dsRNA-induced immunity targets plasmodesmata and is suppressed by viral movement proteins

Abstract

AbstractEmerging evidence indicates that in addition to the well-recognized antiviral RNA silencing, dsRNA elicits responses of pattern-triggered immunity (PTI), likely contributing plant resistance against virus infections. However, compared to bacterial and fungal elicitor-mediated PTI, the mode-of-action and signaling pathway of dsRNA-induced defense remain poorly characterized. Here, using multi-colorin vivoimaging by GFP mobility, staining of callose and plasmodesmal marker lines, we show that dsRNA-induced PTI restricts the progression of virus infection by triggering callose deposition at plasmodesmata, thereby likely limiting the macromolecular transport through these cell-to-cell communication channels. The plasma membrane-resident kinase module of SERK1 and BIK1/PBL1, plasmodesmata-localized proteins PDLP1/2/3 and calmodulin-like CML41, and Ca2+signals are involved in the dsRNA-induced signaling leading to callose deposition at plasmodesmata and antiviral defense. In addition, unlike classical bacterial elicitor flagellin, dsRNA does not trigger detectable reactive oxygen species (ROS) burst, further substantiating a partially shared immune signaling framework with distinct features triggered by different microbial patterns. Likely as a counteract strategy, viral movement proteins from different viruses suppress the dsRNA-induced host response leading to callose deposition to achieve infection. Thus, our data support the new model of how plant immune signaling constrains the virus movement by inducing callose deposition at plasmodesmata and how viruses counteract this layer of immunity.One-sentence summarydsRNA-induced antiviral PTI targets plasmodesmata for callose deposition and is suppressed by virus-encoded movement proteins.IN A NUTSHELLBackgroundPlants use different defense mechanisms pathogens. The major mechanism that plants use for defense against viruses is known as RNA silencing. This mechanism is triggered by the presence of viral double-stranded (ds)RNA and uses small RNAs to inhibit viral replication by targeting the viral genome for degradation. Recently, it was found that dsRNA elicits antiviral defense also through a protein-mediated mechanism known as pattern-triggered immunity (PTI). However, the underlying mechanism of antiviral PTI and how viruses overcome this plant defense mechanism to cause infection is unknown.QuestionIn this study we asked how dsRNA-induced PTI acts to inhibit virus infection and whether we can identify components of the PTI signaling pathway. Moreover, we wanted to know how viruses overcome this plant host defense response in order to cause infection.FindingsWe demonstrate that dsRNA-induced PTI targets plasmodesmata (PD), the intercellular communication conduits in plant cell walls that viruses use to spread infection from cell to cell. By inducing the deposition of callose, dsRNA-induced PTI reduces PD permeability, thus restricting virus movement. We identified PTI signaling components required for dsRNA-induced PD callose deposition and delineate a PTI pathway showing important difference to PTI pathways triggered by microbial elicitors. Moreover, viral movement proteins (MPs) suppress the dsRNA-induced callose deposition response at PD. This leads to a new model of how plant immune signaling constrains virus movement and how viruses counteract this layer of immunity.Next stepsThis study calls upon the identification of the PTI dsRNA receptor and the mechanisms of PTI signaling (involving identified components such as SERK1, BIK1, calcium channels, CML41, PDLP1/2/3) and PTI suppression by MPs, and how dsRNA-induced PTI and RNA silencing are controlled during the spread of infection.