Shuhao Lu

Rehmannia glutinosa, a crucial medicinal plant native to China, is extensively cultivated across East Asia. We used high-throughput sequencing to identify viruses infecting R. glutinosa with mosaic, leaf yellowing, and necrotic symptoms. A novel Torradovirus, which we tentatively named “Rehmannia torradovirus virus” (ReTV), was identified. The complete sequences were obtained through reverse-transcription polymerase chain reaction (RT-PCR), 5′ and 3′ rapid amplification of cDNA ends, and Sanger sequencing. The amino acid sequence alignment between the ReTV-52 isolate and known Torradovirus species in the Pro-Pol and coat protein regions were 51.3–73.3% and 37.1–68.1%, respectively. Meanwhile, the amino acid sequence alignment between the ReTV-8 isolate and known Torradovirus species in the Pro-Pol and coat protein regions were 52.7–72.8% and 36.8–67.5%, respectively. The sequence analysis classified ten ReTV strains into two variants. The ReTV-52 genome has two RNA segments of 6939 and 4569 nucleotides, while that of ReTV-8 consists of two RNA segments containing 6889 and 4662 nucleotides. Sequence comparisons and phylogenetic analysis showed ReTV strains clustered within the Torradovirus, exhibiting the closet relation to the squash chlorotic leaf spot virus. The RT-PCR results showed a 100% ReTV detection rate in all 60 R. glutinosa samples. Therefore, ReTV should be classified as a novel Torradovirus species. ReTV is potentially dangerous to R. glutinosa, and necessitating monitoring this virus in the field.

Rehmannia glutinosa (also known as Chinese foxglove) is a perennial dicotyledonous herb, which plays an important role in traditional Chinese medicine. Its active ingredients have a wide range of pharmacological effects on the blood system, endocrine system, immune system, cardiovascular system, and nervous system (Zhang et al. 2008). In May 2022, leaf blight was observed on 45-day-old R. glutinosa in a seedling nursery in Jiaozuo City (35°01'44.20″N, 113°05'30.63″E), Henan Province, China with an approximate disease incidence up to 54% (~1,300 plants). Irregular brown lesion initially appeared on the tips of basal leaves, then progressed to the entire leaf causing leaf drying out (Supple. Fig. 1-A, B, C). The same symptoms appeared successively in the leaves from the base to the top of the plant, which eventually caused the whole plant to die. To identify the pathogen, eight symptomatic leaves were randomly collected from eight individual plants, and cut into small pieces (5 × 5 mm) at the border of lesions. The pieces were surface disinfected in 75% ethanol for 15 s, followed by 1% NaClO for 1 min, rinsed in sterile water three times, and placed on potato dextrose agar (PDA) medium in the dark for 3 days at 25℃. Finally, 12 purified isolates (DHY1-DHY12) were obtained by using single spore method. Leaves of R. glutinosa seedlings were inoculated with conidial suspension (106 conidia/ml), three plants were inoculated per isolate. Controls were treated with sterilized water. All inoculated and control plants were incubated in a greenhouse at 25℃ under 80 ± 10% humidity and a 8-h/16-h dark/light cycle. This experiment was repeated three times. After 5 days, similar symptoms to those of diseased leaves in the seedling nursery appeared on leaves inoculated with DHY4-DHY10, while plants inoculated with DHY1-DHY3, DHY11-DHY12, and the controls remained asymptomatic (Supple. Fig.1-D, E). The same fungi were re-isolated from diseased leaves, fulfilling Koch's postulates. The causal agents DHY4 to DHY10, showed similar morphology, which were morphologically identified as Aspergillus sp. (Visagie et al. 2014). Isolate DHY5 was selected for further study. On PDA plates, the colonies were covered with white velutinous mycelia (Supple. Fig.1-F). Conidia were ochre yellow and outwards concentric circles. Vesicles were globose, and about 20.1-26.6 μm in diameter (Supple. Fig.1-G). Conidiophore stipes were smooth walled and hyaline, with conidial heads radiating. The conidia were light yellow to orange, exudate clear to orange droplets. The conidia were (2.53-3.25) μm × (2.58-3.47) μm in diameter (n=50) (Supple. Fig.1-H). For further molecular identification, the ITS and TUB gene sequences were amplified with primer pairs ITS1/ITS4 and BT2a/BT2b (Glass and Donaldson. 1995), respectively. BLASTn searches of the ITS (PP355445) and TUB (PP382788) sequences showed 100% and 98.42% similarity to those of A. westerdijkiae (OP237108 and OP700424), respectively. Phylogenetic analysis based on the concatenated sequences of ITS and TUB confirmed that the fungus was A. westerdijkiae, (Supple. Fig.2). A. westerdijkiae was mainly reported on its secondary metabolite ochratoxin A contamination of agricultural products, fruits, and various food products, such as coffee beans (Alvindia et al 2016), grapes (Díaz et al. 2009), oranges and fruit juice (Marino et al. 2009), etc. To our knowledge, this is the first report of A. westerdijkiae causing leaf blight on R. glutinosa in China.

