Modulation of plant defense responses by Salicylate hydroxylase of Candidatus Liberibacter asiaticus

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Citrus huanglongbing (HLB) associated with the pathogen Candidatus Liberibacter asiaticus (Las) is a devastating disease for the US citrus industry (1, 2). To gain knowledge on the mechanism(s) by which Las evades host defense responses, we first expressed salicylate hydroxylase (sahA) of Las in Escherichia coli. Our data indicate that Las encodes a functional salicylate hydroxylase, which converts salicylic acid (SA) into catechol, a product that does not induce resistance. The sahA gene was highly induced in planta compared to psyllid vector suggesting its important role in disease progression. To determine expression level of defense related genes after Las infection, Xanthomonas axonopodis pv. citri strain AW (Xac AW) was used to induce PR gene expression. The PR-1 gene expression in Xac Aw-challenged plants, which were previously infected with Las, was lower than Xac Aw-challenged healthy plants. Using SA biosensor strain (Acinetobacter sp. ADPWH_lux), 4 fold reduction in SA accumulation was observed in the Las-infected as compared to healthy plants. To understanda possible synergistic effect of the presence of Las on the citrus canker [caused by X. citri subsp. citri (Xcc)] we inoculated Xcc in Las-infected and healthy leaves of grapefruit. The population levels of Xcc were significantly higher during all the observation time points (up to 14 days) in Las-infected as compared to healthy citrus indicating that modulation of SA production and subsequent regulation of defense related genes such as PR-1 gene could be one of the mechanisms deployed by Las to evade plant defense responses. The Las-infected plants compromised with defense responses could further succumb to the infection by other pathogens. We also conducted experiments to restore the SA level in Las infected plants using SA hydroxylase inhibitors and test their effect in controlling HLB.