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Enhanced virulence of Xanthomonas citri subsp. citri after coinfection with Apple stem grooving virus in citrus trees

Hisae Hirata: Graduate School of Integrated Science and Technology, Shizuoka University

<div>Plants may be exposed to or infected with multiple pathogens simultaneously, but there is little information on coinfections by different pathogens. The influence of coinfection with a plant virus and plant pathogenic bacterium was studied by challenge inoculation with <em>Xanthomonas citri</em> pv. <em>citri</em> (Xcc), causal agent of citrus canker, on citrus trees (<em>Citrus natsudaidai</em>) infected with <em>Apple stem grooving virus</em>. After pin-prick inoculation with Xcc, a water-soaked lesion, the first visible symptom of citrus canker, was observed earlier than on leaves without ASGV infection. Similarly, a population of Xcc and the size of citrus canker lesions on ASGV-infected leaves were larger than those on ASGV-uninfected leaves. Enhanced virulence of Xcc with ASGV coinfection was confirmed by coinoculation of healthy citrus with ASGV and Xcc. When a citrus leaf was preinoculated with ASGV using an <em>Agrobacterium</em> infection system then challenge-infiltrated or pinpricked with Xcc, Xcc symptoms were relatively more severe than those in the zone preinoculated with <em>Agrobacterium</em> harboring empty vector as a control. Expression of genes related to canker formation was highly upregulated and that of genes related to plant defense responses was repressed after coinoculation with ASGV and Xcc, suggesting that the defense response against Xcc was suppressed by ASGV infection; thus, controlling latent viral infections is important to control secondary challenge by Xcc.</div>