Identification and analysis of E3 ligases and helper NLRs in plant immunity

Plant immunity is usually initiated with two types of immune receptors: 1) pattern recognition receptors (PRRs) recognize the conserved molecular features of pathogens (pathogen-associated molecular patterns, PAMPs) and trigger PTI (PAMP-triggered immunity) and; 2) nucleotide-binding/leucine-rich repeats (NLRs) serve as intracellular immune receptors with the ability to recognize the presence of relatively diverse pathogen effectors and trigger ETI (effector-triggered immunity). The Arabidopsis thaliana mutant snc1 contains a gain-of-function mutation in a Toll/interleukin-1 (TIR)-type NLR (TNL) gene and displays a dwarf morphology. Here, I report on the results of a snc1-influencing plant E3 ligase reverse genetic (SNIPER) screen that looked for snc1 plants with altered dwarfism in the presence of overexpressed E3 ligases. Six SNIPER genes were identified with four snc1-suppressors and two snc1-enhancers. SNIPER1/2/3 were selected for further characterization. The analysis of SNIPER1/2 is incomplete, thus is not included in this thesis. Chapter 3 describes SNIPER3, previously known as SAUL1 (Senescence-Associated E3 Ubiquitin Ligase 1) or PUB44 (Plant U-box 44), which encodes a U-box-type E3 ligase. Our data suggests that SAUL1 plays a dual role in plant immunity: on one hand, SAUL1 positively regulates basal resistance; on the other hand, SAUL1 suppresses a typical TNL immune receptor SUSA1 (Suppressor of saul1) to prevent its autoimmunity. ADR1, ADR1-L1 and ADR1-L2 are three homologous coiled-coil (CC)-type NLRs (CNLs), which were previously shown to work as helper NLRs. Chapter 4 further explores the specificity of the genetic requirement of ADR1s for typical TNLs, SNC1 and CHS2 (CHILLING SENSITIVE 2). Among the three ADR1 members, ADR1 is the leading contributor while ADR1-L1 is the least. Moreover, loss-of-function mutation of ADR1-L1 leads to over compensation of the transcript expression level of ADR1 and ADR1-L2 and results in the enhancement of snc1-mediated immunity. Overall, the studies I completed as part of my Ph.D. thesis expand our knowledge of the roles of E3 ligases and ADR1s in plant defense and help us to better understand the sophisticated regulatory mechanisms of plant innate immunity.Science, Faculty ofBotany, Department ofGraduat