VIRP1 bromodomain shapes nuclear condensate formation and has a positive effect on PSTVd accumulation

Viroids are small, non-coding RNAs that rely on host factors for replication, intracellular trafficking and systemic movement. VIRP1, a Bromodomain and Extra-terminal domain (BET) protein, has previously been implicated in Potato spindle tuber viroid (PSTVd) infection, yet its precise role and mode of action remain unresolved.

In this work, we highlight VIRP1 as the only Solanaceae BET protein containing a proline-rich domain, which overlaps with the PSTVd-binding site. VIRP1-deficient plants exhibit delayed flowering and increased ABA sensitivity, with differentially expressed genes enriched in stress-related pathways.

VIRP1 forms condensates in planta and in vitro , consistent with phase-separation behaviour. Condensate morphology is altered by PSTVd RNA, deletion of the intrinsically disordered CTD and bromodomain mutations.

VIRP1 is particularly important for the early establishment of PSTVd infection, while nuclear localization and bromodomain integrity are required for efficient viroid accumulation. By contrast, the disordered CTD region is dispensable for complementation of PSTVd accumulation.

Our results support a model in which VIRP1 acts as a host nuclear factor that links stress-related functions, nuclear condensate formation and early viroid infection.