High throughput RNA sequencing discovers symptomatic and latent viruses: an example from ornamental Hibiscus

Hibiscus rosa-sinensis L. (Hibiscus, Malvaceae) is an ornamental species grown widely in amenity plantings. We collected leaves on an urban roadside pavement (sidewalk) near a market in Guangzhou which showed multiple symptoms of leaf rolling, deformation and chlorosis. Initial evaluation by electron microscopy using negative staining of drip preparations revealed the presence of tobamovirus-like particles. Total RNA was extracted, and, unusually, without any RNA selection based on sequence, was used for cDNA library construction and high-throughput survey sequencing. From the 814 Mb of clean sequence data (from 2,712,161 paired reads of 150 bp) reads representing chloroplast, ribosomal, and mitochondrial genes were filtered out, eliminating 79.1% of reads. 1,135,848 × 150 bp of the sequence was retained and screened for viral sequences. Assembly of these sequences detected nine virus species from seven virus genera comprising three tobamoviruses, namely, Tobacco mosaic virus, Tobacco mild green mosaic virus and Hibiscus latent Singapore virus, Turnip mosaic virus (Potyvirus), Potato virus M (Carlavirus), Hibiscus chlorotic ringspot virus (Betacarmovirus), Fabavirus sp (Fabavirus), Cotton leaf curl Multan virus (Begomovirus) and a putative mitoviruses replicating in mitochondria, Chenopodium quinoa mitovirus 1. Mapping the reads to complete virus reference sequences showed high and uniform coverage of the genomes from 3,729 x coverage for Turnip mosaic virus to 22 x for Cotton leaf curl Multan virus. By comparison, nuclear reference genes actin showed 14 x coverage and polyubiquitin 27 x. Notable variants from reference sequences (SNPs) were identified. With the low cost of sequencing and potential for semi-automated bioinformatic pipelines, the whole-RNA approach has huge potential for identifying multiple undiagnosed viruses in ornamental plants, resulting in the ability to take preventive measures in production facilities against spread and to product quality for the mutual benefit of producers and consumers.


INTRODUCTION
Hibiscus rosa-sinensis (H. rosa-sinensis)is a common ornamental plant in subtropical and tropical areas, including China, where the species is widely planted as single specimens, in hedges, or in landscaping, in gardens and for amenity horticulture, due to their dense growth and attractive flowers (Adkins et al., 2006). Hibiscus extracts are also used for tea, and in skin ointments with traditional healing uses (Ryoo et al., 2010;Tsai, 2016). Related species are used as textile fibres (kenaf) and planted in temperate gardens. Both RNA and DNA viruses infect H. rosa-sinensis worldwide (Sastry et al., 2019)  betacarmo, rhabdo-, tobamo and begomoviruses (Table 1) where they may reduce quality and cause economic losses.
Virus-infected H. rosa-sinensis can display various symptoms. Infection with Hibiscus chlorotic ringspot virus (HCRSV) exhibits mottling or chlorotic spots, rings, or vein-banding symptoms under natural conditions, which may disappear during the summer months (Jones and Behncken, 1980). Infection with Cotton leaf curl Multan virus (CLCuMuV), exhibits leaf curling, vein swelling, vein darkening and enations on the veins on the undersides of the leaves (She et al., 2017). Visual in/spection of symptomatic phenotypes needs verification by additional analysis since symptom expression can be influenced by various abiotic and biotic parameters. Symptoms may also vary (and typical symptoms may be modified) in plants with mixed infections, and expression can vary depending on the cultivar.

Plant virology and virus diagnostics use multiple technologies including bioassays and
serological testing (ELISA), electron microscopy and PCR methods (RT-PCR, qPCR) as well as transmission experiments for virus characterization. In 2009, high-throughput sequencing technology (HTS) entered the field of plant virology (Kreuze et al., 2009) and it has been a valuable addition to plant virus detection and identification tools, along with the reference sequences in databases such as GenBank. Like electron microscopy, HTS does not need previous knowledge on the viruses' infection characteristics, nor specific antibodies or sequence amplification primers. The International Committee on Taxonomy of Viruses (ICTV) has recognized the use of metagenomic sequencing data and applied it to virus taxonomy (Simmonds et al., 2017 In this study, we aimed to detect viruses by high-throughput survey sequencing of RNA, without previous knowledge of the viruses present or their infection characteristics.
Analyzing the viral genome sequence and establishing a rapid and sensitive virus detection method is of great significance for the prevention and control of viral diseases.