Rehmannia glutinosa is one of the most important medicinal plants in China and is affected by viral diseases. In this study, a new virus tentatively named Rehmannia Allexivirus virus (ReAV) was identified through high-throughput sequencing, reverse-transcription polymerase chain reaction (RT-PCR), and Sanger sequencing. The complete genome length was 7297 nt and it contained five open reading frames (ORFs) encoding replicase, triple gene block 1(TGB1), TGB2, TGB3, and coat protein (CP). The replicase and CP presented nucleotide homology ranges of 59.9–65.2% and 47.5–55.5% between the nine ReAV isolates and the other 12 species of the genus Allexivirus. In the nine isolates, ReAV-20 and ReAV-31 isolates showed breakpoints in the replicase and CP regions, respectively. The other isolates shared 87.2–96.5% nt with the whole genome nucleotide identity. The phylogenetic tree showed that seven ReAV isolates based on replicase, CP, and whole genome sequences were clustered in the same branch and were related to the genus Allexivirus. The ReAV detection rates for 60 R. glutinosa samples were 73.3–81.7% through RT-PCR using primers targeting the replicase or CP genes. These results demonstrate that ReAV is the dominant virus in R. glutinosa. This study provides important evidence for understanding viruses infecting R. glutinosa and for establishing efficient strategies to prevent viral spread.

