Plant YTHDF proteins are direct effectors of antiviral immunity against an N6 ‐methyladenosine‐containing RNA virus

Abstract

In virus–host interactions, nucleic acid‐directed first lines of defense that allow viral clearance without compromising growth are of paramount importance. Plants use the RNA interference pathway as a basal antiviral immune system, but additional RNA‐based mechanisms of defense also exist. The infectivity of a plant positive‐strand RNA virus, alfalfa mosaic virus (AMV), relies on the demethylation of viral RNA by the recruitment of the cellular N6 ‐methyladenosine (m 6 A) demethylase ALKBH9B, but how demethylation of viral RNA promotes AMV infection remains unknown. Here, we show that inactivation of the Arabidopsis cytoplasmic YT521‐B homology domain (YTH)‐containing m 6 A‐binding proteins ECT2, ECT3, and ECT5 is sufficient to restore AMV infectivity in partially resistant alkbh9b mutants. We further show that the antiviral function of ECT2 is distinct from its previously demonstrated function in the promotion of primordial cell proliferation: an ect2 mutant carrying a small deletion in its intrinsically disordered region is partially compromised for antiviral defense but not for developmental functions. These results indicate that the m 6 A‐YTHDF axis constitutes a novel branch of basal antiviral immunity in plants. image Plant YTHDF m 6 A‐binding proteins (ECTs) are required for basal antiviral defense, yet stimulate cellular proliferation. These opposite functions differ in their requirement for a tyrosine‐rich element in the intrinsically disordered region (IDR) that favours phase separation. ECTs are required for basal defense against alfalfa mosaic virus – a positive strand RNA virus containing m 6 A modifications. The m 6 A‐binding activity of ECTs is required for their antiviral function. ECT2 associates with viral RNA in vivo . The antiviral, but not the developmental, ECT2 function and its ability to phase‐separate are compromised upon deletion of a short tyrosine‐rich element (N5) in its IDR.