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UBC Theses and Dissertations

Diverse roles of the ubiquitin proteasome system in regulation of plant immunity Copeland, Charles Craig Johnson


Plants rely on a variety of immune signaling pathways to recognize pathogens and defend against pathogen attacks. In order to effectively recognize pathogens without causing autoimmunity, homeostasis of the proteins in these pathways must be maintained at optimal levels. The ubiquitin proteasome system (UPS) is an important mechanism by which the turnover of proteins, including immune system components, is achieved. This thesis describes novel roles for three UPS-related proteins in the regulation of plant immunity. The conserved E3 ligase C-TERMINUS OF HSP70 INTERACTING PROTEIN (AtCHIP) is a positive regulator of immunity in Arabidopsis, as overexpression of AtCHIP causes enhanced disease resistance. Loss of AtCHIP function causes enhanced susceptibility to virulent pathogens, but does not affect resistance mediated by Nucleotide-binding domain and leucine-rich repeat (NLR) receptors. The stability of the chaperone HEAT SHOCK PROTEIN 90.3 (HSP90.3) is also normal in atchip plants. Previous studies have shown that UPS also functions as a negative regulator of immunity, by preventing excessive accumulation of NLR proteins such as SUPPRESSOR OF NPR1, CONSTITUTIVE 1 (SNC1). CELL DIVISION CYCLE 48A (AtCDC48A) was identified as a negative regulator of immunity in a forward genetic screen for enhancers of autoimmunity caused by a gain-of-function allele of snc1. atcdc48A-4 plants exhibit dwarf morphology and enhanced disease resistance to the oomycete pathogen Hyaloperonospora arabidopsidis (H.a.) Noco2. SNC1 protein level is increased in atcdc48A plants, and AtCDC48A interacts with the E4 enzyme MUTANT, SNC1 ENHANCING 3 (MUSE3) in co-immunoprecipitation experiments, supporting a role for AtCDC48A in NLR turnover. Loss of PROTEASOME REGULATOR 1 (PTRE1), a positive regulator of protein turnover through the UPS, also causes enhanced disease resistance and increases SNC1 protein stability, further supporting the importance of proteasomal degradation in NLR homeostasis. Thus, the UPS plays both positive and negative roles in immune regulation.

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