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UBC Theses and Dissertations

Molecular studies on plant NLR immune receptors Wu, Zhongshou


In both plants and animals, nucleotide-binding leucine-rich repeat (NLR) receptors play crucial roles in the recognition of pathogen-derived molecules and the activation of defense. Sensor NLRs (sNLRs) are polymorphic with the ability to recognize relatively diverse pathogenic effectors. Helper NLRs (hNLRs) with their characteristic RPW8 domain function downstream of diverse sNLRs and are evolutionarily conserved. Proper regulation of immune responses mediated by NLRs is important as over-activation results in growth defects, while under-activation leads to vulnerability to pathogens. Two hNLR families, ADR1s (ACTIVATED DISEASE RESISTANCE 1) and NRG1s (N REQUIREMENT GENE 1), are present in Arabidopsis. Previous studies have shown that the ADR1 family is required for immune response mediated by multiple sNLRs. However, the function of the NRG1 family is unclear. Here, we discovered that the NRG1 family specifically operates downstream of Toll/interleukin-1 (TIR) type sNLRs (TNLs). ADR1s and NRG1s function in two distinct parallel pathways contributing to TNL-specific immunity. Synergistic effects on basal and TNL-mediated defense were detected among ADR1s and NRG1s. In summary, different sensor TNLs differentially use two groups of hNLRs to transduce downstream defense signals. NLRs are controlled at multiple levels to ensure immune responses are effective and timely. Ubiquitin proteasome system (UPS)-mediated NLR turnover plays a pivotal role in the fine-tuned regulation of immune responses. snc1 harbors a gain-of-function mutation in the TNL SNC1 (Suppressor of npr1, Constitutive 1) and exhibits autoimmune phenotype. A snc1-influencing plant E3 ligase reverse (SNIPER) genetic screen identified two snc1-suppressors, SNIPER1 and SNIPER2, both of which completely suppress snc1-mediated autoimmunity upon overexpression. Interestingly, functionally redundant SNIPER1 and SNIPER2 can control the protein levels of diverse sNLRs and the interactions between SNIPER1 and sNLRs appear to be through the common nucleotide-binding (NB) domains of sNLRs. In support, SNIPER1 can ubiquitinate the NB domains of multiple sNLRs in vitro. Our study thus reveals a novel process where global turnover of sNLRs by two master E3 ligases serves to immediately attenuate immune output to effectively avoid autoimmunity. Overall, studies in this dissertation provide new mechanistic insights on NLR activation and the global homeostasis control of NLRs.

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