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An effector from the Huanglongbing-associated bacterium targets a specific family of proteases in citrus

Kelley Clark: University of California, Riverside

<div>The citrus industry continues to be threaten by Huanglongbing (HLB) disease. All citrus cultivars can be affected by the HLB-associated bacterium, <em>Candidatus </em>Liberibacter asiaticus (<em>C</em>Las), and effective management strategies are still needed. Insect-transmitted and phloem-colonizing pathogens, like <em>C</em>Las, utilize the general Sec secretion system to deliver virulence proteins into the host. These proteins can aid infection by manipulating plant physiology and subverting host immunity, thereby promoting bacterial colonization and disease progression. We investigated the virulence function of a <em>C</em>Las effector, Sec-delivered effector 1 (SDE1), which is conserved among all <em>C</em>Las isolates. SDE1 has ~10-folds higher expression in plant hosts compared to the insect vector, implicating a possible role in HLB progression. Using yeast two-hybrid screening, we found that SDE1 directly interacts with several members belonging to a specific family of proteases, known as papain-like cysteine proteases (PLCPs). PLCPs in other pathosystems have been shown to contribute to plant defense against pathogens including bacteria, fungi and <em>Phytophthora</em>. Interestingly, all these different pathogens produce effectors to inhibit PLCP activity and promote infection. We found that SDE1 also inhibits the protease activity of PLCPs <em>in vitro</em> and in citrus. Using a surrogate system, we further showed that SDE1 can promote bacterial infection. These findings support a model that SDE1 acts as a PLCP inhibitor in order to suppress immune response in citrus and promote <em>C</em>Las infection. We will discuss ongoing progress on the characterization of SDE1 and the potential of exploiting SDE1-PLCP interaction for the development of HLB management.</div>

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