IMMUNOGOLD LABELING AND ELECTRON-MICROSCOPIC SCREENING OF BEET NECROTIC YELLOW VEIN VIRUS IN THE FUNGUS POLYMYXA-BETAE INFECTING BETA-VULGARIS ROOT CORTICAL PARENCHYMA CELLS

Immunocytochemistry and EM were used to assess the relationship between the beet necrotic yellow vein virus (BNYVV) and its presumed vector Polymyxa betae in cortical parenchyma of sugarbeet lateral roots. Polyclonal antibodies raised against purified BNYVV were used to localize the virus antigens at the EM level. The spread of BNYVV from the initial P. betae infection site into the cortical parenchyma of lateral roots was very irregular, and no gradient was obvious. The BNYVV was not found in plasmodesmata. Virions appeared in the cytoplasm of host cells infected by P. betae, but always in low quantities. Inside fungi, virions were seen at the young zoosporangia stage, and labeling occurred in the resting spores. Strong labeling always occurred in cytoplasmic residues of cells containing cystosori. Our data support the view that P. betae is an internal carrier of BNYVV and that viral multiplication inside the fungus could occur at the young zoosporangia stage, i.e., just before the differentiation of zoospores which represent the infectious units.