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UBC Theses and Dissertations

Dissection of molecular events involved in saul1-mediated autoimmunity Liang, Wanwan

Abstract

Plant intracellular nucleotide binding and leucine-rich repeat proteins (NB-LRR, NLRs) function as immune receptors to detect microbial pathogens directly or indirectly. Most NLRs guard host proteins that are the direct targets of pathogen effectors. A typical NLR sometimes cooperates with another atypical NLR for effector recognition. Upon effector recognition, plant NLRs oligomerize for defense activation, the mechanism of which is poorly understood. As a positive regulator of plant immunity, E3 ligase SAUL1 is guarded by NLR protein SOC3. The mutant saul1-1 displays seedling lethality from its autoimmunity, which makes it a suitable background for genetic screens. In chapter 2, by using CRISPR/Cas9 gene editing, genetic analysis and biochemical assays, I identified the differential pairings of typical NLR receptor SOC3 with atypical NLR proteins CHS1 or TN2 to guard the homeostasis of the E3 ligase SAUL1. The differential pairing results in autoimmunity with distinct morphological outcomes. SOC3 forms a head-to-head genomic arrangement with CHS1 and TN2, indicative of transcriptional co-regulation. Such cooperative interactions can likely enlarge the recognition spectrum and increase the functional flexibility of NLRs. In chapter 3, SUSA2 and SUSA3 were identified from the saul1-1 suppressor screen. SUSA2 encodes an F-box protein Actin-Related Protein 8 (ARP8) and SUSA3 encodes the chaperone protein HSP90.3. Further characterization showed that susa2-2 only suppresses the autoimmunity mediated by either CHS1-SOC3 or TN2-SOC3 paired NLR proteins, indicating that SUSA2 is specifically involved in NLR protein SOC3-mediated immunity. Moreover, both SUSA proteins are parts of an SCFSUSA2 E3 ligase, forming SCFSUSA2-NLR complex with CHS1-SOC3 or TN2-SOC3 paired NLR proteins. This is the first example of the assembly of plant NLRs into an SCF complex, which likely enables the ubiquitination and degradation/activation of an unknown downstream component to activate defense, a mechanism with remote similarity to mammalian NLR inflammasome activation. Overall, my Ph.D. thesis provides a better understanding of the molecular mechanisms by which NLR immune receptors are activated upon pathogen recognition. Such knowledge will be critical in the future design of pathogen-resistant crops.

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