First report of Cactus virus X infecting Hylocereus undatus in Korea

Hylocereus undatus is widely produced in the tropics, and was first introduced to Juju Island, South Korea, from Vietnam in 1999. In October 2010, 13 symptomless H. undatus were collected at greenhouses in A and B regions of Jeju Island. Viral particles were observed in all five samples from region B alone, by the direct negative staining (Horne and Wildy 1964) and ultrathin sections staining method (Choi et al. 2005) using transmission electron microscopy (Carl Zeiss, Oberkochen, Germany). Flexuous rod particles were observed in the sap of the fruit samples. The lengths of the particles ranged from 430 to 520 nm, and the infected H. undatus cells formed inclusion bodies. According to the large-scale oligonucleotide (LSON) chip assay, a method that efficiently identifies viruses in plants (Nam et al. 2014), the infecting virus in H. undatus was Cactus virus X (CVX) based on spot intensity. Total RNA was extracted from eight and five samples from regions A and B, respectively, using an Easy-Spin Total RNA Kit (iNtRON, Sungnam, Korea). CVX was tested for by RT-PCR (primers, F: 5′-CCACCAACCTCACTCCACTATCAC-3′ and R: 5′-CCTTGGAGCCCCTTGGTGA-3′) with a Suprimescript RT-PCR premix (Genet Bio, Daejeon, Korea). RT-PCR results showed the eight samples from region A were negative for CVX, whereas the five samples from region B were positive for CVX. Whole-genome sequencing was performed on one of the CVX isolates from region B, designated CVX-dragonfruit [LC128411], and its size was 6,618 nucleotides, excluding the poly(A) tail. We compared the genomic sequence of CVX-dragonfruit [LC128411] with the CVX-Hu [AF308158] isolate from Taiwan. The two isolates had 96.3% nucleotide sequence homology and 95.3% amino acid sequence homology. The coat protein (CP) of CVX-dragonfruit displayed homologies of 94.2%, 97.5%, and 76.9%, compared with those of the CVX-Hu [AF308158], CVX-TW [KM365479], and CVX-NTU [JF937699] H. undatus isolates from Taiwan, respectively. Based on these data, our isolate was identified as a member of CVX within the genus Potexvirus. To determine the host range of the CVX-dragonfruit, 29 indicator plants from 28 species were mechanically inoculated with crude sap of virus-infected H. undatus suspended in 0.01 M potassium phosphate buffer (pH 7.2). The experiment was repeated three times. In our sap transmission tests, CVX symptoms were observed 20 days after the inoculation. Chenopodium amaranticolor and C. quinoa developed chlorotic spots or lesions and Gomphrena globosa formed necrotic lesions with red margins; however, the other 25 indicator plants species did not show any symptoms. The first report of CVX in South Korea was in cacti in 1990 (Choi et al. 1990), and the first report of CVX in H. undatus was in Taiwan in 2000 (Liou et al. 2000). In this study, we show the first isolation of CVX from H. undatus in South Korea and report its characteristics in relation to the other CVX strains. This report suggests there should be additional work to investigate and describe the biological significance of these strains. Although no visible symptoms were present in the infected fruit and infection did not cause considerable economic loss, CVX could be easily introduced into other greenhouses by cutting or grafting.