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Ca. Liberibacter asiaticus peroxiredoxin and peroxidase are virulence factors critical for survival and colonization of citrus

Dean Gabriel: Institute of Food and Agricultural Sciences, University of Florida

<div>A seminal feature of plant defense reactions in response to an attempted pathogen invasion is engagement of an oxidative (H<sub>2</sub>O<sub>2</sub>) burst. In <em>Ca</em>. Liberibacter asiaticus (Las) strain UF506, the expression of an SC2 peroxidase increases bacterial fitness and delays symptom development in the host plant. Two peroxiredoxin genes, CLIBASIA_00980 (C980) and CLIBASIA_00485 (C485) are conserved among all sequenced Las strains (including those lacking prophages). Both peroxiredoxins were functionally characterized in <em>L. crescens</em>, a culturable proxy for Las. C980 appears to be a housekeeping gene with similar moderate transcript abundance in both Las-infected psyllids and citrus, whereas C485 expression completely repressed in psyllids, reminiscent of SC2 peroxidase expression. Both SC2 peroxidase and C485 were demonstrated to be non-classically secreted. C485 conferred significant resistance against both H<sub>2</sub>O<sub>2 </sub>and an organic oxidizing agent<em> t</em>BOOH. Transient overexpression of C485 in tobacco resulted in (a) transcriptional inactivation of <em>RbohB</em>, and (b) suppression of oxidative damage to host cell lipid membranes. In addition to mitigating the RBOH-mediated long-distance systemic defense signaling cascades, C485 may also play a critical role in alleviating the chain reaction like spread of lipid peroxidation, and subsequent accumulation of oxylipins that are both antimicrobial as well as positive transcriptional regulators of hypersensitive cell death response.</div>