Translating lessons from effector biology to fungal effector screens, plant resistance mechanisms, and bacterial-fungal interactions

217 pagesIn a close interaction between host and symbiont, organisms exchange molecules to manipulate each other. Bacterial pathogens of plants and animals secrete effector proteins to promote infection and disable host defense responses. Plants, in turn, evolve mechanisms to detect effector presence or activity and prevent infection. Possibly one of the most integral model systems in plant pathology is that of Arabidopsis thaliana and the bacterium Pseudomonas syringae, which has been foundational to the study of effector biology and plant immunity. In this thesis, I have applied lessons and tools from P. syringae and other bacterial plant pathogens to more complex systems to fast-track discovery. First, I focused on how knowledge of bacterial pathogens can be helpful for screening the hundreds of putative fungal effector candidates per species. While other researchers have used bacteria to deliver fungal effectors for functional characterization in planta, I found that using phytopathogenic bacteria can introduce confounding results, necessitating careful controls for experiments. To reduce false positives, I employed an effector polymutant of P. syringae for effector delivery. Next, I used a known interaction between the P. syringae effector protease AvrPphB and two Arabidopsis proteins as a blueprint to investigate this same resistance mechanism in barley. I found that barley responds to the activity of AvrPphB with effector-triggered immunity dependent on an NLR, PBR1 (AvrPphB Response 1). My evidence supports the hypothesis that barley convergently evolved the same mechanism to detect AvrPphB as A. thaliana. My final chapter focuses on how transcription activator-like (TAL) effectors from phytopathogenic bacteria informed the characterization of an effector from a fungal endosymbiont. TAL effectors act as eukaryotic transcription factors that can specifically upregulate gene expression once secreted into host plants by Xanthomonas and Ralstonia spp. Burkholderia TAL-like (Btl) proteins from Mycetohabitans spp. affect the transcriptome and stress tolerance of its fungal host, R. microsporus, but without the same transcriptional control mechanism as TAL effectors. In each of these studies, knowledge about a model system was translated to new systems resulting in accelerated discovery