Beet Soil-Borne Virus Is a Helper Virus for the Novel Beta vulgaris Satellite Virus 1A

Abstract

Sugar beet (Beta vulgaris L.) is grown in temperate regions around the world as a source of sucrose. Sugar beet is susceptible to a number of viral diseases, but identification of the causal agent(s) under field conditions is often difficult due to mixtures of viruses that may be responsible for disease symptoms. In this study, the application of RNAseq to RNA extracted from diseased sugar beet roots obtained from the field and from greenhouse-reared plants grown in soil infested with the virus disease rhizomania (causal agent, Beet necrotic yellow vein virus; BNYVV) yielded genome-length sequences from BNYVV as well as Beet soil-borne virus (BSBV). Nucleotide identities of the derived consensus sequence of BSBV RNAs ranged from 95.3 to 99.3% compared to published BSBV sequences. Based on the BSBV genome consensus sequence, clones of the genomic RNAs were obtained to produce RNA copies of the genome through in vitro transcription. Capped RNA produced from the clones was infectious when inoculated to leaves of Chenopodium quinoa and B. vulgaris, and extracts from transcript-infected C. quinoa leaves could infect sugar beet seedling roots through a vortex inoculation method. Subsequent exposure of these infected sugar beet seedling roots to aviruliferous Polymyxa betae, the protist vector of both BNYVV and BSBV, confirmed that BSBV derived from the infectious clones could be transmitted by the vector. Co-inoculation of BSBV synthetic transcripts with transcripts of a cloned satellite virus designated Beta vulgaris satellite virus 1A (BvSat1A) demonstrated that BSBV is a helper virus for this novel satellite.