The location of virus‐like particles in the male gametophyte of birch, walnut and cherry naturally infected with cherry leaf roll virus and its relevance to vertical transmission of the virus

Abstract

Virus‐like particles (VLPs) in close‐packed paracrystalline arrays were observed in cells forming the anther walls and in the sperm cell cytoplasm of immature pollen grains developing within cherry leaf roll virus (CLRV)‐infected birch (Betula pendula Roth.). VLPs within tubules, that were in some instances multiple and membrane bound, were also observed in anther cells and in pollen grains of CLRV‐infected walnut (Jugians regia L.). VLPs rarely coated the outer surfaces of developing grains. Washings from intact freshly collected pollen did not contain infective agents but pollen triturates were infectious after 12 months storage at‐70°C. Purified CLRV (concentration 6·4 ng/ml) was readily detected by enzyme‐linked immunosorbent assay (ELISA). CLRV‐specific antigens (detected by ELISA) and VLPs (detected on grids coated with an antiserum prepared against CLRV) were readily removed by washing intact pollen grains from infected birch, walnut and cherry (Prunus avium L. cv. F12/1). The antigen was less tenaciously held to the surfaces of anemophilous (birch and walnut) than entomophilous (cherry) pollen. Treatment of grains before ELISA testing with CLRV‐specific γ globulin virtually eliminated the antigenicity of pollen washings whereas γ globulin from a pre‐immune serum had no such effect. When anti‐CLRV γ globulin‐treated pollen grains were disrupted, CLRV‐specific antigens were liberated. VLPs trapped on CLRV‐antiserum coated grids to which pollen washings or extracts from disrupted grains had been applied were identified by decoration; a halo of antibody molecules enveloped VLPs treated with CLRV‐antiserum but not those treated with antiserum prepared against poplar mosaic virus.