Molecular characterisation of a novel sadwavirus infecting cattleya orchids in Australia.

The complete genome sequence of a novel sadwavirus infecting cattleya orchids in South East Queensland is described. Isometric virions of c. 27 nm diameter were observed in sap extracts viewed under a transmission electron microscope, and the genome sequence of this virus was determined by high-throughput sequencing. The viral genome consists of two RNA components, 5,910 and 4,435 nucleotides (nt) in length, each encoding a long polyprotein, with predicted cleavage sites at H/Y, E/G, Q/S, and Q/G for the RNA1 and T/G for the RNA2 translation products, respectively. RNA2 has an additional small ORF of 684 nt near the 3' untranslated region. Phylogenetic analysis based on an amino acid sequence alignment of the Pro-Pol region suggested that this virus is most closely related to pineapple secovirus A, a member of the subgenus Cholivirus, but warrants classification as a member of a new species because it exhibited no more than 64% amino acid identity in pairwise sequence comparisons. Because of the prominent purple ringspots that were observed on the leaves of some of the plants, we propose the name "cattleya purple ringspot virus" for this virus (suggested species name: "Sadwavirus cattleyacola").

Surveillance for Avocado Sunblotch Viroid Utilizing the European Honey Bee (Apis mellifera).

Avocado is one of the world's fastest growing tropical fruit industries, and the pathogen avocado sunblotch viroid (ASBVd) is a major threat to both production and access to international export markets. ASBVd is seed transmissible, with infection possible via either the male (pollen) or female gametes. Surveillance for ASBVd across commercial orchards is a major logistical task, particularly when aiming to meet the stringent standards of evidence required for a declaration of pest freedom. As with many fruit crops, insect pollination is important for high avocado yields, and honey bee (Apis mellifera) hives are typically moved into orchards for paid pollination services. Exploiting the foraging behavior of honey bees can provide a complementary strategy to traditional surveillance methods. High-throughput sequencing (HTS) of bee samples for plant viruses shows promise, but this surveillance method has not yet been tested for viroids or in a targeted plant biosecurity context. Here, we tested samples of bees and pollen collected from pollination hives in two ASBVd orchard locations, one in Australia, where only four trees in a block were known to be infected, and a second in South Africa, where the estimated incidence of infection was 10%. Using real-time RT-PCR and HTS (total RNA-seq and small RNA-seq), we demonstrated that ASBVd can be confidently detected in bees and pollen samples from hives within 100 m of infected trees. The potential for using this approach in ASBVd surveillance for improved orchard management and supporting market access is discussed.

Untangling the taxonomy of dahlia mosaic virus.

In this brief note, we review the taxonomic history of dahlia mosaic virus (DMV) and related viruses. DMV is the only officially recognized caulimovirus known to infect dahlia (Dahlia variabilis) plants, although this virus appears to be relatively rare as a pathogen compared to a more recently described but unclassified caulimovirus called dahlia common mosaic virus (DCMV). We have undertaken a new set of analyses to test the hypothesis that DCMV represents a new caulimovirus species whose members infect dahlia, but we ultimately reject this hypothesis. A probable sequencing error was identified in the reference genome sequence of DMV, and consequently, we recommend that an alternative virus isolate be nominated as the exemplar for this species. In accordance with the new binomial nomenclatural system, it is proposed that the virus species be called "Caulimovirus dahliae".

Bermuda grass latent virus in Australia: genome sequence, sequence variation, and new hosts.

Bermuda grass latent virus (BGLV; genus Panicovirus) is identified for the first time in Australia and in only the second country after the USA. A full-length genome sequence was obtained, which has 97% nucleotide sequence identity to that of the species exemplar isolate. Surveys for BGLV, utilising a newly designed universal panicovirus RT-PCR assay for diagnosis, demonstrated widespread infection by this virus in a broad variety of Bermuda grass cultivars (Cynodon dactylon and C. dactylon × C. transvaalensis) grown in both New South Wales and Queensland. The virus was also detected in Rhodes grass (Chloris gayana) and Kikuyu grass (Cenchrus clandestinus), which are both important pasture grasses in subtropical Australia, and the latter is also grown as turf. Furthermore, the Rhodes grass plant, which had strong mosaic symptoms, was also infected with sugarcane mosaic virus, warranting further investigations as to whether synergistic interactions occur between these two viruses.

