Phosphorylation Alters the Condensation State of the Potato Transcription Factor bZIP18 to Enhance Plant Immunity

Biomolecular condensates formed by transcription factors (TFs) via liquid-liquid phase separation enable precise spatiotemporal control of eukaryotic gene expression. Although phosphorylation is a well-established regulator of TF activity, its capacity to remodel condensate organization and transcriptional function in plant immunity remains largely undefined. Here, we report that pathogen-induced phosphorylation converts a pre-existing TF condensate into a dynamic transcriptional hub to enhance the immune response in potato. Upon infection by the oomycete pathogen Phytophthora infestans, the mitogen-activated protein kinase StMPK9 phosphorylates the TF StbZIP18 at the conserved residue threonine 337. This modification triggers rapid changes in the nuclear StbZIP18 condensates, transforming pre-existing condensates into numerous smaller assemblies. The phospho-null StbZIP18^T337A mutant maintains its presence in low-activity condensates and compromises immunity, whereas the phospho-mimetic variant StbZIP18^T337E constitutively forms active condensates and enhances resistance. The gene encoding the TF StMYB44 was identified as the direct downstream gene target of StbZIP18. TurboID-based proximity labeling revealed that both wild-type StbZIP18 and StbZIP18^T337E, but not the phospho-null variant StbZIP18^T337A, selectively interact with the transcriptional cofactor StMBF1c to form transcriptional activation hubs that drive defense gene expression, including StMYB44. Together, our findings establish that phosphorylation-directed condensate reprogramming operates as a tunable phase-separation switch, converting a low-activity assembly into a transcriptional amplifier. This study unveils a biophysical mechanism for signal amplification in plant immunity and provides insight into how post-translational modifications modulate transcriptional condensate function.