-Viroids are small, non-coding RNAs that rely on host proteins 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 show that VIRP1 is the only Solanaceae BET protein containing a proline-rich domain overlapping the PSTVd-binding site. VIRP1-deficient plants exhibit delayed flowering and increased ABA sensitivity, with differentially expressed genes enriched in stress-related pathways. In parallel, CRISPR/Cas9-generated virp1 mutants and RNAi lines showed increased ABA sensitivity and delayed flowering accompanied by gene expression changes, indicating that VIRP1 is linked to endogenous stress-responsive gene regulation. -VIRP1 forms condensates in planta and in vitro, consistent with phase-separation behaviour. Condensate morphology was altered by PSTVd RNA, by deletion of the intrinsically disordered CTD and by mutations in a conserved bromodomain residue. -Functional assays showed that 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 was dispensable for complementation of PSTVd accumulation. -These results support a model in which VIRP1 acts as a host nuclear factor that links chromatin-related functions, nuclear condensate formation and early viroid infection.
Bardani, E., Ostendorp, S., Andronis, C., Asch, F., Ostendrop, A., Katsarou, K., Kehr, J., Kalantidis, K.
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