Tulip virus X (tulip virus X, TVX) is a member of the genus Potexvirus (family Alphaflexiviridae) and is a positive single-stranded RNA virus. TVX was described first in Scotland (Mowat 1982), followed by several countries (Yamaji et al. 2001; Tzanetakis et al. 2005; Ward et al. 2008; Dees et al. 2011; Sochacki and Komorowska 2012; Wylie et al. 2019). In April 2021, 86 whole tulip plants showing viral symptoms in leaves (mosaic, yellowing, and malformation) and flowers (color breaking) were collected in Chilgok, Chuncheon, Goseong, Yecheon and Yesan, Korea. Furthermore, high-throughput sequencing was performed to identify viruses that infect tulips in Korea. Total RNA was extracted from pooled the leaves and petals using a Maxwell® 16 LEV Plant RNA Kit (Promega, Madison, USA). We constructed a single library using the TruSeq Stranded Total RNA LT Sample Prep Kit for Plant (Illumina, San Diego, USA). The library was 100 bp paired-end sequenced using Illumina's NovaSeq 6000 (Macrogen, Seoul, Korea) and was assembled de novo using Trinity software version trinityrnaseq_r20140717, with default parameters. The contigs were annotated as in previous study (Lee et al. 2020), revealing a single contig each related to TVX, lily symptomless virus (LSV), and tulip breaking virus (TBV) was generated from 648 million total reads. The TVX-related contig (GenBank ON205948) consisting of 6,076 bp showed 99.52% nucleotide identity (6027/6056 bp) with TVX-J (GenBank AB066288). We conducted an RT-PCR assay to validate the presence of viruses with specific primers as TVX-F5093/R5624 (5'-CTATCCGGACTCATTCTACTTC/GTGCGTTCCAGATAAGCTTG-3'), LSV-F7013/R7338 (5'-CTTGGTCGACAGGGACATAAC/GATTGGAATTGTGCTTTTCAGC-3'), and TBV-F7515/R8116 (5'-GTGTGTCATGGATGATTGTTG/CAACTGATTTGCTACCGCTAG-3'). Consequently, TVX were detected in 13 of 86 samples. Moreover, LSV and TBV were detected in 15 and 26 samples, respectively. However, the yellowing and mosaic observed in the TVX infected samples were not observed in the LSV and TBV infected samples. Subsequently, two TVX amplicons were selected, cloned and sequenced. The obtained sequences were 532 bp and were named YS24 and YS38 (GenBank LC664027 and LC664028), respectively. The Korean isolates showed 98.68% (525/532 bp) and 99.62% (530/532 bp) identity with Australian isolate (GenBank MH886522) in BLASTn analysis. To bioassay for TVX, the infected tulip leaf tissue from which YS24 was obtained was used to sap-inoculate, in triplicates, 15 species of indicator plants (Nicotiana benthamiana, N. clevelandii, N. debneyi, N. glutinosa, N. rustica, N. tabacum, Datura stramonium, Glycine max, Phaseolus vulgaris, Chenopodium amaranticolor, C. quinoa, Cucumis sativus, Cu. melo, Gomphrena globosa, and Tetragonia tetragonioides). After 14 days of inoculation, we observed distinct chlorotic spots on inoculated and upper leaves of C. quinoa, but no symptoms were observed in other indicator plants. In RT-PCR assay using TVX-specific primers, only C. quinoa showed a positive reaction. In previous studies, C. amaranticolor, C. quinoa, G. globosa, and N. benthamiana were known as the experimental host of TVX (Dees et al. 2011; Tzanetakis et al. 2005), but only C. quinoa was confirmed to be susceptible to the Korean isolate. Furthermore, transmission electron microscopy revealed typical flexuous rod-shaped viral particles in the inoculated C. quinoa. To our knowledge, this is the first report of TVX infecting tulips in Korea.
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