First Report of Soybean Mosaic Virus of Sword Bean in South Korea

Abstract

Soybean mosaic virus (SMV) is a member of the genus Potyvirus in the family Potyviridae. Legume crops are often infected by SMV. SMV has not been naturally isolated from sword bean (Canavalia gladiata) in South Korea. In July 2021, 30 samples of sword bean were collected at the field located in Hwasun and Muan, Jeonnam, Korea to investigate viruses infecting sword bean. The samples exhibited symptoms typical of viral infection such as mosaic pattern and, mottling of leaves. Reverse transcription polymerase chain reaction (RT-PCR) and reverse transcription loop-mediated isothermal amplification (RT-LAMP) techniques were employed to identify the agent of viral infection in sword bean samples. Total RNA was extracted from the samples using the Easy-SpinTM Total RNA Extraction Kit (Intron, Seongnam, Korea). Out of the 30 samples, seven were found to be infected by the SMV. RT-PCR was performed using RT-PCR Premix (GeNet Bio, Daejeon, Korea) with SMV-specific primer set, forward primer (SM-N40, 5′-CATATCAGTTTGTTGGGCA-3′) and the reverse primer (SM-C20, 5′-TGCCTATACCCTCAACAT-3′), yielding a product of 492 bp (Lim et al., 2014). RT-LAMP was performed using RT-LAMP Premix (EIKEN Chemical, Tokyo, Japan) with SMV-specific primer set, the forward primer (SML-F3, 5′-GACGATGAACAGATGGGC-3′, SML-FIP, 5′-GCATCTGGAGATGTGCTTTTGTGGTTATGAATGGTTTCATGG-3′) and reverse primer (SML-B3, 5′-TCTCAGAGTTGGTTTTGCA-3′, SML-BIP, 5′-GCGTGTGGGTGATGATGGATTTTTTCGACAATGGGTTTCAGC-3′) for diagnosis of viral infection (Lee et al., 2015). The full coat protein genes of seven isolates were amplified using RT-PCR to determine their nucleotide sequence. The standard nucleotide BLAST (blastn suite) showed that the seven isolates had approximately 98.2–100% homology with SMV isolates (FJ640966, MT603833, MW079200, and MK561002) in NCBI GenBank. The sequences of seven isolates were deposited in the GenBank database under the accession numbers: OP046403–9. For the pathogenicity assay of the isolate, the crude saps from SMV-infected samples were mechanically inoculated into sword bean. Fourteen days after inoculation, the mosaic symptoms were observed on the upper leaves of sword bean. As a result of the RT-PCR diagnosis in the upper leaves, SMV was reconfirmed in sword bean. This is the first report of natural SMV infection in sword bean. As sword beans are increasingly consumed for teas, transmitted seeds are resulting in a decrease in pod production and quality. It is necessary to develop efficient methods of seed processing and management strategies to control SMV infection in sword bean.