Genome sequence of pineapple secovirus B, a second sadwavirus reported infecting Ananas comosus.

The complete genome sequence of pineapple secovirus B (PSV-B), a new virus infecting pineapple (Ananas comosus) on the island of Oahu, Hawaii, was determined by high-throughput sequencing (HTS). The genome comprises two RNAs that are 5,956 and 3,808 nt long, excluding the 3'-end poly-A tails, both coding for a single large polyprotein. The RNA1 polyprotein contains five conserved domains associated with replication, while the RNA2 polyprotein is cleaved into the movement protein and coat protein. PSV-B is representative of a new species in the subgenus Cholivirus (genus Sadwavirus; family Secoviridae), as the level of amino acid sequence identity to recognized members of this subgenus in the Pro-Pol and coat protein regions is below currently valid species demarcation thresholds.

Characterisation of novel endogenous geminiviral elements in macadamia.

BACKGROUND: The presence of geminivirus sequences in a preliminary analysis of sRNA sequences from the leaves of macadamia trees with abnormal vertical growth (AVG) syndrome was investigated. RESULTS: A locus of endogenous geminiviral elements (EGE) in the macadamia genome was analysed, and the sequences revealed a high level of deletions and/or partial integrations, thus rendering the EGE transcriptionally inactive. The replication defective EGE in the macadamia genome indicates its inability to be the source of new viral infections and thus cause AVG or any other disease in macadamia. The EGE sequences were detected in two edible Macadamia species that constitute commercial cultivars and the wild germplasm of edible and inedible species of Macadamia. This strongly suggests that the integration preceded speciation of the genus Macadamia. A draft genome of a locus of EGE in Macadamia was developed. The findings of this study provide evidence to suggest the endogenization of the geminiviral sequences in the macadamia genome and the ancestral relationship of EGE with Macadamia in the Proteaceae family. Random mutations accumulating in the EGE inform that the sequence is evolving. CONCLUSIONS: The EGE in Macadamia is inactive and thus not a direct cause of any diseases or syndromes including AVG in macadamia. The insertion of the EGE in the macadamia genome preceded speciation of the genus Macadamia.

Complete genome sequence of aucuba ringspot virus.

A new badnavirus, aucuba ringspot virus (AuRV), was identified in plants of Aucuba japonica showing mild mosaic, vein banding, and yellow ringspot symptoms on the leaves. The complete nucleotide sequence of the AuRV genome was determined and found to be 9,092 nt in length, and the virus was found to have a genome organization typical of members of the genus Badnavirus. ORF3 was predicted to encode a polyprotein containing conserved movement protein, coat protein, aspartic protease, reverse transcriptase (RT), and RNase H domains. Phylogenetic analysis suggested that this virus is most closely related to codonopsis vein clearing virus but belongs to a distinct species, based on only 69.6% nucleotide sequence identity within the part of ORF 3 encoding the RT and RNase H domains. The vector of AuRV is unknown, but based on phylogenetic relationships, it is predicted to be a type of aphid.

Identification of putative viroplasms within banana cells infected by banana streak MY virus.

