Phase separation and phosphorylation of sfCactin orchestrate antiviral defense against Southern rice black‐streaked dwarf virus in <i>Sogatella furcifera</i>

Southern rice black-streaked dwarf virus (SRBSDV) is a devastating plant reovirus transmitted by the white-backed planthopper (WBPH: Sogatella furcifera) in a persistent-propagative manner. However, the antiviral mechanisms in this insect vector remain poorly understood. Here, we identify sfCactin as a critical antiviral factor in WBPH, revealing its role in restricting viral replication through liquid-liquid phase separation (LLPS) and phosphorylation-dependent regulation. Evolutionary analysis shows that sfCactin shares conserved antiviral activity with Drosophila Cactin, suppressing replication of Drosophila C virus in heterologous systems. Mechanistically, sfCactin undergoes LLPS in vivo, forming dynamic nuclear condensates driven by its intrinsically disordered region 1 (IDR1), which contains an arginine/serine-rich domain and a coiled-coil domain. Unlike Drosophila Cactin, sfCactin lacks conserved phosphorylation sites (Ser₉₉/Ser₁₀₄) but interacts with phosphoglycerate kinase, which phosphorylates Thr₁₂₀ within IDR1. Phospho-mimetic mutations (T120D) enhance sfCactin's phase separation dynamics and antiviral activity, while non-phosphorylatable mutants (T120A) impair these functions. In S. furcifera, sfCactin expression is upregulated during SRBSDV infection, particularly in the midgut - a primary site of viral entry. RNAi-mediated knockdown of sfCactin significantly increases SRBSDV replication, acquisition, and transmission, highlighting its role in antiviral defense. This study uncovers a conserved yet evolutionarily adaptable mechanism by which Cactin employs LLPS and phosphorylation to combat viral infections in insect vectors, offering insights for managing plant virus transmission through vector immunity modulation.