First Report of Alternanthera Mosaic Virus Infecting Epiphyllum spp.

Epiphyllum spp. (orchid cactus) is a group of epiphytic plants in the cactus family grown as flowering ornamentals. Symptoms of virus infections are frequently observed in them (Chessin 2002). In 2015, 13 Epiphyllum hybrids (seven with chlorotic ring spots, mottle, or mosaic symptoms and six without any symptoms) were tested by RT-PCR/PCR using specific or group primers for cactus mild mottle virus, cactus virus X, impatiens necrotic spot virus, saguaro cactus virus, zygocactus virus X, badnavirus, and potyvirus, respectively. None of these viruses were detected from a hybrid ‘Dragon Heart’ showing irregular chlorotic patches on its stems. This plant was tested in subsequent high-throughput sequencing analysis for identification of viruses and viroids. Total RNAs were extracted from stem tissue using an RNeasy Plant Mini Kit (Qiagen, Germantown, MD) and subjected to Illumina NextSeq 500 sequencing (SeqMatic, Fremont, CA). Analysis of the RNA reads of 75 nucleotides (nt) revealed the presence of alternanthera mosaic virus (AltMV, 213,038 reads, 2,445x) and a new cavemovirus in the Dragon Heart plant. Infection of AltMV-DH was confirmed by RT-PCR using AltMV-detF6c (5′-TCACYCAGGAACAGATGRACG-3′) and AltMVR6 (5′-TAGGTCTACAGAAGCGGAAGC-3′). To obtain the genomic sequence of this AltMV isolate, RT-PCR was conducted using six overlapping primer pairs designed according to the alignment of the AltMV type species (GenBank no. NC_007731) and AltMV contig obtained in this study. The RT-PCR amplicons were cloned into a pGEM-T easy vector (Promega, Fitchburg, WI) and sequenced (MCLAB, South San Francisco, CA). The complete genomic sequence of 6,633 nt (GenBank no. MH423501) was assembled from six overlapped amplicon sequences. BLAST search showed that this isolate had the highest nucleotide sequence identity of 94 to 95% with four AltMV isolates available in the GenBank databases. Stem tips from the Dragon Heart plant were grafted onto three plants of hybrid ‘King’s Ransom’, a hybrid that tested negative for the nine viruses included in this study. Three grafted plants showed mottling symptoms 6 months after inoculation. AltMV was detected in new growth of all three grafted plants, indicating the virus was graft transmissible. A total of 30 additional Epiphyllum hybrids were tested by RT-PCR, and AltMV was detected from four symptomatic hybrids (‘Clarabelle’, ‘Mark Twin’, ‘Professor Ebert’, and ‘Over the Top’) and two asymptomatic hybrids (‘Cream and Gold’ and ‘Montrose’). AltMV is a member of the genus Potexvirus in the family Alphaflexiviridae. The virus was first reported from Alternanthera pungens in Australia (Geering and Thomas 1999) and has been found from plants of at least eight families in the United States, Europe, and Brazil (Hammond and Reinsel 2015). To our knowledge, this is the first report of AltMV in orchid cactus. It is not clear if AltMV alone causes symptoms in Epiphyllum spp., because two of the positive hybrids were asymptomatic, but the infected cactus may serve as a reservoir from which AltMV may spread to other hosts. We are currently characterizing the new cavemovirus detected in the hybrid Dragon Heart.