SUMO E3 ligase SIZ1 sustains nuclear condensate-mediated immune activation in plants

SIZ1, a major plant SUMO E3 ligase, has diverse roles in development and immunity. Although loss-of-function studies have positioned SIZ1 as a negative immune regulator, the pleiotropic effects of its broad substrate repertoire may obscure its core function. Here we report that SIZ1 overaccumulation unexpectedly triggers robust pro-immune responses and cell death, dependent on its E3 ligase activity. We demonstrate that SIZ1 functionally converges on the MOS4-Associated Complex (MAC), a key immune signaling module, with both SIZ1 and the immune receptor SNC1 recruited to MAC-dependent nuclear condensates (MDNCs) upon pathogen challenge. These condensates serve as essential platforms for potentiating immune activation and cell death. Within the MDNC, SIZ1 promotes SUMOylation and stabilizes MAC components, thereby sustaining MDNC integrity during immune responses. Our findings reveal a previously unrecognized pro-immune role for SIZ1 and establish a mechanistic link between SUMOylation, MAC stability, and condensate-mediated immune signaling in plants. RNA-Seq profiling of 4-week-old soil grown Col-0, siz1-2 loss-of-function mutant and SIZ1 overexpression mutant. For RNA-Seq, four-week-old WT and siz1-2 pSIZ1:SIZ1g-GFP seedlings were collected, and total RNA was extracted using the Direct-zol RNA Miniprep kit. Library construction, quality control, and RNA sequencing were performed by Novogene (https://en.novogene.com/) on an Illumina NovaSeq X Plus Series (PE150) platform. The raw RNA-seq reads generated were filtered using trim-galore v0.6.6 with the default parameters to remove low-quality reads. Cleaned data were then mapped to the Arabidopsis thaliana genome TAIR10 using HISAT2 v2.2.051 with default parameters. BAMs were sorted by Samtools v1.9.52.