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UBC Theses and Dissertations

Genetic and molecular analysis of resistance protein mediated plant immunity Fang, Xu


Upon recognition of pathogen effectors, plant resistance proteins trigger strong defence responses that restrict the growth and spread of pathogens. Most of these immune receptors are nucleotide-binding (NB) and leucine-rich repeat (LRR) domain-containing proteins (NLRs). SNC1 (Suppressor of npr1, constitutive 1) is an Arabidopsis Toll/interleukin-1 Receptor (TIR)-type NLR that was originally identified through a gain-of-function autoimmune mutant, snc1, from a forward genetic screen. My Ph.D. thesis is comprised of three projects, all taking advantage of the unique autoimmune phenotypes of snc1 to enable efficient genetic screening. Firstly, MOS12 was identified from the snc1 suppressor screen, which encodes a protein homologous to cyclin L of mouse and human. MOS12 is required for both SNC1- and RPS4- mediated defense response through its involvement in alternative splicing of SNC1 and RPS4. MOS12 associates with the MAC in planta and MAC components also contribute to proper splicing of SNC1 and RPS4. This study provides the emerging regulatory details of alternative splicing of certain NLR genes. Secondly, the transcription factor bHLH84 was isolated from a reverse genetic screen by its ability to confer enhanced immunity when overexpressed. bHLH84 and its homologs function redundantly in SNC1- and RPS4-mediated defense response. While bHLH84 does not seem to regulate the expression of NLR-encoding genes directly, it associates with SNC1 and RPS4 to fulfil their function in plant immunity. Being a transcription activator, bHLH84 associates with nuclear NLR proteins, probably in parallel with NLR-associated transcription repressors, enabling potentially fast and robust transcriptional reprogramming upon pathogen recognition. Lastly, the identification and functional study of MUSE6 revealed the crucial impact of N-terminal acetylation on the turnover of SNC1. Biochemical analysis uncovered that SNC1 undergoes alternative translation initiation, which provides respective substrates for NatA and NatB. SNC1 peptides translated from the first Met are targeted by NatA, and the acetylation serves as a degron, while SNC1 peptides initiated from the second Met are targeted by NatB, which stabilizes the protein. Different acetylation events on SNC1 are speculated to provide more flexibility to maintain its homeostasis. Overall, my Ph.D thesis research contributes to the better understanding of NLR protein regulation and activation.

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