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SPECIAL SESSION: 18th I. E. Melhus Graduate Student Symposium: Molecular Basis of Plants, Pathogens, And Plant-Microbe Interactions: Today’s Students Build The Foundation for Next Level Plant Disease Resistance

An effector of the citrus Huanglongbing-associated pathogen provides mechanistic insight into pathogenesis and enhances disease management
Kelley Clark - University of California, Riverside. Jessica Franco- University of California, Davis, Simon Schwizer- University of California, Riverside, Nian Wang- Citrus Research and Education Center, University of Florida, Wenbo Ma- University of California, Riverside, Gitta Coaker- University of Califo

Citrus industries worldwide are facing unprecedented devastation from Huanglongbing (HLB) disease which, in the US, is associated with the bacterium Candidatus Liberibacter asiaticus (CLas). A majority of plant pathogens rely on secreted “effector” proteins to directly target specific cellular processes in the host, aiding in pathogen colonization and disease development. Effectors have been used as molecular probes to uncover fundamental mechanism of the host-pathogen arms race, thus setting the foundation for the development of disease resistance. CLas possesses a Sec secretion system and encodes ~40 Sec-delivered effectors (SDEs). Expression profiling revealed an effector, named SDE1, has significantly higher expression levels in the citrus host compared to the insect vector, implicating a role in HLB disease progression. Expression of SDE1 in planta results in a yellowing phenotype reminiscent of a typical symptom of HLB diseased leaves. We found that SDE1 interacts with several papain-like cysteine proteases (PLCPs), which have been shown to contribute to defense against bacterial, fungal, and Phytophthora pathogens. Importantly, SDE1 inhibits the protease activity of PLCPs and promotes bacterial infection. Our results support a model where SDE1 contributes to CLas infection by suppressing citrus defense through its inhibitory effect on PLCP activities. Our current research is focused on the detailed characterization of SDE1 and citrus PLCPs, which will contribute to the generation of tolerant or resistant citrus varieties via genome editing.