Post-transcriptional gene silencing suppression study of Beet soil-borne mosaic virus: characterization of p14 and production of chimeric isolates of Benyviruses

Beet soil-borne mosaic virus (BSBMV) belongs to the Benyvirus genus, together with Beet necrotic yellow vein virus (BNYVV). Both viruses possess a multipartite genome formed by four ssRNAs(+). BSBMV and BNYVV are closely related since they possess the same host range, vector and genome organization. Recent studies demonstrated a possible amplification and transmission of BSBMV RNAs by BNYVV. In the United States of America, both benyviruses are frequently present in the same cultivated field, infecting the same plant but no chimeric forms have been described from field isolates so far. Chenopodium quinoa infection has been carried out using in vitro infectious transcripts of both BNYVV and BSBMV RNA-1 and -2 and the behavior of BSBMV/BNYVV combinations and wild type isolates has been compared. In parallel, the properties of the BSBMV VSR, a cysteine-rich protein (CRP) of 14 kDa expressed by RNA-2, have been investigated and compared to the BNYVV p14 RNA silencing suppressor. P14 has a zinc-finger domain able to bind nucleic acids and agroinfection of Nicotiana benthamiana plants demonstrated that p14 is able to suppress the PTGS downstream of the Dicer proteins, without interfering with the transitivity. Moreover, both p14 are localized in the nucleolus, forms homodimers and binds the “coremin” sequence, a stretch of 20 nucleotides present in the RNA-3 of Benyviruses and necessary for the systemic spread of viruses in the plant. Experiments performed to investigate relationships between BSBMV/BNYVV p14s and VSR activity, “coremin” sequence, long distance movement and absence of natural chimeras of Benyviruses will be presented.