No abstract

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.

Plant viruses have evolved a wide array of strategies to ensure efficient transfer from one host to the next. Any organism feeding on infected plants and traveling between plants can potentially act as a virus transport device. Such organisms, designated vectors, are found among parasitic fungi, root nematodes and plant-feeding arthropods, particularly insects. Due to their extremely specialized feeding behavior -exploring and sampling all plant tissues, from the epidermis to the phloem and xylem -aphids are by far the most important vectors, transmitting nearly 30% of all plant virus species described to date. Several different interaction patterns have evolved between viruses and aphid vectors and, over the past century, a tremendous number of studies have provided details of the underlying mechanisms. This article presents an overview of the different types of virusaphid relationships, state-of-the-art knowledge of the molecular processes underlying these interactions, and the remaining black boxes waiting to be opened in the near future.

Virions of beet western yellows luteovirus contain a major capsid protein (P22.5) and a minor readthrough protein (P74), produced by translational readthrough of the major capsid protein sequence into the neighboring open reading frame, which encodes the readthrough domain (RTD). The RTD contains determinants required for efficient virus accumulation in agroinfected plants and for aphid transmission. The C-terminal halves of the RTD are not well conserved among luteoviruses but the N-terminal halves contain many conserved sequence motifs, including a proline-rich sequence separating the rest of the RTD from the sequence corresponding to the major coat protein. To map different biological functions to these regions, short in-frame deletions were introduced at different sites in the RTD and the mutant genomes were transmitted to protoplasts as transcripts and to Nicotiana clevelandii by agroinfection. Deletions in the nonconserved portion of the RTD did not block aphid transmission but had a moderate inhibitory effect on virus accumulation in plants and abolished symptoms. Deletion of the proline tract and the junction between the conserved and nonconserved regions inhibited readthrough protein accumulation in protoplasts by at least 10-fold. The mutants accumulated small amounts of virus in plants, did not induce symptoms, and were nontransmissible by aphids using agroinfected plants, extracts of infected protoplasts, or purified virus as a source of inoculum. Other deletions in the conserved portion of the RTD did not markedly diminish readthrough protein accumulation but abolished its incorporation into virions. These mutants accumulated to low levels in agroinfected plants and elicited symptoms, but could not be aphid-transmitted. A preliminary map has been produced mapping these functions to different parts of the RTD.