Characterization of the MOS4-associated complex in plant defense signaling

• Main Author: Monaghan, Jacqueline
• Language: English
• Published: University of British Columbia 2010
• Online Access: http://hdl.handle.net/2429/23638

The plant immune system is governed in part by Resistance (R) proteins that recognize pathogenic microorganisms and initiate enduring defense responses. While the terminal outputs of R protein activation are fairly well understood, information about signaling components involved in plant immunity is scarce. We previously showed that MODIFIER OF SNC1, 4 (MOS4) associates with the transcription factor AtCDC5 and the WD-40 protein PRL1 in vivo, forming the MOS4-Associated Complex (MAC). The MAC is required for plant defense responses, including those activated in the autoimmune mutant snc1, in which the R protein SNC1 is constitutively active. To identify additional MAC proteins, hemagglutinin-tagged MOS4 was purified by affinity chromatography and over 20 associated proteins were subsequently identified by mass spectrometry. In addition to MOS4, AtCDC5, and PRL1, we identified two homologous U-box proteins as well as several nucleic-acid binding proteins and snRNP subunits predicted to be integral components of the spliceosome. This thesis describes the characterization of selected MAC proteins in plant defense as well as EDS17, a gene unrelated to the MAC but that is likewise required for innate immunity in Arabidopsis. MAC3A and MAC3B encode highly similar E3 ubiquitin ligases with homology to the yeast and human protein Prp19. Through the analysis of loss-of-function mutants, we found that these loci are genetically redundant and are required for innate immunity in plants. MAC5A and MAC5B encode highly homologous putative RNA-binding proteins similar to the human protein RBM22. Analysis of these loci by reverse genetics revealed that they are partially redundant in a dosage-dependent manner and that both genes are essential for viability in Arabidopsis. Importantly, the loss of either MAC3A/3B or MAC5A suppresses snc1-associated autoimmune responses, indicating that these loci function in the snc1 pathway similar to MOS4. Even though the MAC is closely associated with the spliceosome, we could not detect obvious splicing defects in MAC mutants, indicating that this protein complex is probably not required for general splicing in plants. Together, our data suggest that the MAC likely functions as a transcriptional regulator to fine-tune plant immune responses.