1USDA-ARS, Ithaca, NY, USA; 2Department of Plant Pathology, Cornell University, Ithaca, NY, USA; 3Department of Plant Pathology, Pennsylvania State University, University Park, PA, USA

Background and objectives
The barley yellow dwarf luteoviruses are transmitted by aphids in a circulative, non-propagative manner. Each of the virus strains is efficiently transmitted by only a single or a few aphid species. The ability of the aphid to transmit the virus is determined by the ability of the virus to successfully negotiate several potential barriers in the aphid. (i) The ingested virus must attach to and be transported across the hindgut epithelial cells and be released into the hemocoel. (ii) The virus must survive in the aphid haemolymph. (iii) The virus must specifically interact with and accumulate at the basal lamina surrounding the accessory salivary gland (ASG-BL). (iv) The virus must move through the ASG-BL and into the pockets created by the invagination of the ASG cell plasmalemma. (v) The virus must be transported across the ASG cells and deposited into the salivary canal where it can be injected into phloem tissues along with salivary secretions during prolonged feeding. The inability of an aphid to transmit BYDV can be traced to the inability of the virus to complete one or more of these steps. The barrier to transmission can differ for the same virus in different aphid species or for different viruses in the same aphid species. Our work has focused on determining the virus and aphid components that regulate virus-aphid interactions at each of the steps in the circulative pathway.

Materials and methods
A combination of biological, molecular, biochemical and ultrastructural assays has been used to identify virus protein domains that interact with aphid membranes and to identify genetically regulated elements in aphids that are linked to the ability of the aphid to transmit virus.

Results and conclusions
The virus capsid contains two proteins: a predominant 22-kDa coat protein and lesser, but variable amounts of a 72-kDa coat protein readthrough. The carboxy-terminal half of the readthrough domain is often removed in purified preparations of the virus. Virus particles can assemble from 22-kDa coat protein in the absence of the readthrough, but the virus is not transmissible by aphids. The virus particles assembled from 22-kDa protein are able to move from the gut into the haemolymph. There is also evidence that these particles can move from the haemolymph into the accessory salivary gland. These results suggest that the 22-kDa coat protein contains two different domains that regulate the movement of the virus through all the cellular barriers in the aphid. The readthrough protein contains multiple domains that are involved in virus-plant interactions as well as influencing the stability of the virion structure. The virion stability appears to influence the longevity and survival of the virus in the aphid tissues, especially the haemolymph, and this in turn influences the transmission efficiency of the virus.

Clonal variation in the aphid Schizaphis graminum for its ability to transmit BYDV has provided a means to investigate genetically controlled elements in aphids that are linked to virus transmission. Clones from the south-eastern USA are unable to vector any BYDV tested, whereas clones from other geographic areas vector virus isolates from at least three BYDV serotypes. The non-vector clones are able to acquire similar amounts of virus, but transmission is blocked at sites beyond the hindgut. Virus stability in haemolymph, interactions with haemolymph-associated factors, and virus movement into the ASG of non-vector biotypes will be discussed in relation to similar studies in biotypes able to vector BYDV.