Background: Laodelphax striatellus transmits rice stripe virus (RSV) during sap feeding on the rice plant. The insect saliva proteins have direct and indirect roles in mediating RSV transmission; however, the function of most saliva proteins remains unclear.Results: In this study, we sequenced L. striatellus saliva proteins using shotgun liquid chromatography-electrospray ionizationtandem mass spectrometry. We identified 41 secreted saliva proteins, among which a saliva mucin-like protein, designated LssaMP, was the most abundant. In silico analysis revealed the sequence conservation among planthoppers. We revealed that the LssaMP gene is specifically expressed in the salivary glands and the protein is secreted as a component of gel saliva. Using LssaMP-specific double-stranded RNA (dsRNA) to silence gene expression, we revealed that LssaMP is required for formation of the salivary sheath, an important structure for sap feeding. Disrupting LssaMP expression resulted in inefficient formation of the feeding structure, thereby stopping insects from secreting watery saliva and acquiring sufficient nutrients from the phloem sap. We confirmed that RSV is mainly released via the watery saliva, which passes through the salivary sheathes into the plant phloem. An insufficient feeding structure results in decreased release of watery saliva, as well as the arboviruses.Conclusion: This study clarified the function of an insect saliva protein in mediating insect feeding, as well as arbovirus transmission.
Plant arboviruses rely heavily on insects’ feeding activities for successful transmission. Insect salivary proteins have been suggested to be essential for successful viral infection, but their exact mechanisms are largely unknown. In this study, we reveal that salivary factors from Laodelphax striatellus are necessary for infection of Rice stripe virus (RSV) in plants. A salivary carbonic anhydrase (LssaCA) is identified as an essential factor in promoting RSV infection. LssaCA interacts with a rice thaumatin-like protein (OsTLP) that has endo-β-1,3-glucanase activity and can degrade callose in plants. RSV infection induces callose deposition, which can be reversed by LssaCA. Furthermore, LssaCA directly binds to the RSV nucleocapsid protein (NP) in salivary glands, and the LssaCA-RSV NP complex still binds OsTLP and further increases its glucanase activity. This study provides new insights into the tripartite virus-insect vector-plant interaction, which is relevant to many agriculturally important plant arboviruses whose transmission is facilitated by insect salivary proteins.
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