The badnavirus replication cycle is poorly understood and most knowledge is based on extrapolations from model viruses such as Cauliflower mosaic virus (CaMV). However, in contrast to CaMV, badnaviruses are thought not to produce viroplasms and therefore it has been a mystery as to where virion assembly occurs. In this study, ultrathin sections of a banana leaf infected with a badnavirus, banana streak MY virus (BSMYV), were examined by transmission electron microscopy. Electron-dense inclusion bodies (EDIBs) were sporadically distributed in parenchymatous tissues of the leaf, most commonly in the palisade and spongy mesophyll cells. These EDIBs had a characteristic structure, comprising an electron-dense core, a single, encircling lacuna and an outer ring of electron-dense material. However, much less frequently, EDIBs with two or three lacunae were observed. In the outer ring, densely packed virions were visible with a shape and size consistent with that expected for badnaviruses. Immunogold labelling was done with primary antibodies that detected the N-terminus of the capsid protein and strong labelling of the outer ring but not the central core or lacuna was observed. It is concluded that the EDIBs that were observed are equivalent in function to the viroplasms of CaMV, although obviously different in composition as there is not a paralogue of the transactivation/viroplasm protein in the badnavirus genome. It is postulated that production of a viroplasm could be a conserved characteristic of all members of the Caulimoviridae.

ICTV Virus Taxonomy Profile: Caulimoviridae.

Caulimoviridae is a family of non-enveloped reverse-transcribing plant viruses with non-covalently closed circular dsDNA genomes of 7.1-9.8 kbp in the order Ortervirales. They infect a wide range of monocots and dicots. Some viruses cause economically important diseases of tropical and subtropical crops. Transmission occurs through insect vectors (aphids, mealybugs, leafhoppers, lace bugs) and grafting. Activation of infectious endogenous viral elements occurs in Musa balbisiana, Petunia hybrida and Nicotiana edwardsonii. However, most endogenous caulimovirids are not infectious. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Caulimoviridae, which is available at ictv.global/report/caulimoviridae.

Spatiotemporal Spread of Abnormal Vertical Growth of Macadamia in Australia Informs Epidemiology.

Australian macadamia production is threatened by a disorder known as abnormal vertical growth (AVG), for which the etiology is unknown. AVG is characterized by vigorous upright growth and reduced lateral branching, flowering, and nut set that results in over 70% yield loss annually. Six commercial macadamia orchards were surveyed in 2012 and again in 2018 to examine spatiotemporal dynamics of the epidemic. Data were subjected to point-pattern and geostatistical analyses. AVG incidence in all orchards showed a better fit to the beta-binomial distribution than the binomial distribution. AVG incidence in the different orchards varied between 5 and 47% in 2012, and 13 and 55% in 2018 and the rate of spread was slow, averaging at about 2% increase in disease incidence per annum. Spatial patterns of AVG were highly aggregated on both survey years and spread was mainly between neighboring trees in a row or trees that were opposite to each other in different rows. Semivariograms showed large range values (approximately 15 to 120), indicating aggregation of AVG-affected trees beyond quadrat levels. Furthermore, clusters of disease were mainly at the edge of the orchard on the first survey date and the disease progressed toward the center of the orchard over time. It is concluded that AVG is caused by an infectious agent, and based on patterns of spread, we hypothesize that spread is facilitated by root grafting or root-to-root contact. Furthermore, a vascular-limited pathogen could be involved that modulates plant hormone production.

Questions surrounding the taxonomic validity of the species Garlic mite-borne filamentous virus (genus Allexivirus).

Garlic mite-borne filamentous virus is one of the oldest recognized allexivirus species but, paradoxically, one with the least well studied member viruses. In this paper, we review the history of this taxon and highlight problems in designating a holotype (exemplar isolate). Analyses are presented that suggest that GarMbFV is conspecific with Garlic virus A, and therefore the former taxon should be abolished.

Characterization of the banana streak virus capsid protein and mapping of the immunodominant continuous B-cell epitopes to the surface-exposed N terminus.

