Hierarchical Action and Inhibition of Plant Dicer-Like Proteins in Antiviral Defense

TLDR

RNA silencing induction and suppression in plant‑virus interactions remain poorly understood. Arabidopsis DCL4 generates virus‑derived small RNAs that trigger antiviral immunity, while DCL2 provides a subordinate, dosage‑dependent defense; loss of both DCL2 and DCL4 restores systemic infection, defining a hierarchical silencing pathway that blocks viral movement from the vasculature.

Abstract

The mechanisms underlying induction and suppression of RNA silencing in the ongoing plant-virus arms race are poorly understood. We show here that virus-derived small RNAs produced by Arabidopsis Dicer-like 4 (DCL4) program an effector complex conferring antiviral immunity. Inhibition of DCL4 by a viral-encoded suppressor revealed the subordinate antiviral activity of DCL2. Accordingly, inactivating both DCL2 and DCL4 was necessary and sufficient to restore systemic infection of a suppressor-deficient virus. The effects of DCL2 were overcome by increasing viral dosage in inoculated leaves, but this could not surmount additional, non-cell autonomous effects of DCL4 specifically preventing viral unloading from the vasculature. These findings define a molecular framework for studying antiviral silencing and defense in plants.