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The nucleocytoplasmic exchange is of fundamental importance to eukaryotic life and is mediated by karyo- pherins, a superfamily of nuclear transport receptors. However, the function and cargo spectrum of plant kar- yopherins are largely obscure. Here, we report proximity-labeling-based proteomic profiling of in vivo sub- strates of KA120, a karyopherin-b required for suppressing autoimmune induction in Arabidopsis. We identify multiple components of the MOS4-associated complex (MAC), a conserved splicing regulatory pro- tein complex. Surprisingly, we find that KA120 does not affect the nucleocytoplasmic distribution of MAC proteins but rather prevents their protein condensation in the nucleus. Furthermore, we demonstrate that MAC condensation is robustly induced by pathogen infection, which is sufficient to activate defense gene expression, possibly by sequestrating negative immune regulators via phase transition. Our study reveals a noncanonical chaperoning activity of a plant karyopherin, which modulates the nuclear condensation of an evolutionarily conserved splicing regulatory complex to coordinate plant immune activation.