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UBC Theses and Dissertations

Regulation of plant immunity : lessons from SNIPER4 and CDK8 Huang, Jianhua


Plants employ sophisticated innate immune systems to ward off the invasion of pathogens. Upon perception of pathogens, plants transduce signals to downstream components and activate defense responses. To investigate the regulation of immune responses and the mechanisms of immune signaling, my Ph.D. projects focus on the characterization of immune regulators isolated from two genetic screens conducted in autoimmune mutants snc1 and camta1/2/3. SNIPER4, identified from the snc1-influencing plant E3 ligase reverse genetic screen (SNIPER), encodes an F-box protein being part of an SCF E3 complex. Two redundant tumor necrosis factor receptor associated factors (TRAF) proteins MUSE13 and MUSE14 serve as adaptors in the SCFCPR1 E3 complex to facilitate the degradation of NLRs including SNC1 and RPS2. Accumulation of MUSE13 and MUSE14 is decreased by overexpression of SNIPER4, but is increased when dominant-negative (DN)-SNIPER4 is overexpressed. In addition, SNIPER4 interacts with MUSE13 and MUSE14. Collectively, my data suggest that SNIPER4 fine-tunes the output of NLR proteins by modulating the turnover of MUSE13 and MUSE14 of the SCFCPR1 E3 complex. Three calmodulin binding transcription activators (CAMTAs), including CAMTA1, CAMTA2 and CAMTA3, play redundant roles in plant immunity. However, their major function in immune responses remains ambiguous. By conducting the Suppressor of camta1/2/3 (SUCA) screen, I found that loss-of function ICS1, a gene indispensable for SA biosynthesis, almost fully suppresses the drastic autoimmunity of camta1/2/3 triple mutant, suggesting that the major role of CAMTAs is to inhibit SA biosynthesis. In support of this, SA levels are decreased in the gain-of-function camta3-3D mutant. Transcriptional analysis revealed that expression of SA-related genes, including ICS1, EDS5 and PBS3, is compromised in camta3-3D plants. On the other hand, mutations in Cyclin-Dependent Kinase 8 (CDK8), isolated from the SUCA screen, compromise SA accumulation and systemic required resistance (SAR). cdk8 mutants exhibit reduced transcript levels of ICS and EDS5. Taken together, my results indicate that CAMTAs and CDK8 function oppositely in transcriptional regulation of SA biosynthesis. Overall, my thesis work adds to the current literature on the regulation of plant immunity. Such knowledge will assist the future development of sustainable methods for controlling diseases of our crop plants.

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