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A sneak peek into the citrus defense response affected by Candidatus Liberibacter effectors

Shujian Zhang: New Mexico Consortium

<div>Huanglongbing (HLB) disease (a.k.a. Citrus greening), caused by <em>Candidatus </em>Liberibacter spp., is the most devastating disease of citrus. As in other bacterial pathogens, Liberibacter secretes effectors that play important roles in disease onset and progression. At present, however, the molecular details of interaction between Liberibacter effectors and host citrus targets remain unresolved. Here, we describe the identification of two putative effector proteins (designated as LasP<sub>235, </sub>LasE<sub>3</sub>) with a eukaryotic nuclear localization signal (NLS) in the prophage region of the Liberibacter asiaticus (Las) psy62 genome. The purified recombinant protein was used in pull-down assays with total protein from both healthy and <em>Las</em>-infected citrus leaves. The interacting complexes were subsequently purified using TALON affinity chromatography, and submitted to LC MS/MS, in order to identify citrus proteins that are unique to the healthy, infected, as well as several proteins that were expressed in both the healthy and infected samples. Among the identified interactors, we selected Aspartyl protease (AP), glycosyl hydrolases, LTP (lipid transfer proteins) and SOD (superoxide dismutase) for further validation. Using a similar approach, we also identified Kunitz-type Trypsin Inhibitor Protein (KTI), Asocorbate Peroxidase, Alcohol Dehydrogenase and Photosystem II proteins. Further biochemical assays, including characterization of AP, KTI, SOD and hydrolases, antimicrobial activity of LTPs, etc., in presence and absence of effectors, will be presented in detail. Our understanding of the mechanism of between Liberibacter effectors and citrus proteins should lead to the development of much-needed HLB therapies.</div>