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UBC Theses and Dissertations

Study of the avirulence effector of Ustilago hordei, UhAVR1, during susceptible and resistant host interactions Montenegro Alonso, Ana Priscilla


Plant pathogens secrete virulence factors known as effectors to aid in infection and host colonization. Plants possess a multi-tier defense system to detect and halt pathogens. The basidiomycete fungus Ustilago hordei causes covered smut disease of barley. The previously identified effector UhAVR1 from U. hordei causes avirulence in barley cultivars carrying the resistance gene, Ruh1, whereas Ruh1 absence leads to virulence. To date, UhAVR1 is the only proven avirulence effector of smuts. This study aimed to functionally characterize UhAVR1 and to understand the underlying molecular mechanism leading to susceptibility or resistance in barley. Using fungal strains expressing UhAVR1+SP:mCherry, the secretion of UhAVR1 via the Brefeldin A-sensitive ER-Golgi pathway and by the action of its signal peptide was shown. UhAvr1 transcripts were detected only early during infection of barley seedlings confirming in planta induction upon host sensing. This infection-specific induction contributes to susceptibility in cultivar Odessa (ruh1) or to complete immunity in cultivar Hannchen (Ruh1). Transient expression of UhAVR1 in barley, Nicotiana benthamiana, and Arabidopsis thaliana showed it localizes to the cytosol, the site where it likely performs its functions. The delivery of UhAVR1 via foxtail mosaic virus, Pseudomonas bacteria, and Agrobacterium-mediated suppression of cell death inducers in N. benthamiana and barley support a role in the suppression of a conserved component(s) of plant immunity. RNA-seq analysis at 48 hpi of cultivar Odessa infected with U. hordei revealed that UhAvr1 induces plant fatty acids and suppresses plant defense. Whereas in cultivar Hannchen, UhAvr1 induces ETI and down-regulates PTI. Co-immunoprecipitation assays coupled with mass spectrometry from cultivar Odessa and N. benthamiana transiently expressing UhAVR1-SP:GFP revealed candidate host interactors with a chloroplast role. Previous work genetically located Ruh1 on barley chromosome 7H. This information was used to localize Ruh1 to a 10 million base pair region where 21 predicted resistance genes reside. Although identification of Ruh1 among the candidates was unsuccessful, this works paves the way for further studies on host resistance in this pathosystem. Ultimately, generating knowledge on how pathogens cause disease and how plants defend themselves is pivotal in generating control strategies against plant pathogens.

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