Postdoctoral Research Associate TEXAS A&M UNIVERSITY COLLEGE STATION, Texas
Body of Abstract: The plant immunity is triggered by detecting pathogen-derived molecules by two immune receptors: cell surface-resident pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat receptors (NLRs). PRR-activated immunity triggers the MAPK cascade, MEKK1-MKK1/2-MPK4, and disruption of this pathway induces the NLR SUMM2-mediated cell death as autoimmunity. Ceramides also play crucial roles in plant immunity by influencing cell death regulation. The levels and balance of ceramides, along with their phosphorylated derivatives, are essential for controlling cell death processes. Dysfunction of ceramide kinase ACD5 in plants leads to cell death caused by the accumulation of non-phosphorylated ceramides. To investigate the involvement of ceramides in MEKK1 pathway-regulated cell death, we employed a VIGS-based transient RNAi approach. Our results showed that RNAi-ACD5-triggered cell death was inhibited by suppressors of the mekk1 cell death, and both mekk1 and mpk4 cell death mutants exhibited elevated levels of ceramides. Moreover, the mycotoxin FB1, known to induce cell death through ceramide accumulation, displayed partially reduced cell death levels in suppressors of the mekk1 and increased NLR SUMM2 protein levels. Intriguingly, elevating ceramide levels through MEKK1/ACD5 silencing and FB1 treatment resulted in increased MPK4 protein abundance. To better understand the functional roles of MPK4 in ceramide synthesis, the TurboID-based proximity labeling analysis involving MPK4 was performed; the lipid kinase and lyase, which catalyze long-chain sphingoid bases for ceramide synthesis, were identified as potential substrates of MPK4. Furthermore, we also confirmed that MPK4 phosphorylates these enzymes, indicating their involvement in MEKK1 pathway-mediated cell death regulation. In summary, our findings provide valuable insights into the interplay between ceramides and MEKK1 pathway-mediated cell death regulation in plants.