SUMO E3 ligase SIZ1 promotes nuclear condensate-mediated immune activation in Arabidopsis

SIZ1, a major plant SUMO E3 ligase, has diverse roles in development and immunity. Although loss-of-function phenotypes have categorized SIZ1 as a negative immune regulator, its broad substrate range complicates interpretation of its core function. Here we show that SIZ1 overaccumulation unexpectedly activates robust immune responses and cell death, dependent on its E3 ligase activity. Proximity-labeling proteomics revealed that SIZ1 function converges on the MOS4-Associated Complex (MAC), a critical immune signaling module that forms pro-immune nuclear condensates during pathogen attack. Both SIZ1 and the immune receptor SNC1 are recruited to MAC-dependent nuclear condensates (MDNCs) upon pathogen challenge, where they synergistically potentiate immune responses and cell death. SIZ1 SUMOylates and stabilizes MAC components, reinforcing MDNC formation and sustaining immune signaling. This mechanism counteracts the previously identified MDNC-inhibitory mechanism mediated by karyopherin KA120, establishing a regulatory system that balances immune-promoting condensate assembly with disassembly to prevent autoimmunity while maintaining rapid defense capacity.