Beet necrotic yellow vein virus (BNYVV) is an economically important pathogen of sugar beet and has been found worldwide, probably as the result of recent worldwide spread. The BNYVV genome consists of four or five RNA components. Here, we report analysis of sequence variation in the RNA3-p25, RNA4-p31, RNA2-CP, and RNA5-p26 genes of 73 worldwide isolates. The RNA3-p25 gene encodes virulence and avirulence factors. These four sets of gene sequences each fell into two to four groups, of which the three groups of p25 formed eight subgroups with different geographical distributions. Each of these subgroup isolates (strains) could have arisen from four original BNYVV population and their mixed infections. The genetic diversity for BNYVV was relatively small. Selection pressure varied greatly depending on the BNYVV gene and geographical location. Isolates of the Italy strain, in which p25 was subject to the strongest positive selection, were able to overcome the Rz1-host resistance gene to differing degrees, whereas other geographically limited strains could not. Resistance-breaking variants were generated by p25 amino acid changes at positions 67 and 68. Our studies suggest that BNYVV originally evolved in East Asia and has recently become a pathogen of cultivated sugar beet followed by the emergence of new resistance-breaking variants.

About half of Japanese isolates of beet necrotic yellow vein virus (BNYVV) were found to contain RNA 5 molecules, which were also detected in virus isolates from China and France. Sequence comparisons of RNA 5 (nucleotides 327 to 1171) in 25 isolates showed that there are up to 8% sequence differences, and that RNA 5 variants fall into three groups: group I contains most of the Japanese and Chinese isolates, group II two Japanese isolates, and group III four French isolates. The group I isolates fall into three small clusters. In the 26 kDa coding region of RNA 5, there was a maximum of 1.5% nucleotide sequence differences (6 amino acid changes) within the group and 8.4% nucleotide sequence differences (17 amino acid changes) between the groups. Comparisons of the coat protein gene of RNA 2 revealed that most of the Japanese and Chinese isolates belonged to the A type strain, but some isolates were of the B type. The French isolates (P type) were closely related to those of the A type. Mixed infections of the two types of virus and the two groups of RNA 5 were detected in a small area of Hokkaido. BNYVV might have been introduced into Japan and China by a similar route from at least two origins. These results, together with other evidence, suggest that the three groups of RNA 5 variants separated from an original population a long time ago and, thereafter, the group I population diverged further into three clusters, which may have been associated with the A type strain rather than the B type.

The RNA3-encoded p25 protein of beet necrotic yellow vein virus (BNYVV) is responsible for the production of rhizomania symptoms of sugar beet roots (Beta vulgaris subsp. vulgaris). Here, it was found that the presence of the p25 protein is also associated with the resistance response in rub-inoculated leaves of sugar beet and wild beet (Beta vulgaris subsp. maritima) plants. The resistance phenotype displayed a range of symptoms from no visible lesions to necrotic or greyish lesions at the inoculation site, and only very low levels of virus and viral RNA accumulated. The susceptible phenotype showed large, bright yellow lesions and developed high levels of virus accumulation. In roots after Polymyxa betae vector inoculation, however, no drastic differences in virus and viral RNA accumulation levels were found between plants with susceptible and resistant phenotypes, except at an early stage of infection. There was a genotype-specific interaction between BNYVV strains and two selected wild beet lines (MR1 and MR2) and sugar beet cultivars. Sequence analysis of natural BNYVV isolates and site-directed mutagenesis of the p25 protein revealed that 3 aa residues at positions 68, 70 and 179 are important in determining the resistance phenotype, and that host-genotype specificity is controlled by single amino acid changes at position 68. The mechanism of the occurrence of resistance-breaking BNYVV strains is discussed.

The W8 isolate of the phytopathogenic fungus, Rosellinia necatrix that causes white root rot, contained three segments of double-stranded (ds) RNA, namely L1, L2 and M. Purified viral particles of about 25 nm in diameter contained an RNA segment with almost the same mobility as M-dsRNA, but the band was sensitive to S1 nuclease. Molecular analysis revealed that M-dsRNA consisted of two (RNA 1 and RNA 2) similarly sized species of 2299 and 2279 bp excluding an interrupted poly (A or U) tail of 16-51 bp. The predicted largest open reading frame in RNA 1 and RNA 2 was similar to those of RNA dependent RNA polymerase (RdRp) and coat protein (CP), respectively, encoded by the family Partitiviridae. The non-coding regions (NCR) of the two segments were similar (approximately 70% base identity) at the 5' end, but different at the 3' end. The NCR at the 3' end contained adenosine-uracil rich elements (AREs) in both segments. Northern analyses revealed RNA 1 and RNA 2 in mycelial and viral particle fractions. We coined the name Rosellinia necatrix partitivirus 1-W8 (RnPV1-W8) for M-dsRNA based on viral particle morphology and sequence information.