UBC Theses and Dissertations
Dissecting signaling pathways mediated by plant TIR-type immune receptors Tian, Lei
Plants utilize sophisticated innate immune systems to fight against pathogens and pests. They have multiple types of immune receptors to detect the presence of pathogens and initiate defense responses. Among them, Toll/interleukin-1 receptor (TIR)-type nucleotide-binding leucine-rich repeat (NLR) immune receptors (TIR-NLRs, TNLs) recognize pathogen effectors intracellularly and activate downstream signaling components, including helper NLRs (hNLRs) consisting of ADR1 (ACTIVATED DISEASE RESISTANCE 1) and NRG1 (N REQUIREMENT GENE 1) families, and two distinct lipase-like protein dimers EDS1 (ENHANCED DISEASE SUSCEPTIBILITY 1)-PAD4 (PHYTOALEXIN DEFICIENT 4) and EDS1-SAG101 (SENESCENCE-ASSOCIATED GENE 101) through enzymatic activities from their TIR domains. However, immune signal transduction from TNL receptors to these downstream modules remains largely unclear. Studies in this dissertation first confirmed EDS1-PAD4-ADR1s and EDS1-SAG101-NRG1s as two distinct modules downstream of TNLs by genetic analysis. Further biochemical experiments indicated that TIR/TNL activation promotes the association between EDS1-PAD4 and one of the ADR1 family members ADR1-L1 (ADR1-Like 1). Enhanced self-association of ADR1-L1 was also detected upon TIR/TNL activation, suggesting a potential EDS1-PAD4-ADR1s protein complex formation for TNL immune signal transduction. Furthermore, an N-terminally truncated hNLR NRG1C was identified with a negative role in plant immunity, which antagonizes immune responses mediated by the TNLs that are dependent on full-length NRG1A/1B. However, NRG1C could not affect the transcript or protein levels of NRG1A/1B, and it did not associate with them either. The interaction of NRG1C with EDS1 and SAG101 suggested that it may interfere with the EDS1-SAG101 complex that signals together with full-length NRG1s to modulate TNL signaling. Furthermore, NRG1Cs are evolutionarily conserved and are functionally exchangeable in Brassicaceae. Finally, the reported NADase and 2’,3’-cAMP/cGMP synthetase activities of plant TIR domains were examined for their importance in TNL-mediated immunity. Through protein structure-based mutagenesis analysis of a typical Arabidopsis TNL SNC1 (SUPPRESSOR OF NPR1-1, CONSTITUTIVE 1), I revealed that the TNL SNC1 requires NADase activity to trigger plant immune responses, while its 2’,3’-cAMP/cGMP synthetase activity is not needed. Altogether, studies in this dissertation contribute to the understanding of TNL signaling pathways in plant immunity. These findings will be helpful for future design of broad-spectrum resistant crops.
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