Suppression of a dsRNA-induced plant immunity pathway by viral movement protein

The virome of plants is dominated by RNA viruses 1 and several of these cause devastating diseases in cultivated plants leading to global crop losses 2. To infect plants, RNA viruses engage in complex interactions with compatible plant hosts. In cells at the spreading infection front, RNA viruses replicate their genome through double-stranded RNA (dsRNA) intermediates and interact with cellular transport processes to achieve cell-to-cell movement of replicated genome copies through cell wall channels called plasmodesmata (PD) 3. In order to propagate, viruses also must overcome host defense responses. In addition to triggering the antiviral RNA silencing response, RNA virus infection also elicits pattern-triggered immunity (PTI) 4 whereby dsRNA, a hallmark of virus replication, acts as an important elicitor 5. This innate antiviral immune response is also triggered when dsRNA is applied externally and does not require sequence homology to the virus 5. However, the mechanism by which PTI restricts virus infection is not known. Here, we show that dsRNA inhibits the progression of virus infection by triggering callose deposition at plasmodesmata and the inhibition of transport through these cell-to-cell communication channels. The dsRNA-induced signaling pathway leading to callose deposition is independent of ROS production and thus distinguished from pathways triggered by bacterial and fungal elicitors. The dsRNA-induced host response at plasmodesmata is suppressed by the Tobacco mosaic virus movement protein (MP). Thus, the virus uses MP to inhibit innate dsRNA-induced immunity at plasmodesmata, which could be a general strategy of phytoviruses to overcome plant defenses and spread infection.