂O₂ concentration increased in kumquat whereas in grapefruit it increased from 0-4 days after inoculation (dai) but declined below or were similar to the controls during the rest of the disease cycle.  Chloroplastic superoxide dismutate (SOD) increased temporarily in both species by 1 dai and the resultant H₂O₂ produced was considered a signaling molecule for downstream disease resistance.  Mitochondrial SOD activity was higher than the controls for the entire disease process after 1 dai in kumquat, but were higher only from 1-4 dai in grapefruit, patterns of activity that corresponded with the pattern of H₂O₂.  Catalase (CAT) and ascorbate peroxidase (APOD) activities were lower 6 and 1 dai, respectively, in kumquat, whereas guaiacol peroxidase (POD) activity was higher than the controls throughout the study.   CAT, APOD, and POD activities were higher from 2, 4, and 2 dai, respectively, to the end of the disease cycle in grapefruit.  We propose that kumquat is able to maintain high levels of H₂O₂ throughout pathogenesis, which moves to the apoplast and coupled with higher POD activity changes the apoplastic environment that makes it less favorable for Xcc growth.  In grapefruit, however, we propose that Xcc interferes with H₂O₂ production in the mitochondria and peroxisomes that serve as the sources for apoplastic H₂O₂.  The resultant lower apoplastic H₂O₂ levels coupled with higher POD activity favors cell growth necessary for rupture of the epidermis in the last stages of the disease cycle.