The dehydrin protein COR15 enhances antiviral RNA silencing by preventing viral protein-mediated SGS3 degradation and promoting SGS3 condensate formation.

RNA silencing, a conserved gene-regulatory mechanism mediated by small interfering RNAs (siRNAs), is a major component of plant antiviral immunity. The plant RNA-binding protein SUPPRESSOR OF GENE SILENCING 3 (SGS3) forms condensates that drive siRNA body assembly and promote siRNA biogenesis however, the regulators of SGS3 condensate formation and their roles in antiviral responses remain largely unknown. Here, we show that the cold-regulated 15-kDa dehydrin protein (COR15) homologs from Citrus aurantifolia and Nicotiana benthamiana interact with the citrus tristeza virus (CTV)-encoded p20 protein. COR15 abolishes the RNA silencing suppressor activity of p20, enhances antiviral RNA silencing, and confers resistance to CTV in N. benthamiana. COR15 also restricts infection by other positive-sense RNA viruses, suggesting that it functions as a broad-spectrum antiviral factor. Mechanistically, CTV infection induces NbCOR15 expression and promotes the formation of COR15 granules in the cytoplasm. COR15 attenuates p20-mediated SGS3 degradation by disrupting the p20-SGS3 interaction, thereby stabilizing SGS3. COR15 is recruited into siRNA bodies and undergoes liquid-liquid phase separation within these structures. Notably, COR15 promotes SGS3 condensate formation and efficient siRNA processing. CRISPR-Cas9-mediated knockout of COR15 in N. benthamiana plants reduces the number of SGS3 condensates, leads to abnormally sized SGS3 condensates, and impairs SGS3-dependent siRNA synthesis, indicating that COR15 is a previously unrecognized component of siRNA bodies. Together, our findings reveal the function and mechanism of a dehydrin protein in plant antiviral immunity and provide new insights into the molecular arms race underlying plant-virus co-evolution.