First report of Tomato spotted wilt orthotospovirus infecting agapanthus (Agapanthus praecox) in South Africa.

Abstract

Agapanthus praecox Willd. is an ornamental flowering plant that is indigenous to southern Africa and was reported to be a host of tomato spotted wilt orthotospovirus (TSWV) in Australia in 2000 (Wilson et al. 2000). Tomato spotted wilt orthotospovirus (TSWV) belonging to the genus Orthotospovirus of the family Tospoviridae is a single-stranded negative sense RNA virus known to cause disease symptoms in many crops and ornamental plant species. This virus is in the top 10 of most economically important plant viruses worldwide (Rybicki 2015; Scholthof et al. 2011). In May 2021, leaf material from three agapanthus (Agapanthus praecox) plants displaying chlorotic mottling, and yellow lesions (Supplementary material 1A) was collected in Mbombela, South Africa. One gram of symptomatic leaf material was used for total RNA extraction from each of the three samples using a CTAB extraction protocol (Ruiz-García et al. 2019). The three RNA extracts were pooled, and a sequencing library was constructed using the Ion Total RNA-Seq Kit v2.0 and RiboMinus™ Plant Kit for RNA-Seq (ThermoFisher Scientific) (Central Analytical Facility (CAF), Stellenbosch University). The RNA library was sequenced on an Ion Torrent Proton Instrument (CAF). A total of 34,392,939 single-end reads were obtained. Data was trimmed for quality with Trimmomatic (CROP:250, MINLEN:50). De novo assembly was performed on the remaining 32,281,645 trimmed reads (average readlength: 100 nt, range: 50-250 nt) using SPAdes 3.13.0 and resulted in 4,788 contigs. BLASTn analysis identified viral contigs longer than 1,000 nucleotides (nts) with high nucleotide (nt) identity to TSWV (6 contigs), as well as to the newly discovered viruses, agapanthus tungro virus (AgTV) (1 contig), and agapanthus velarivirus (AgVV) (4 contigs) (Read et al 2021). Read mapping was performed against the relevant reference sequence with the highest nt identity to the contigs. For TSWV, 4995, 21221 and 14574 reads mapped to segment L (KY250488), M (KY250489) and S (KY250490) of isolate LK-1, respectively resulting in 99.97%, 100.00% and 99.97% genome coverage of the reference accessions. The nt identity between the reference accessions and the consensus sequences generated (OP921761-OP921763) were 97.26%, 97.64% and 97.82% for segment L, M and S. The presence of TSWV was confirmed in the HTS sample using an RT-PCR assay (primers L1 and L2) targeting the L segment of TSWV (Mumford et al. 1994). In July 2022, additional leaf samples displaying symptoms of chlorotic mottling, streaking, and ringspots were collected from 31 symptomatic and 3 asymptomatic agapanthus plants in public gardens in Stellenbosch, South Africa. Using the above-mentioned RT-PCR assay, 13 of the symptomatic samples tested positive for TSWV. All six plants displaying ring spot symptoms (Supplementary material 1B) were infected with TSWV. However, plants that displayed yellow streaking (five samples) and chlorotic mottling (two samples) (Supplementary material 1C-D) were also positive for TSWV which could be due to the presence of other viruses, plant growth stage, infection time or just variable symptom expression in a single host species as reported previously (Sherwood et al. 2003). The 275 bp RT-PCR amplicons of the HTS sample and three additional positive samples were validated with bidirectional Sanger sequencing (CAF) and had 96% identity to accession KY250488. The pairwise nt identity between amplicons was 98.55-100%. This is the first report of TSWV infecting agapanthus in South Africa. This study contributes information towards the distribution and incidence of TSWV and highlights the need for nurseries to screen plant material before propagation.