Mobile virus-derived siRNAs drive plant antiviral silencing through reiterated 3-4 cell immunization

The movement of virus-derived small interfering RNAs (vsiRNAs) between plant cells has prompted the widely held assumption that they help immunize as-yet-uninfected tissues. Ascertaining this role is challenging because it requires uncoupling vsiRNA mobility from viral movement while preserving viral virulence. Consequently, the contribution of non-cell-autonomous RNAi to plant antiviral immunity remains essentially unknown. Here, we investigate the naturally phloem-restricted infection of turnip yellows virus (TuYV) in the stereotypical Arabidopsis root system at cellular resolution. We uncover intertwined, complex mechanisms operating in both vsiRNA-emitting and -recipient cells, which drive reiterated short-range (∼3-4 cell) immunization events. We find that, while intracellular antiviral RNAi is essentially nullified by TuYV, non-cell-autonomous immunization by vsiRNAs accounts mainly-if not exclusively-for the containment of viral titers and tissue invasion at the whole-plant scale. Mobile vsiRNA-mediated immunization likewise spatially confines a TuYV-unrelated virus over ∼3-4 cells, uncovering a generic, essential, yet previously unrecognized component of plant antiviral defense.