Crucifer crops (Brassica spp.) are important winter vegetables in Taiwan. Five viruses, including Turnip mosaic virus (TuMV), Cucumber mosaic virus (CMV), Radish mosaic virus (RaMV), Beet western yellows virus (BWYV), and Cauliflower mosaic virus (CaMV), have been detected in a range of domestic-grown crucifers during past decades (1). Field mustard plants (Brassica chinensis) showing mosaic in the leaves were collected in the ChiaYi area in December 2007. Spherical virus-like particles, approximately 30 nm in diameter, were readily observed in crude sap of symptomatic plants. Tests by ELISA failed to detect any of the aforementioned viruses. A spherical agent was isolated through mechanical inoculation onto Chenopodium quinoa, and a virus culture was established and inoculated mechanically back to the original host as well as other crucifers. Systemic mosaic appeared on inoculated B. campestris, B. chinensis, and B. juncea, whereas ringspots appeared on inoculated leaves of B. oleracea. Total RNA was extracted from symptomatic leaves and used for reverse transcription (RT)-PCR amplification using degenerate primers for comoviruses (2). Other successive fragments of RNAs 1 and 2 were amplified by specific or degenerate primers designed on the basis of sequences of published Turnip ringspot virus (TuRSV). The RNA 1 (GenBank Accession No. GU968732) and RNA 2 (No. GU968731) of the isolated virus consisted of 6,076 and 3,960 nucleotides, respectively. The number of nucleotides and the arrangement of open reading frames on both RNA 1 and RNA 2 were similar to those of comoviruses. Sequence analysis revealed that the nucleotide sequences of RNA 1 and RNA 2 shared 54.2 to 82.5% and 50.2 to 79.3% similarities, respectively, to those of comoviruses and were most similar to Turnip ringspot virus. The deduced peptides of large and small coat proteins (LCP and SCP) contain 375 amino acids (41.2 kDa) and 251 amino acids (28.5 kDa), respectively. The deduced amino acid sequences of RNA-dependent RNA polymerase (RdRp), LCP, and SCP share 92.0 to 94.5%, 93.1 to 93.3% and 87.3 to 89.6% similarity, respectively, to those of published TuRSV isolates, i.e., -B (GenBank Accession No. GQ222382), -M12 (No. FJ516746), and -Toledo (No. FJ712027) indicating that the newly isolated virus from field mustard in Taiwan is an isolate of TuRSV, hence TuRSV-TW. Comparison of LCP and SCP between current TuRSV-TW and Radish mosaic virus (RaMV; GenBank Accession No. AB295644) showed 74% similarity, which is below the species demarcation level of 75% (3), indicating its discrimination from RaMV. To our knowledge, this is the first report of the occurrence of TuRSV in Taiwan and in the subtropics.
References: (1) T. H. Chen et al. Plant Pathol. Bull. 9:39, 2000. (2) V. Maliogka et al. J. Phytopathol. 152:404, 2004. (3) K. Petrzik and I. Koloniuk. Virus Genes 40:290, 2010.