Simon Lab

The first RNA viruses were likely transferred from organisms feeding on plants, thus requiring the use of host movement proteins (MPs) for transiting through plasmodesmata to establish systemic infection. Before the discovery of umbra-like viruses (ULVs), encoding of specialized MPs was accepted as a required feature of all modern plant viruses. Umbraviruses (family Tombusviridae) encode a 30K-type cell-to-cell MP and a long-distance MP for systemic infection, but rely on a helper virus for trans-encapsidation and plant-to-plant transmission. The recent discovery of two groups of novel viruses sharing umbravirus-like RNA-dependent RNA polymerases and 3ʹ-terminal structures, but which do not  necessarily encode MPs is expanding our understanding of the minimum requirements for a modern plant RNA virus. ULV CY1 from citrus uses host protein PHLOEM PROTEIN 2 for systemic movement, and related ULVs encode a capsid protein thereby providing an explanation for the lack of helper viruses in many ULV infected plants.