PhD defence by Paraskevi Doukoudaki

Blumeria hordei (Bh) is the barley powdery mildew fungus, one of the most severe pathogens for the crop, that can directly penetrate barley cells and establish feeding haustoria to uptake nutrients. Specific types of plant immune responses are activated upon Bh attack, characterized by the formation of defence structures called papillae and encasements. Both rely on secretion of extracellular vesicles (EV) carrying various cargo but are regulated by two independent secretory pathways. EVs in papillae are recruited through an unconventional recycling mechanism, dependent on the ARF-GEF GNOM and independent from Rab5 and Rab7 GTPases, that regulate secretion to encasements. 

A protein that is localized in both EV-populations and is therefore used as a marker, is the plasma membrane syntaxin ROR2 in barley and its homolog PEN1 in Arabidopsis. The secretory mechanism of EVs to the papilla is poorly characterized and has been the main question of the current study. The data in this thesis show that ROR2-rich EVs secreted to the papilla are not dependent on the endosomal sorting complex required for transport (ESCRT) and require the sphingolipid ceramide. 

Moreover, secretion of ROR2 to the papilla was compromised in barley cells that overexpressed the Bh effector CSEP0254. Subsequent Split Ubiquitin yeast-two-hybrid studies revealed several potential interactors of CSEP0254 in barley. Two of these proteins were selected for further investigation – cytochrome B5 and a papain-like cysteine protease – for their role in lipid metabolism and plant immunity, respectively. Finally, Arabidopsis ceramide synthase mutants and autophagy deficient mutants were tested for their involvement in penetration immunity.​

Everybody is welcome!