Beet necrotic yellow vein virus RNA-1 and 2 replicate and encapsidate Beet soil- borne mosaic virus RNA-3 in planta

Beet soil-borne mosaic virus (BSBMV) and Beet necrotic yellow vein virus (BNYVV) are members of Benyvirus genus. BSBMV has been reported only in the United States while BNYVV has a worldwide distribution. Both viruses are vectored by Polymyxa betae, possess similar host ranges, particles number and morphology. Both viruses are not serologically related but have similar genomic organizations. Field isolates consist of four RNA species but some BNYVV isolates contain a fifth RNA. RNAs 1 and 2 are essential for infection and replication while RNAs 3 and 4 play important roles on plant and vector interactions, respectively. Nucleotide and amino acid analyses revealed BSBMV and BNYVV are different enough to be classified in two different species. Considering RNA-3 involvement on symptoms development, we synthesised biologically active cDNA clones from which complete copies of the BSBMV genomic RNA-3 are transcribed. Sequence and structure similarities were found in BNYVV and BSBMV RNA-3 UTR domains. Recombination experiments showed capability of BNYVV RNA-1 and 2 to replicate and encapsidate BSBMV RNA-3 in planta and allow us to demonstrate that BSBMV RNA-3 support long-distance movement of BNYVV RNA-1, -2 when inoculated on Beta macrocarpa plants. However, competition occurred when BSBMV RNA-3 derived replicons were used together with BNYVV derived RNA-3 but not when RNA-5-derived component was used. Our data demonstrate the close relation between BSBMV and BNYVV RNA-3 even if the BSBMV RNA-3 encoded p29 is much closer to RNA-5-encoded p26 than to BNYVV RNA-3-encoded p25. Molecular interactions between sugar beets and Benyviruses will be better-investigated exploiting similarities and divergences between BSBMV and BNYVV.