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, but the regulators of SGS3 condensate formation and their roles in antiviral responses remain largely unknown. Here, we show that the dehydrin protein COR15 from Citrus aurantifolia and Nicotiana benthamiana interacts with citrus tristeza virus (CTV)-encoded p20 protein. COR15 abolishes the RNA silencing suppressor activity of p20, positively regulates antiviral RNA silencing, and confers resistance to CTV in N. benthamiana plants. Furthermore, COR15 also restricts the infection of other positive-sense RNA viruses, suggesting that COR15 functions as a broad-spectrum antiviral factor. Mechanistically, CTV infection induces NbCOR15 expression and promotes COR15 granule formation 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 bodies. Notably, COR15 promotes SGS3 condensates formation and efficient siRNA processing. Knockout of COR15 via CRISPR/Cas9 in N. benthamiana plants reduces SGS3 condensate number, causes abnormally sized SGS3 condensates, and impairs SGS3-dependent siRNA synthesis, indicating that COR15 is a previously unrecognized component of the siRNA bodies. Together, this work reveals the function and mechanism of a dehydrin protein in plant antiviral immunity and provides new sight into the molecular arms race during plant-virus co-evolution.
ZHANG et al. (Wed,) studied this question.