Emerging evidence indicates that in addition to its well-recognized functions in antiviral RNA silencing, dsRNA elicits pattern-triggered immunity (PTI), likely contributing to 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-color in vivo imaging, analysis of green fluorescent protein (GFP) mobility, callose staining and plasmodesmal marker lines in Arabidopsis thaliana and Nicotiana benthamiana, 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 SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (SERK1) the BOTRYTIS INDUCED KINASE1 (BIK1)/AVRPPHB SUSCEPTIBLE1 (PBS1)-LIKE KINASE1 (BIK1/PBL1) kinase module, PLASMODESMATA-LOCATED PROTEINs (PDLPs)1/2/3, as well as CALMODULIN-LIKE 41 (CML41) and Ca2+ signals are involved in the dsRNA-induced signaling leading to callose deposition at plasmodesmata and antiviral defense. Unlike the classical bacterial elicitor flagellin, dsRNA does not trigger detectable reactive oxygen species (ROS) burst, substantiating the idea that different microbial patterns trigger partially shared immune signaling frameworks with distinct features. Likely as a counter strategy, viral movement proteins from different viruses suppress the dsRNA-induced host response leading to callose deposition to achieve infection. Thus, our data support a model in which plant immune signaling constrains virus movement by inducing callose deposition at plasmodesmata and reveals how viruses counteract this layer of immunity.

Southern tomato virus (STV) is a double-stranded RNA (dsRNA) virus belonging to genus Amalgavirus (family Amalgamaviridae). STV has been detected in tomato plants showing different symptoms although it has not been demonstrated that STV is the causal agent. To study the STV incidence and its pathogenic role, a sensitive and quantitative real-time reverse transcription-polymerase chain reaction assay (RT-qPCR) was developed. The standard curve performed with viral RNA transcripts allowed a wide dynamic range for STV quantitation from 104 to 1011 copies/ng of total RNA. STV detection by RT-qPCR was 102-fold more sensitive than conventional RT-PCR or RT-LAMP and 104-fold more sensitive than molecular hybridization. STV was detected in different tomato plant tissues, as well as in the coat and the embryo of individual seeds. Also, viral concentration remained constant over time in leaf tissues of STV-infected tomato plants. Surveys on different tomato fields from Spain revealed that STV was widespread. In addition, the virus was detected in almost every tomato variety and nursery analyzed. STV-infected tomato plants did not show any disease-related symptom suggesting that the virus was not directly the causal agent of any tomato disease. However, there is no information about the STV effect in mixed infections or in abiotic stressed conditions and further studies must be performed to clarify it. The RT-qPCR assay developed in this work could be implemented on sanitation programs in order to limit the virus spread and could be used to study the effect of STV in mix infections or abiotic stressed conditions.

Emerging 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-color in vivo imaging 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.

Southern tomato virus (STV) is a double-stranded RNA virus (genus Amalgavirus, family Amalagaviridae) with a persistent lifestyle, transmitted only vertically by seed.STV is widely distributed showing a high incidence in tomato crops from different production regions. Acute viruses usually induce plant development and cell ultrastructural changes related to the production of plant symptoms. Some of these changes are the consequence of alterations in the expression of endogenous plant microRNAs