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Autophagic Maintenance and Specificity of Plant Immune Receptors
Plant cell surface pattern recognition receptors (PRRs) detect the presence of microbes by recognizing pathogen-associated molecular patterns (PAMPs). The most studied PRR in plants is Arabidopsis flagellin-sensing 2 (FLS2). FLS2 detects flagellin from many bacteria and initiates diverse immune responses against pathogen infection. To prevent an overwhelming immune response, a plant must eliminate activated FLS2. It does so by ubiquitinating FLS2 resulting in it being translocated to vacuole for degradation. However, it remains unknown how plants degrade and recycle non-activated FLS2. In this study, I showed that non-activated FLS2 is degraded by selective autophagy mediated by Arabidopsis orosomucoid (ORM) proteins that act as selective autophagy receptors. ORMs are known as negative regulators of sphingolipid biosynthesis that can bind and suppress the activity of serine palmitoyltransferase (SPT). I found that Arabidopsis plants over-expressing ORM do not accumulate FLS2 and are more susceptible to the bacterial pathogen Pseudomonas syringae. Conversely, ORM RNAi knockdown plants and CRISPR orm mutants enhanced FLS2 abundance and become more resistant to P. syringae infection. The ORM-mediated FLS2 breakdown is not due to their role in sphingolipid regulation. ORM proteins bind to FLS2 and autophagy-related protein ATG8. ORMs induce autophagosome formation and cause autophagic degradation of FLS2. These results suggest that selective autophagy plays an essential role in maintaining of non-activated FLS2. My second focus in this dissertation is on plant immunity mediated by EFR, a plant PRR that recognizes the bacterial elongation factor Tu (EF-Tu). EF-Tu from Agrobacterium tumefaciens (Atu) is well recognized by EFR whereas EF-Tu from Pseudomonas syringae pv. tomato (Pto) DC3000 is only weakly recognized. I generated Pto Δef-tu mutants that express Atu-like EF-Tu, and found these strains triggered an enhanced immunity and grew to lower numbers and produced less disease symptoms compared to the wild type strain. In contrast, Atu Δef-tu mutants that express Pto-like EF-Tu elicited weaker immune responses compared to wild type in an EFR-dependent manner. Atu mutants with Pto-like EF-Tu showed increased transformation efficiency when they transiently delivered T-DNA into plants. These results suggest that EFR-dependent immunity can be altered by genetically manipulating EF-Tu protein sequence.
Yang, Fan, "Autophagic Maintenance and Specificity of Plant Immune Receptors" (2019). ETD collection for University of Nebraska - Lincoln. AAI13862438.