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UBC Theses and Dissertations

Functional analyses of three SUPPRESSOR OF mkk1 mkk2 proteins in plant immunity Liu, Yanan


Plant immunity relies on two groups of pathogen receptors, plasma membrane-localized pattern-recognition receptors (PRRs) and intracellular nucleotide binding-leucine rich repeat receptors (NLRs). Mitogen-activated protein kinase (MAPK) cascades, which generally consist of MAPK kinase kinase (MAPKKK), MAPK kinase (MAPKK) and MAPK, play critical roles in plant immunity. Disruption of the MEKK1-MKK1/MKK2-MPK4 cascade in Arabidopsis results in an autoimmune phenotype in the mekk1, mkk1 mkk2 and mpk4 mutants. A mkk1 mkk2 suppressor screen led to the identification of the NLR SUMM2 (SUPPRESSOR OF MKK1 MKK2 2), suggesting that the MEKK1-MKK1/MKK2-MPK4 cascade is guarded by SUMM2 and that disruption of this cascade can lead to the activation of SUMM2-mediated immunity. This thesis reports the identification and characterization of three additional SUMM proteins, SUMM3, SUMM5 and SUMM6. SUMM3 encodes CALMODULIN-BINDING RECEPTOR-LIKE CYTOPLASMIC KINASE 3 (CRCK3). crck3 mutants suppress the constitutively activated immune responses in mekk1, mkk1 mkk2 and mpk4 mutants. CRCK3 was found to be a substrate of MPK4 and it interacts with SUMM2 in planta, suggesting that SUMM2 utilizes CRCK3 as a “guardee” or “decoy” to monitor the integrity of the MEKK1-MKK1/MKK2-MPK4 cascade. SUMM5 encodes protein phosphatase 5 (PP5), a highly conserved atypical phosphatase with an N-terminal tetratricopeptide domain. SUMM2 was shown to interact with Heat Shock Protein 90 (Hsp90) and PP5 in planta. Accumulation of SUMM2 is reduced in the pp5 knockout mutant, suggesting that PP5 acts as a co-chaperone to regulate the accumulation of SUMM2. SUMM6 encodes the malectin-like domain containing kinase MEDOS1 (MDS1). Mutations in MDS1 and its close homolog MDS2 have additive effects on suppression of the mkk1 mkk2 mutant phenotypes, suggesting that MDS1 and MDS2 have overlapping roles in promoting SUMM2-mediated immunity. MDS1 and CRCK3 were found to associate in planta, indicating that MDS1/MDS2 might promote SUMM2-mediated immunity by targeting CRCK3. Overall, my thesis work provides new insights on how plants sense pathogens using receptor proteins and how they maintain appropriate levels of these receptor proteins to ensure adequate defense response.

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