White clover (Trifolium repens L.) is a herbaceous, perennial plant that has become one of the most widely distributed legumes in the world. It is extensively used in grass-legume pastures, but also has the potential to invade agricultural lands and natural ecosystems. White clover is a well-known natural host for Alfalfa mosaic virus (AMV), Clover yellow vein virus (ClYVV), Soybean dwarf virus (SbDV), Beet western virus (BWYV), Tomato spotted wilt virus (TSWV), Zucchini yellow mosaic virus (ZYMV), etc (1). In July 2013, during a survey to determine the presence of different viruses infecting weed plants in South Korea, three white clover leaf samples showing yellow mosaic symptoms were collected from Taean County, South Chungcheong Do Province, South Korea. In order to identify the infecting virus, total RNA from three leaf samples was extracted using the Tri-reagent (MRC Reagent, Inc., OH) as described by the manufacturer, and was applied to the large-scale oligonucleotide (LSON) chip (3), wherein probes specific to a ClYVV isolate produced a positive reaction. All three samples tested were positive for ClYVV. To confirm this result, ClYVV-specific primers were designed using the sequences of four ClYVV isolates from NCBI (GenBank Accession Nos. AF185959, AF203536, DQ333346, and NC003536). Total RNA was extracted from symptomatic white clover samples using Easy-Spin Total RNA Extraction Kit (iNtRon, Daejeon, Korea) and used as template for RT-PCR. The positive control RNA was used from ClYVV GM isolate (KF975894) and negative control RNA used symptomless white clover plants. The ClYVV coat protein (CP) gene was amplified by RT-PCR using the specific primer pairs ClYVV-CP-F / ClYVV-CP-R (5′-CAAGAGCAGCACGATGAG-3′ and 5′-CTCGCTCTATAAAGATCAGAT-3′). DNA fragments of the expected size (1,042 bp) were obtained from the white clover Korea isolate (AB930132), and the PCR product was cloned into a T&A cloning vector (RBC Bioscience, Taipei, Taiwan) and sequenced directly in both directions. BLAST analyses of the nucleotide sequence CP gene fragments revealed the highest identity with 98% with other ClYVV isolates (AF203536). To determine the experimental host range of the ClYVV Korea isolate, we inoculated five species (Chenopodium amaranticolor, C. quinoa, Nicotiana clevelandii, N. benthamiana, and Trifolium repens) in three families using this isolate. All test plants were mechanically inoculated with 0.1 M phosphate buffered saline (Takara, Tokyo, Japan). Each test plant was inoculated nine times and grown in a greenhouse maintained at 27 to 33°C. Necrotic local lesions were produced on inoculated leaves of C. amaranticolor, C. quinoa, and N. clevelandii 4 to 6 days post-inoculation. After 10 to 14 days, C. amaranticolor and C. quinoa showed systemic chlorotic spot symptoms, and N. clevelandii, N. benthamiana, and T. repens showed chlorotic spot, mild mosaic, and mosaic in the upper leaves, respectively. Up to now, in South Korea, ClYVV has been detected in gladiolus (Gladiolus gandavensis) (3) and soybean (Glycine max) (4). ClYVV can be easily transmitted by insect, aphid, or mechanical inoculation and has a host range including tobacco, soybean, etc. The presence of ClYVV could become an important threat to crop production in South Korea. To our knowledge, this is the first report of a ClYVV infection of the white clover plant in South Korea.
References: (1) B. L. Denny and P. L. Guy. Australas. Plant Pathol. 38:270, 2009. (2) M. Nam et al. Plant Pathol. J. 30:51, 2014. (3) I. S. Park et al. Korean J. Plant Pathol. 14:74, 1998. (4) J. C. Shin et al. Plant Dis. 98:1283, 2014.
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