This study identified the structural proteins of two badnavirus species, Banana streak MY virus (BSMYV) and Banana streak OL virus (BSOLV), and mapped the distribution of continuous B-cell epitopes. Two different capsid protein (CP) isoforms of about 44 and 40 kDa (CP1 and CP2) and the virion-associated protein (VAP) were consistently associated with purified virions. For both viral species, the N terminus of CP2 was successfully sequenced by Edman degradation but that of CP1 was chemically blocked. De novo peptide sequencing of tryptic digests suggested that CP1 and CP2 derive from the same region of the P3 polyprotein but differ in the length of either the N or the C terminus. A three-dimensional model of the BSMYV-CP was constructed, which showed that the CP is a multi-domain structure, containing homologues of the retroviral capsid and nucleocapsid proteins, as well as a third, intrinsically disordered protein region at the N terminus, henceforth called the NID domain. Using the Pepscan approach, the immunodominant continuous epitopes were mapped to the NID domain for five different species of banana streak virus. Anti-peptide antibodies raised against these epitopes in BSMYV were successfully used for detection of native virions and denatured CPs in serological assays. Immunoelectron microscopy analysis of the virion surface using the anti-peptide antibodies confirmed that the NID domain is exposed on the surface of virions, and that the difference in mass of the two CP isoforms is due to variation in length of the NID domain.

Genome organization and host range of a Brazilian isolate of johnsongrass mosaic virus.

This work reports the complete genome sequence, production of a polyclonal antiserum, and host range of a Brazilian strain of johnsongrass mosaic virus (JGMV) found infecting Panicum maximum in the state of São Paulo, Brazil. The complete genome sequence of this potyvirus, comprising 9874 nucleotides, showed 82 % amino acid sequence identity in the polyprotein to that of an isolate of JGMV from Australia. The experimental host range of this virus included mainly fodder species. Cultivated species such as rice, oats, sugarcane, rye, corn and wheat were not infected, suggesting that current isolates of this potyvirus do not represent a threat to these crops in Brazil.

An inordinate fondness for Fusarium: phylogenetic diversity of fusaria cultivated by ambrosia beetles in the genus Euwallacea on avocado and other plant hosts.

Ambrosia beetle fungiculture represents one of the most ecologically and evolutionarily successful symbioses, as evidenced by the 11 independent origins and 3500 species of ambrosia beetles. Here we document the evolution of a clade within Fusarium associated with ambrosia beetles in the genus Euwallacea (Coleoptera: Scolytinae). Ambrosia Fusarium Clade (AFC) symbionts are unusual in that some are plant pathogens that cause significant damage in naïve natural and cultivated ecosystems, and currently threaten avocado production in the United States, Israel and Australia. Most AFC fusaria produce unusual clavate macroconidia that serve as a putative food source for their insect mutualists. AFC symbionts were abundant in the heads of four Euwallacea spp., which suggests that they are transported within and from the natal gallery in mandibular mycangia. In a four-locus phylogenetic analysis, the AFC was resolved in a strongly supported monophyletic group within the previously described Clade 3 of the Fusarium solani species complex (FSSC). Divergence-time estimates place the origin of the AFC in the early Miocene ∼21.2 Mya, which coincides with the hypothesized adaptive radiation of the Xyleborini. Two strongly supported clades within the AFC (Clades A and B) were identified that include nine species lineages associated with ambrosia beetles, eight with Euwallacea spp. and one reportedly with Xyleborus ferrugineus, and two lineages with no known beetle association. More derived lineages within the AFC showed fixation of the clavate (club-shaped) macroconidial trait, while basal lineages showed a mix of clavate and more typical fusiform macroconidia. AFC lineages consisted mostly of genetically identical individuals associated with specific insect hosts in defined geographic locations, with at least three interspecific hybridization events inferred based on discordant placement in individual gene genealogies and detection of recombinant loci. Overall, these data are consistent with a strong evolutionary trend toward obligate symbiosis coupled with secondary contact and interspecific hybridization.

Endogenous Caulimovirids: Fossils, Zombies, and Living in Plant Genomes.

The Caulimoviridae is a family of double-stranded DNA viruses that infect plants. The genomes of most vascular plants contain endogenous caulimovirids (ECVs), a class of repetitive DNA elements that is abundant in some plant genomes, resulting from the integration of viral DNA in the chromosomes of germline cells during episodes of infection that have sometimes occurred millions of years ago. In this review, we reflect on 25 years of research on ECVs that has shown that members of the Caulimoviridae have occupied an unprecedented range of ecological niches over time and shed light on their diversity and macroevolution. We highlight gaps in knowledge and prospects of future research fueled by increased access to plant genome sequence data and new tools for genome annotation for addressing the extent, impact, and role of ECVs on plant biology and the origin and evolutionary trajectories of the Caulimoviridae.

