First report of black currant-associated rhabdovirus in blackcurrants in Latvia

Blackcurrants (Ribes nigrum) are among the most important commercial berry crops in Latvia and, together with redcurrants and gooseberries, have a long history of local cultivation and breeding (Zuļģe et al. 2018). So far at least 20 viruses were reported to infect Ribes plants (Špak et al. 2021). Blackcurrant-associated rhabdovirus (BCaRV) was previously identified in USA by high throughput sequencing (HTS) of blackcurrant germplasm accession introduced from Russia (isolate Veloy) and now serves as an exemplar isolate for BCaRV (Wu et al. 2018). Presence of BCaRV was also confirmed by RT-PCR in blackcurrant germplasm accession of cv. Burga from France during the same study by Wu et al. (2018). Currently Blackcurrant betanucleorhabdovirus is one of the nine species recognized by ICTV in genus Betanucleorhabdovirus of family Rhabdoviridae, but the impact of BCaRV on the host still remains unknown. Leaf tissue from twelve asymptomatic blackcurrant cv. Mara Eglite plants that negatively tested for blackcurrant reversion virus from Dobele, Latvia (56°36'31.9"N, 23°18'13.6"E) was collected on May 17, 2019 and used for HTS study of local Ribes resistance genes. Total RNA from the leaf tissue of sampled plants was isolated following a method described by Kalinowska et al. (2012) with minor modifications. Briefly, RNeasy Plant Mini Kit (QIAGEN) was used with RLC lysis buffer being supplemented with 2% PVP and 1% β-mercaptoethanol. Plant rRNA was subsequently removed by a RiboMinus Plant Kit for RNA-Seq (Thermo Fisher Scientific (TFS)) prior to cDNA library construction. HTS libraries were prepared using MGIEasy RNA Directional Library Prep Set for 16 reactions (MGI), following a protocol for 150 bp pair-end reads. According to the manufacturers guidelines libraries were pooled, circularized and cleaned before being subjected to sequencing on DNBSEQ-G400 (MGI) using PE150 flow cell (MGI). The sequencing run yielded a total of 393660492 150 bp long read pairs. Reads were assembled into transcripts using rnaSPAdes v 3.13.1 (Bushmanova et al. 2019) and a 14424 base long contig with an average coverage of 684x was found to be 99.5% identical (14358/14432 identities and 8 gaps in the pairwise alignment) to the previously reported first complete genome of BCaRV (MF543022.1) using EMBOSS Needle (Madeira et al., 2019). This contig representing the genome of BCaRV isolate Mara Eglite, onto which 66768 of the raw reads could be mapped, was subsequently deposited at European Nucleotide Archive under accession number OU015520. All of the twelve individual samples were also tested for the presence of BCaRV by RT-PCR, using Verso cDNA Synthesis Kit with random hexamer primers (TFS) for first strand cDNA synthesis followed by PCR with N protein nested primers BCaRV-N-F (5’ AGATGTGCTTCATCGATGGCTAGTTCTGCT 3’) and BCaRV-N-R (5’ TGCATTCCCACGGGTTAGGAATACATTGGTACT 3’) resulting in a 243 bp long fragment for six of the samples. RT-PCR products from six BCaRV positive samples were directly sequenced by Sanger-based method using BigDye Terminator v3.1 Cycle Sequencing Kit (Applied Biosystems) with BCaRV-N-F and BCaRV-N-R primers. Acquired RT-PCR product sequences matched the corresponding region of BCaRV isolate Mara Eglite genome assembled from HTS data. In this report, we have documented the natural occurrence of BCaRV in Latvia, which makes it a second evidence on the presence of BCaRV in Europe.