Beet western yellows luteovirus is obligately transmitted by the aphid Myzus persicae in a circulative, non‐propagative fashion. Virus movement across the epithelial cells of the digestive tube into the hemocoel and from the hemocoel into the accessory salivary glands is believed to occur by receptor‐mediated endocytosis and exocytosis. Virions contain two types of protein; the major 22 kDa capsid protein and the minor read‐through protein, P74, which is composed of the major capsid protein fused by translational read‐through to a long C‐terminal extension called the read‐through domain. Beet western yellows virus carrying various mutations in the read‐through domain was tested for its ability to be transmitted to test plants by aphids fed on agro‐infected plants and semi‐purified or purified virus preparations. The results establish that the read‐through domain carries determinants that are essential for aphid transmission. The findings also reveal that the read‐through domain is important for accumulation of the virus in agro‐infected plants.

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Beet western yellows luteovirus, like other luteoviruses, cannot be trnted to host plants by medanical inoculation but requires an aphid vector, a feature that has heretofore presented a serious obstacle to the study of such viruses. In this paper we describe use of agroinfection to infect hosts with beet western yellows virus without recourse to aphids. Agroinfection is a procedure for introducing a plant virus into a host viaAgrobacterium twmefaciens harboring a Ti plasmid, which can efflclently transfer a portion of the p id (T-DNA) to plant celis near a wound. The viral genome must be inserted into the T-DNA insuch a way that it can escape and begin autonomous replication, a requirement that has, so far, limited agroinfection to pathogens with a circular genome. We have cloned cDNA corresponding to the complete beet western yellows virus RNA genome between the cauliflower mosaic virus 35S promoter and the nopaline synthase transcription termination signal. In one construct, a self-cleaving (ribozyme) sequence was included so as to produce a transcrpt in planta with a 3' extremity almost identical to natural viral RNA. When inoculated mechanically to host plants, the naked plasmid DNA was not infectious but, when introduced into T-DNA and agroinfected to plants, both the construct with and without the ribozyme produced an infection. This approach should be applicable to virtually any plant virus with a linear plus-strand RNA genome.