Paspalum striate mosaic virus: an Australian mastrevirus from Paspalum dilatatum.

Three monocot-infecting mastreviruses from Australia, all found primarily in pasture and naturalised grasses, have been characterised at the molecular level. Here, we present the full genome sequence of a fourth, Paspalum striate mosaic virus (PSMV), isolated from Paspalum dilatatum from south-east Queensland. The genome was 2816 nt long and had an organisation typical of other monocot-infecting mastreviruses. Its nearest relative is Bromus cartharticus striate mosaic virus (BCSMV), with which it shares an overall genome identity of 75%. Phylogenetic analysis of the complete genome and each of the putative viral proteins places PSMV in a group with the other three Australian striate mosaic viruses. PSMV, BCSMV and Digitaria didactyla striate mosaic virus all contain a similar, small recombinant sequence in the small intergenic region.

Development of a one-step RT-qPCR detection assay for the newly described citrus viroid VII.

An apscaviroid, tentatively named citrus viroid VII (CVd-VII), was recently discovered in citrus in Australia. A diagnostic assay using real-time reverse transcription polymerase chain reaction was developed and validated to detect the viroid in citrus plants. The assay showed a high level of sensitivity, reliably detecting 2000 plasmid copies per reaction, while down to 20 plasmid copies per reaction were occasionally detected. The assay showed high specificity, producing no false positives or cross-reactivity with a range of other citrus graft-transmissible pathogens, including viroids, viruses and bacteria. The real-time assay was also found to be more sensitive than the available end-point reverse transcription polymerase chain reaction assay by a factor of 100,000 and could be a useful tool for the rapid detection of CVd-VII in diagnostic and research environments.

Adaptation of a filter paper method for RNA template preparation for the detection of avocado sunblotch viroid by reverse transcription qPCR.

An easy, rapid and inexpensive method of preparing RNA template for a reverse transcription qPCR assay for avocado sunblotch viroid (ASBVd) is described. This method depends on the principle of reversible binding of viroid RNA to filter paper under different concentrations of monovalent cation. Lysis buffers containing either sodium chloride or lithium chloride were compared, and 1.5 M lithium chloride was shown to be optimal for the adsorption of the viroid RNA to the filter paper. The extraction method was validated using field samples and equivalent yields of viroid RNA were obtained using this method and either a commercial RNA extraction kit or a dsRNA chromatography method. The filter paper method of RNA extraction is ideally suited for the large-scale surveillance for ASBVd.

Viruses Infecting Greenhood Orchids (Pterostylidinae) in Eastern Australia.

The Australasian biogeographic realm is a major centre of diversity for orchids, with every subfamily of the Orchidaceae represented and high levels of endemism at the species rank. It is hypothesised that there is a commensurate diversity of viruses infecting this group of plants. In this study, we have utilised high-throughput sequencing to survey for viruses infecting greenhood orchids (Pterostylidinae) in New South Wales and the Australian Capital Territory. The main aim of this study was to characterise Pterostylis blotch virus (PtBV), a previously reported but uncharacterised virus that had been tentatively classified in the genus Orthotospovirus. This classification was confirmed by genome sequencing, and phylogenetic analyses suggested that PtBV is representative of a new species that is possibly indigenous to Australia as it does not belong to either the American or Eurasian clades of orthotospoviruses. Apart from PtBV, putative new viruses in the genera Alphaendornavirus, Amalgavirus, Polerovirus and Totivirus were discovered, and complete genome sequences were obtained for each virus. It is concluded that the polerovirus is likely an example of an introduced virus infecting a native plant species in its natural habitat, as this virus is probably vectored by an aphid, and Australia has a depauperate native aphid fauna that does not include any species that are host-adapted to orchids.