Broad bean wilt virus 2 (BBWV-2), a member of the genus Fabavirus in the family Secoviridae mainly transmitted by aphids, has been recognized as a severe pathogen affecting the production of horticultural and ornamental plants worldwide (Xia et al. 2020). The virus was reported to infect many plant species mostly belonging to the family Fabaceae in China (Wang et al. 2017). In August 2018, marigold plants with the symptom of mosaic were observed in the field of Huairou, Beijing (Figure S1). Total RNA was extracted from symptomatic leaf samples from a single plant with TRIzol reagent (Invitrogen, Carlsbad, CA) with a standard procedure following the manufacturer's instructions, and small RNAs were isolated for deep sequencing library construction with Illumina TruSeq Small RNA Sample Prep Kit (Illumina, San Diego, CA, USA). The high-throughput sequencing was carried out on an Illumina HiSeq2500 platform. After raw data process, 8,911,917 clean reads were gained and further de novo assembled into contigs with CLC Genomics Workbench software. BLASTN and BLASTX analysis against the GenBank database showed that 81 of the 9,495 assembled contigs shared high nucleotide (nt) sequence identity with the bipartite genome of BBWV-2 isolate Gyp (KX686589-KX686590 for RNA1 and RNA2, 89% of the genome coverage and 90% nt identity) and 34 with high nt sequence identity of cucumber mosaic virus(CMV) from Tagetes erecta (EU665000-EU665002 for RNA1-RNA3,89% of the genome coverage and 96% nt sequence identity) with sequence coverage ranging from 24-fold to 8,078-fold at different genome positions. To further confirm the presence of BBWV-2, a RNA2 specific primer pair targeting the coat protein region (F1423-1448,5-CTGACAGAGGAATACTATTTCCAAAG-3;R2692-2719,5-CCTGTAAAATTGATATCTCCGGACAAAC-3) was designed from the obtained HTS sequence and reverse transcription-polymerase chain reaction (RT-PCR) was conducted. The 1.3 kb amplicon was ligated to pMD19-T vector (TaKaRa, Dalian, China) and sequenced. Sequence analysis showed it (BBWV-2-marigold, MW322809) shared 99% nt sequence identity with the Gyp isolate infecting Gynura procumbens from South Korea (LC497425.1). Phylogenetic analysis constructed with MEGA6 with the CP nt sequence of other reported BBWV-2 isolates showed BBWV-2-marigold clustered closely with the isolates from South Korea infecting Gynura procumbens (Figure S2), in accordance with the sequence identity analysis. Further RT-PCR with primer pair targeting the RNA1 (F3025-3050 5-GACAGAGTGATATTCCTAATCGAGAT-3; R4035-4062CACTCAATGC AATAAAGGTCTGGCACCT) was conducted and specific bands with the expected size of 1.0 kb were obtained in the agarose gel (data not shown), which further confirmed the existence of BBWV-2.A total of 16 marigold leaf samples(7 from Huairou and 9 from Yanqing) with mosaic symptom were collected and tested by RT-PCR with the abovementioned primer pair, and 4 from Huairou were BBWV-2 positive. Sequence analysis showed that these 4 isolates shared 100% nt sequence identity with the former sequenced isolateBBWV-2-marigold. Furthermore, CMV specific primer pair targeting the CP (F: 5-ATGGACAAATCTGGATCTCCCAAT-3/R: 5-CTAAGTCGGG AGCATCCGTGAGAT-3) were designed to detect the existence of CMV in these samples and results showed that all these 16 samples were positive for CMV. To the best of our knowledge, this is the first report of BBWV-2 in marigold in China.These findings will assist investigations on the epidemiology of diseases caused by BBWV2 in China.

Marigold plants with symptoms of mosaic, crinkle, leaf curl and necrosis were observed and small RNA and ribo-depleted total RNA deep sequencing were conducted to identify the associated viruses. Broad bean wilt virus 2, cucumber mosaic virus, turnip mosaic virus, a new potyvirus tentatively named marigold mosaic virus (MMV) and a new partitivirus named as marigold cryptic virus (MCV) were finally identified. Complete genome sequence analysis showed MMV was 9811 nt in length, encoding a large polyprotein with highest aa sequence identity (57%) with the putative potyvirus polygonatumkingianum virus 1. Phylogenetic analysis with the definite potyviruses based on the polyprotein sequence showed MMV clustered closest to plum pox virus. The complete genome of MCV comprised of dsRNA1 (1583 bp) and dsRNA2 (1459 bp), encoding the RNA-dependent RNA polymerase (RdRp), and coat protein (CP), respectively. MCV RdRp shared the highest (75.7%) aa sequence identity with the unclassified partitivirus ambrosia cryptic virus 2, and 59.0%, 57.1%, 56.1%, 54.5% and 33.7% with the corresponding region of the definite delta-partitiviruses, pepper cryptic virus 2, beet cryptic virus 3, beet cryptic virus 2, pepper cryptic virus 1 and fig cryptic virus, respectively. Phylogenetic analysis based on the RdRp aa sequence showed MCV clustered into the delta-partitivirus group. These findings enriched our knowledge of viruses infecting marigold, but the association of the observed symptom and the identified viruses and the biological characterization of the new viruses should be further investigated.