Dissecting negative regulation of plant immunity through studying muse (mutant, snc1-enhancing) mutants

Plants respond in various ways to defend themselves against pathogen infections. Resistance (R) protein-mediated defense is one of the most effective mechanisms, through which plants can detect the activity of secreted pathogen-derived molecules (effectors) that promote infection. In Arabidopsis, snc1 encodes a TIR-NB-LRR R-like protein that carries a unique gain-of-function mutation, leading to constitutive activation of defense. Mutant snc1 has been used as a successful tool to dissect R protein-mediated immunity. Over a dozen MOS (Modifier of SNC1) proteins have been identified as positive regulators of plant immunity, indicating a complicated signaling network involved in R protein activation. To study negative regulation of snc1-mediated resistance, genetic screens were performed in mos snc1 backgrounds. Mutants restoring snc1-mediated autoimmunity phenotypes were isolated and named as muse (mutants, snc1-enhancing) mutants. The sensitized mos snc1 background enables us to find mutants that may have subtle defense phenotypes by themselves. This PhD thesis reports the identification and characterization of two muse mutants, muse3 and muse5, both enhancing snc1-associated autoimmune phenotypes in the mos4 snc1 background. MUSE3 is an Arabidopsis ortholog of yeast E4 ubiquitin conjugating factor required for polyubiquitin chain assembly. From the genetic and biochemical data, we found that MUSE3 functions downstream of the E3 complex SCFCPR1 to facilitate the degradation of at least two R proteins, SNC1 and RPS2. My study is the first report on E4 function in plants and adds another key step in R protein turnover pathway. MUSE5 encodes an ortholog of yeast PAM16, part of the mitochondrial inner membrane protein import motor and therefore, is renamed AtPAM16. In yeast pam16 mutants, preprotein import into the matrix is defective. Knocking out AtPAM16 leads to elevated ROS production and enhanced PR gene expression, suggesting that a negative regulator of plant immunity may not be properly imported into mitochondria in Atpam16. This unknown negative regulator is probably involved in preventing ROS accumulation and autoimmunity in mitochondria. This study highlights the significance of negative regulation of plant immunity in mitochondria. In summary, my PhD research contributes to better understanding of the negative regulatory mechanisms plants utilize to defend themselves against pathogen attack.Science, Faculty ofBotany, Department ofGraduat