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UBC Theses and Dissertations

Regulation of the pattern-recognition receptor signaling in Arabidopsis : lessons from sobir7-1 and MC2 Wu, Di


Plants employ a multi-layered innate immune system to ward off invading pathogens. The pattern-recognition receptors (PRRs) play pivotal roles in immune signaling by recognizing the pathogen-derived conserved molecular signatures termed PAMPs (pathogen-associated molecular patterns). Finetuning the activity of PRR signaling is vital for the well-being of plants, as the perturbation of components of the PRR complex typically causes disastrous outcomes on growth and defense. However, how plants achieve such tight regulation of the PRR signaling is not fully understood. BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1), which functions as a coreceptor for PAMP recognition, is an essential component of the PRR complex. BAK1 harbors a mysterious carboxyl-terminal tail (CT) beyond its kinase domain. In the present study, we clarified the biological significance of this CT region using a CT-deletion bak1 mutant allele. Our data showed that the CT is required for BAK1’s function in PAMP-triggered immunity (PTI), but is dispensable for cell death control and brassinosteroid signaling, indicating the differential requirement of the CT in development and immunity. In vitro phosphorylation assay revealed that the CT is required for the autophosphorylation activity of BAK1, suggesting that the CT serves as an intrinsic modulator to promote BAK1’s kinase activity. Plant metacaspases (MCs) are structural homologs of mammalian caspases. Arabidopsis METACASPASE 2 (MC2) carries an N-terminal prodomain, whose function is unclear. Here, we uncovered an imperative role of the MC2 prodomain in activating PTI signaling via genetic analysis of an mc2 mutant allele called mc2-1. Heightened expression of the MC2 prodomain in mc2-1 results in constitutive activation of defense responses dependent on BAK1 and SOBIR1 (SUPPRESSOR OF BIR1 1), suggesting that the MC2 prodomain specifically boosts the immunity mediated by the receptor-like protein (RLP)-type PRRs. These data reveal a novel functional and mechanistic link bridging an evolutionarily conserved metacaspase and the regulation of plant PRR signaling. Overall, my thesis work provides valuable insights into the understanding of the complicated regulatory network of PRR signaling in Arabidopsis. These findings can potentially be utilized to engineer durable and broad-spectrum resistance in crops.

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