First Report of Youcai Mosaic Virus Infecting Yam in China.

Yam (Dioscorea opposita Thunb.) is cultivated mainly as a functional food and for nutritional and medicinal purposes in China (1). It is propagated through tubers and this facilitates the spread and accumulation of viruses in the crop, eventually leading to yield losses (2). At present, different virus species belonging to the genera Aureusvirus, Badnavirus, Carlavirus, Comovirus, Cucumovirus, Fabavirus, Macluravirus, Potexvirus and Potyvirus have been reported in yams (3) and fifteen viruses in these genera have been detected in China. In July 2020, a survey of viral diseases on yam was conducted in plantations of Wenxian and Mengzhou counties in Henan Province, China. Fifty-four leaf samples of Dioscorea opposite showing mosaic and leaf discoloration (Supplementary Fig1) were collected from eight fields (five to ten plants per field). These leaf samples were ground in liquid nitrogen and total RNA was extracted from a portion of the mixed powder using RNAprep Pure Plant Plus Kit (TIANGEN Biotech, Beijing, China). A cDNA library was constructed using NEBNext Ultra RNA Library Prep Kit for Illumina (NEB, USA) after ribosomal RNA depletion using Ribo-off rRNA Depletion Kit (Vazyme Biotech, Nanjing, China), and sequenced on the Illumina NovaSeq 6000 system at the Berry Genomics Corporation (Beijing, China). A total of 87,075 contigs (>200 bp) were generated from de novo assembly (CLC Genomic Workbench 10.0) from a total of 34,656,172 paired-end reads. After BLASTn analysis, three contigs with the length of 1009, 1340 and 1859 nucleotides shared 96.33%, 96.72% and 96.29% nt identity respectively with youcai mosaic virus SX isolate, a tobamovirus (YoMV GenBank accession no. JX422022). In addition to YoMV, broad bean wild virus 2 and yam latent virus were also identified, which had previously been reported in yams in China. To confirm the NGS result, total RNAs were extracted from fifty-four above-mentioned samples and RT-PCR was carried out to amplify a 528 bp fragment of the coat protein (CP) of YoMV by using a pair of specific primers CP gene. PCR products with expected size were obtained from 26 out of 54 samples, and seventeen amplicons of YoMV-CP were sequenced (accession nos. ON052726 to ON052742). The nt sequence identities of CP gene among these seventeen isolates were 99.6%-100%. Furthermore, the near-full-length genomic sequence of YoMV-Do41 isolate was obtained from sample 41 by RT-PCR amplification of four overlapping fragments using the following primer pairs: YoMV-15F/YoMV-1910R, YoMV-1770F/YoMV-3750R, YoMV-3645F/YoMV-5404R and YoMV-4921F/YoMV-6280R (Supplementary Table1). The YoMV-Do41 isolate was 6, 274 nt in length (accession no. ON149803) and shared 89.65% and 97.31% nt identities to As1-2 isolate (GenBank accession no. MW307290) and to SX isolate (accession no. JX422022), respectively.To the best of our knowledge, this is the first report of YoMV infecting yam in China. YoMV has a wide host range including genera Impatiens, Rehmannia, Brassica, Chelidonium, Trifolium, Crossandro, Alstroemeria, Stellaria. This study will serve as an important reference for the host range of YoMV. According to the detection rate infections with YoMV in yam are common in these producing regions. Further studies will be required to determine the infection rate in other producing regions and the potential threat posed by YoMV on yam production should be considered.