Five Newly Collected Turnip Mosaic Virus (TuMV) Isolates from Jeju Island, Korea are Closely Related to Previously Reported Korean TuMV Isolates but Show Distinctive Symptom Development

Abstract

For several years, temperatures in the Korean peninsula have gradually increased due to climate change, resulting in a changing environment for growth of crops and vegetables. An associated consequence is that emerging species of insect vector have caused increased viral transmission. In Jeju Island, Korea, occurrences of viral disease have increased. Here, we report characterization of five newly collected turnip mosaic virus (TuMV) isolates named KBJ1, KBJ2, KBJ3, KBJ4 and KBJ5 from a survey on Jeju Island in 2017. Full-length cDNAs of each isolate were cloned into the pJY vector downstream of cauliflower mosaic virus 35S and bacteriophage T7 RNA polymerase promoters. Their fulllength sequences share 98.9-99.9% nucleotide sequence identity and were most closely related to previously reported Korean TuMV isolates. All isolates belonged to the BR group and infected both Chinese cabbage and radish. Four isolates induced very mild symptoms in Nicotiana benthamiana but KBJ5 induced a hypersensitive response. Symptom differences may result from three amino acid differences uniquely present in KBJ5; Gly(382)Asp, Ile(891)Val, and Lys(2522)Glu in P1, P3, and NIb, respectively.

Fig. 1. Symptoms of the five isolates (KBJ1-KBJ5) inoculated to Nicotiana benthamiana, shown at 7 days post inoculation (dpi) and 21 dpi; and to radish (Raphanus sativus) cultivars Chunghua and Iljin at 14 dpi. Representative plants inoculated with each isolate are shown. TuMV infection of all radish plants was confirmed by RT-PCR.

Fig. 2. Amino acid residues differing among isolates KBJ1-5. Isolate KBJ-5 is marked in red because it can induce systemic necrosis in Nicotiana benthamiana. Differing amino acid residues are shown in red or blue, with unique KBJ5 residues 382 (HC-Pro), 891 (P3) and 2522 (NIb) marked by a red box.

Fig. 3. Phylogenetic tree constructed by the maximum-likelihood method with 1,000 bootstrap replicates, based on the complete genome sequences of 58 TuMV isolates including twenty other Korean isolates (Gong et al., 2019) and the five new isolates from Jeju (indicated by arrows to the right of the isolate/accession number/country label).

Fig. 4. (A) Structure of the chimeras constructed between mild isolate KBJ4 and severe isolate KBJ5. Chimeras AAB and BBA were generated by exchange of a 5ʹ ApaI/SpeI fragment; to create chimeras ABB and ABA, mutagenic PCR was utilized to alter a single nt resulting in I891V in the ApaI/SpeI fragment. (B) Each chimeric construct was then used to infect Nicotiana benthamiana, and radish cultivars Chunghua and Iljin. Images of representative plants were captured at 24 days post inoculation (dpi) for N. benthamiana, and 14 dpi for radish.