Plant material
Symptomatic leaves of H. rosa-sinensis were sampled from plants growing in soil squares on an urban roadside pavement (US: sidewalk) near Changban market, Guangzhou, China (23º10'E, 113º21'N, Xingke Rd/Lu, Tianhe Qu, 23.174231N, 113.361287). Fresh leaves from a single plant, collected without contamination, were rinsed with water and used for RNA extraction.

Transmission electron microscopy
A mixed sample of symptomatic leaves was homogenized in 2-5-fold volume of extraction buffer (0.1 M phosphate buffer pH 7.0 containing 2 % polyvinylpyrrolidone MW 11.000 and 0.2 % sodium sulfite). Viral particles were adsorbed by floating a pioloform carbon-coated copper grid on the leaf extract. For negative contrast the grids were stained with 1 % uranyl acetate in ultrapure water, dried and analysed by transmission electron microscopy (TEM) in a Tecnai G2 Spirit TWIN Transmission Electron Microscope (Czech Republic) with 80 kV accelerating voltage. Digital images were acquired with a GATAN camera and processed using TIA software (Tecnai Imaging and Analysis). Beads, #E7775S) with appropriate barcoded sequencing primers (NEB #E7335-12 indices).

High-throughput sequencing, assembly, and analysis of viral sequences
The cDNA library was sequenced by Wuhan Bena Technology Service using Illumina Hiseq X-ten (2×150 bp paired-end with 350 bp insert size average). A total of 814 Mb of sequence with the appropriate barcode primers was extracted.
The paired-end reads were assembled to known abundant reference sequences of the Hibiscus (https://www.ncbi.nlm.nih.gov/nuccore/?term=) before de novo assembly of the remaining reads (Geneious assembler, Medium-low sensitivity/Fast setting). Contig sequences were analysed using BLAST comparisons (National Centre for Biotechnology Information (NCBI) database). Where the sequence showed homology to known viral genomes, these were used as a reference target for reassembly of the reads as used for the preliminary assembly ( Figure 1).

Sequence comparisons and phylogenetic analyses
The virus complete genome was aligned with those of reference sequences published in NCBI, using Clustal W sequence alignment. Phylogenetic trees were generated by the MAGE X (Kumar, 2018) software with 1,000 bootstrap replicates.

Identification of viruses in Hibiscus rosa-sinensis samples
Leaves of H. rosa-sinensis showing leaf curl and chlorosis (

Sequencing, assembly, and identification of viral sequences
The unselected RNA library from H. rosa-sinensis had a total of 5,424,322 high-quality reads. After mapping to the rDNA sequence (KX709342), chloroplast genome (MK937807) and mitochondrial genome (NC_035549), 20.9% of clean reads remained. The proportion of rDNA, chloroplast and mitochondria removed were 51.0%, 26.1% and 1.1% respectively, plus 0.9% of related unpaired reads. De novo assembly of the remaining reads ("screened RNA reads"), generating 19,776 contigs, including Hibiscus nuclear transcripts as well as viral RNA sequences. Contigs were compared to the NCBI database by BLAST and viral genomes were identified. The screened DNA reads were then assembled to these reference viral sequences.     A phylogenetic analysis ( Figure 5) was carried out for the well-characterised TMGMV using 16 related TMGMV isolates (Table 3) sequences deposited in GenBank. The sequence identified here groups most closely with a TMGMV isolate from Capsicum annuum (chilli) found in the Hunan province, in central and southern China (Table 3). mean that RT-PCR will be at least as expensive as whole RNA sequencing used here. Arguably, the high-throughput results are more accurate with fewer experimental steps and no primer selection.
The data reveal two cases for expansion of the host range of viruses (Table 1 vs Table 2