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Beet necrotic yellow vein virus (BNYVV) A type isolates E12 and S8, originating from areas where resistance-breaking had or had not been observed, respectively, served as starting material for studying the influence of sequence variations in BNYVV RNA 3 on virus accumulation in partially resistant sugar beet varieties. Sub-isolates containing only RNAs 1 and 2 were obtained by serial local lesion passages; biologically active cDNA clones were prepared for RNAs 3 which differed in their coding sequences for P25 aa 67, 68 and 129. Sugar beet seedlings were mechanically inoculated with RNA 1+2/RNA 3 pseudorecombinants. The origin of RNAs 1+2 had little influence on virus accumulation in rootlets. E12 RNA 3 coding for V 67 C 68 Y 129 P25, however, enabled a much higher virus accumulation than S8 RNA 3 coding for A 67 H 68 H 129 P25. Mutants revealed that this was due only to the V 67 'GUU' codon as opposed to the A 67 'GCU' codon.The Polymyxa betae-transmitted beet necrotic yellow vein virus (BNYVV) is the causal agent of the devastating rhizomania disease. Its genome consists of four, and in some geographical areas five, different RNA species. RNA 1 and 2 contain the genetic information necessary to enable replication, encapsidation, cell-to-cell movement and suppression of RNA silencing in local lesion hosts (Koenig, 2008; Dunoyer et al., 2002). The additional presence of RNA 3, which codes for a 25 kDa protein (P25), is necessary for virus movement in infected sugar beet roots and production of typical rhizomania symptoms. RNA 4 enables efficient vector transmission and suppression of gene silencing in Nicotiana benthamiana roots (Rahim et al., 2007). The severity of symptoms in sugar beet may be enhanced by the additional presence of RNA 5. The three major strain groups of BNYVV, i.e. the A, B and P types, have highly conserved genomes. Nucleotides 199-210 of the P25-coding region, however, show considerable sequence variation, especially in A type BNYVV (Tamada et al., 2003;Meunier et al., 2003;Schirmer et al., 2005;Ward et al., 2007;Kutluk Yilmaz et al., 2007;Li et al., 2008;Koenig et al., 2008). This stretch of nucleotides codes for the P25 aa 67-70, which in the following will be referred to as the 'tetrad'. Also for the sake of brevity we will refer to the variability of the tetrad rather than its coding sequence, although we do not know whether the effects described below are due to changes at the protein or the nucleic acid level. More than 15 variants of the tetrad have been identified so far, raising the question whether these variations influence the pathogenicity of the virus and may be involved in resistance-breaking phenomena. Klein et al. (2007) studied the influence of P25 sequence variation on its oligomerization and pathogenicity for Tetragonia expansa leaves. Chiba et al. (2008) investigated the influence of mutations in the P25 aa 68, 69 and 70 on the local lesion response in mechanically inoculated leaves of lines of Beta maritima and of the sugar beet varieties Rizor and Monomidori. Acosta- ...

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The complete nucleotide sequence of RNA 3 of the Ahlum isolate of beet soil-borne virus (BSBV) was determined from cDNAs obtained with immunocaptured virus particles and denatured preparations of dsRNA. BSBV RNA 3 is unique among the plant virus RNAs studied so far in containing apparently only the coding sequences of a triple gene block (TGB). The derived amino acid sequences of the three putative TGB-encoded proteins showed the highest level of sequence similarities with those of the corresponding proteins of potato mop top furovirus (PMTV) followed by those of peanut clump furovirus and barley stripe mosaic hordeivirus. Progressively fewer similarities were found with the TGB-encoded proteins of beet necrotic yellow vein virus (uncertain classification), potato X potexvirus, and potato M carlavirus. The 3'-terminal 78 nucleotides of BSBV RNA 3 can be folded into a tRNA-like structure and a high degree of sequence similarity exists between the 122 3'-terminal nucleotides of BSBV RNA 3 and PMTV RNA 2. In other regions, however, no pronounced sequence similarities were found between the two RNAs, and PMTV RNA 2 contains an additional putative gene for a cysteine-rich protein downstream of the TGB. The two viruses are unrelated serologically. BSBV RNA 3 adds a further variant to the heterogeneity of the gene content of furovirus genomes and of triple gene block-carrying RNAs.

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