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UBC Theses and Dissertations

Dissecting plant innate immunity using SNC1 : a sensitive immune receptor Cheng, Yu Ti


Plants rely on innate immunity to fight pathogens. Among plant defence mechanisms, Resistance (R) proteins play essential roles in recognizing pathogens and mounting robust defence responses. snc1, a mis-regulated plant Resistance (R) gene mutant, exhibits distinctive morphological and autoimmune phenotypes. To identify the signalling components downstream of R proteinmediated defence, forward genetic screens were conducted in the snc1 background. From the screens, fifteen novel mos (modifier of snc1) mutants were identified, of which, I studied mos7 and mos9 for my PhD thesis. Both mos7 and mos9 partially suppress all snc1- related autoimmune phenotypes. MOS7 is an Arabidopsis homolog of human and Drosophila Nucleoporin 88 (Nup88) required for protein nuclear retention. Partial loss-of-function alleles of up88 in Drosophila and mammalian cells fail to accumulate NF-κB in the nucleus and result in immune deficiency. We found that several defence related proteins have altered distribution/abundance in mos7, including snc1. This study highlights the importance of nucleocytoplasmic trafficking in plant immunity. mos9 contains a mutation in a gene encoding a protein of unknown function. Immunoprecipitation followed by mass spectrometry identified a SET domain-containing protein, ATXR7, as an interactor of MOS9. Previous studies showed that ATXR7 methylates lysine 4 of histone H3 (H3K4) and this methylation is required for proper transcriptional activation of Flowering Locus C (FLC). We found reduced expression of two R genes, S C1 and RPP4, in mos9 and atxr7. Also, H3K4 marks close to S C1 are reduced in mos9. My research on MOS9 illustrated the importance of histone modification in regulating plant immune receptor transcription. Serendipitously, I worked on an F-box protein, CPR1. From the genetic and biochemical data, we found that at least two R proteins, SNC1 and RPS2, are being regulated by CPR1. Our study is the first report with evidence suggesting how plant R proteins are negatively regulated by the ubiquitin-26S proteasome. Overall, my PhD thesis research provides evidence that plant immunity is under tight control at multiple levels. My work furthers our knowledge on how plant immune responses are regulated.

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