Transmission mechanism of plant viruses transmitted by the fungus Polymyxa betae

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Beet necrotic yellow vein virus (BNYVV), which is transmitted by the soil-borne fungus Polymyxa betae, contains five ssRNA species (RNAs 1 to 5). BNYVV was transmitted by the biotype of P. batae from sugar beets, but not by the other biotypes from Chenopodium album, Amaranthus retroflexus and Portulaca oleracea. Although host ranges of BNYVV and the vector were thought to be limited to only few species such as Chenopdiaceae species, P. betae zoospores were found to infect a wide range of plant species, and also BNYVV infected roots of several plant species of them. Nicotiana benthamiana is a good indicator host. The coat protein is encoded by the 5' proximal ORF of RNA2, and the terminal codon of this ORF can be suppressed, which results in the production of a 75 kDa readthrough (RT) protein. Mutagenic analysis revealed that a region within the N-terminal half of the RT domain is involved in virus assembly, whereas the C-terminal protein contains sequences important for fungus transmission. Infectious cDNA clones to BNYVV RNAs 3,4 and 5 were established. Fungal inoculation tests with isolates containing different combinations of RNAs 3, 4 and 5 showed that RNA3 and RNA5 are involved in symptom expression. The 25 kDA protein encoded by RNA3 was responsible for rhizomania of susceptible cultivars of sugar beet, and also was involved in resistant reaction in resistant cultivars. When RNA5, encoding the 26 kDA protein, was present, the root symptoms were more intense, Antisera against 25 kDA AND 26 kDA protein, were obtained. It was suggested that expressions of these proteins in infected plant are strongly involved in symptom development. RNA4 was not associated with symptom expression, but acted as increasing transmission efficiency and stability. Synergistic effects were found between RNA4 and RNA3 or RNA5.