Research Institute for Bioresources, Okayama University, Kurashiki 710-0046, Japan

Background and objectives
The vectors for soilborne viruses with rod-shaped, filamentous or isometric particles are fungi. Evidence has been recently obtained on genome organization and transmission of the rod-shaped viruses in the genera Benyvirus(beet necrotic yellow vein virus, BNYVV), Furovirus (soilborne wheat mosaic virus), Pecluvirus (peanut clump virus) and Pomovirus (potato mop-top virus), formerly all classified in the genus Furovirus [1]. The vectors are the plasmodiophoraceous fungi Polymyxa betae, P. graminis and Spongospora subterranea. These viruses can cause several economically important diseases and can survive in soil within the long-lived resting spores of their vectors. BNYVV, which causes rhizomania in sugar beet, has essentially a bipartite ssRNA genome, but in nature isolates contain three other RNA components (RNAs 3, 4 and 5) [2]. These are needed for disease development and virus spread. The viral gene products and protein domains involved in fungal transmission of BNYVV, and the interactions with the vector P. betae, will be described.

Results and conclusions
BNYVV RNAs 1 and 2 are required for viral RNA replication, particle assembly and virus movement [2]. The coat protein is encoded by the 5' proximal ORF of RNA 2. The termination codon of this ORF can be suppressed, which results in the production of a 75 kDa readthrough (RT) protein which is a minor component of virions and is located at the ends of the virus particles. Mutagenic analysis revealed that a region within the N-terminal half of the RT domain is involved in virus assembly, whereas the C-terminal portion contains sequences important for vector transmission. Alanine scanning mutagenesis of the C-terminal region showed a KTER motif to be important for fungus transmission. A coat protein RT protein implicated in fungus transmission is present at a 5' terminus of all furovirus, pomovirus and benyvirus genomes and each contains sequence motifs (ER or QR) also found in the polyprotein coded by RNA 2 of the genus Bymovirus (Potyviridae), which has filamentous particles [3]. Possibly there are sequences common to the interaction between fungi and both rod-shaped and filamentous virus particles, in spite of their distant taxonomic relationship.

Other genome components of BNYVV have an indirect effect on fungus transmission. Tests with isolates containing combinations of deletion mutants of RNAs 3, 4 and 5 showed that the 31 kDa protein encoded by RNA 4 was important for transmission efficiency and that RNA 3 and RNA 5 were not involved in fungus transmission. The 25 kDa protein encoded by RNA3 is responsible for rhizomania of sugar beet, and the abnormal proliferation of fine rootlets seems to be favourable for the multiplication of P. betae. When RNA 5, encoding the 26 kDa protein, was present, the root symptoms were more intense. Synergistic effects are thus found among these RNAS. There is also a symbiotic interaction between BNYVV and P. betae. The effects of either RNA 3 or RNA 4 on transmission and symptom development were less with Nicotiana benthamiana than with chenopodiaceous hosts. Thus BNYVV RNAs 3, 4 and 5 have a host-specific function and play important, but different, roles in the natural infection process of sugar beet roots.

1. Torrance L, Mayo MA, 1997. Archives of Virology 142, 435-439.
2. Richards KE, Tamada T, 1992. Annual Review of Phytopathology 30, 291-313.
3. Peerenboom E, Jacobi V, Antoniw JF, Schlichter U, Cartwright EJ, Steinbiss HH, Adams MJ, 1996. Virus Research 40,149-159.