Localization of Banana bunchy top virus and cellular compartments in gut and salivary gland tissues of the aphid vector Pentalonia nigronervosa

A New Perspective on the Co-Transmission of Plant Pathogens by Hemipterans

Co-infection of plants by pathogens is common in nature, and the interaction of the pathogens can affect the infection outcome. There are diverse ways in which viruses and bacteria are transmitted from infected to healthy plants, but insects are common vectors. The present review aims to highlight key findings of studies evaluating the co-transmission of plant pathogens by insects and identify challenges encountered in these studies. In this review, we evaluated whether similar pathogens might compete during co-transmission; whether the changes in the pathogen titer in the host, in particular associated with the co-infection, could influence its transmission; and finally, we discussed the pros and cons of the different approaches used to study co-transmission. At the end of the review, we highlighted areas of study that need to be addressed. This review shows that despite the recent development of techniques and methods to study the interactions between pathogens and their insect vectors, there are still gaps in the knowledge of pathogen transmission. Additional laboratory and field studies using different pathosystems will help elucidate the role of host co-infection and pathogen co-transmission in the ecology and evolution of infectious diseases.

Small GTPase Rab40C is upregulated by 20-hydroxyecdysone and insulin pathways to regulate ovarian development and fecundity

The insulin and 20-hydroxyecdysone (20E) pathways coordinately regulate insect vitellogenesis and ovarian development. However, the detailed molecular mechanisms such as the genes mediating the cooperation of the interaction of these 2 pathways in regulating insect reproductive development are not well understood. In the present study, a small GTPase, Rab40C, was identified from the notorious agricultural pest Bactrocera dorsalis. In addition to the well-known RAB domain, it also has a unique SOCS-box domain, which is different from other Rab-GTPases. Moreover, we found that Rab40C was enriched in the ovaries of sexually mature females. RNA interference (RNAi)-mediated knockdown of BdRab40C resulted in a decrease in vitellogenin synthesis, underdeveloped ovaries, and low fertility. Furthermore, depletion of insulin receptor InR or the heterodimer receptor of 20E (EcR or USP) by RNAi significantly decreased the transcription of BdRab40C and resulted in lower fecundity. Further studies revealed that the transcription of BdRab40C could be upregulated by the injection of insulin or 20E. These results indicate that Rab40C participates in the insulin and 20E pathways to coordinately regulate reproduction in B. dorsalis. Our results not only provide new insights into the insulin- and 20E-stimulated regulatory pathways controlling female reproduction in insects but also contribute to the development of potential eco-friendly strategies for pest control.

Insights into the capsid structure of banana bunchy top virus

Banana is the major staple food crop for approximately 400 million people. Bunchy top disease of banana is one of the most devastating diseases caused by banana bunchy top virus (BBTV), which results in stunting of plants, bunchy appearance of leaves and a significant loss of yield. While many isolates of BBTV from various regions of India have been characterized by different groups, no structural study exists for this important virus. To bridge this gap, the pET28a clone of the coat protein (CP) gene from BBTV isolate of Hill banana grown in lower Pulney Hills (Virupakshi) of Tamilnadu was expressed in BL21 (DE3) pLysS. Purification of the CP was achieved by Ni-NTA affinity chromatography. In vitro capsid assembly studied using sucrose density gradient centrifugation suggested that the CP did not assemble as a virus-like particle (VLP), but remained as smaller oligomers. Studies using dynamic light scattering (DLS) indicate that the purified protein is poly-dispersed, represented majorly as pentamers. Homology modeling studies provided useful insights into the probable fold of the CP suggesting that it is a β-sandwich, similar to that seen in the majority of plant viruses. In silico capsid reconstruction aided the understanding of the quaternary organization of subunits in the capsid and their molecular interactions. The location of the aphid-binding EAG motif was identified on the surface loops close to the pentameric axis indicating its role in vector-mediated transmission. Comparison with the CP and capsid structure of geminiviruses provided useful insights into the mode of nucleic acid binding and the role of genome during capsid assembly.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03204-4.

Nanovirus Disease Complexes: An Emerging Threat in the Modern Era

Multipartite viruses package their genomic segments independently and mainly infect plants; few target animals. Nanoviridae is a family of multipartite single-stranded DNA plant viruses that individually encapsidate single-stranded DNAs of approximately 1 kb and transmit them through aphids without replication in the aphid vectors, thereby causing important diseases of leguminous crops and banana. Significant findings regarding nanoviruses have recently been made on important features, such as their multicellular way of life, the transmission of distinct encapsidated genome segments through the vector body, evolutionary ambiguities, mode of infection, host range and geographical distribution. This review deals with all the above-mentioned features in view of recent advances with special emphasis on the emergence of new species and recognition of new host range of nanoviruses and aims to shed light on the evolutionary linkages, the potentially devastating impact on the world economy, and the future challenges imposed by nanoviruses.

RNAi technology for management of banana bunchy top disease

Banana bunchy top disease (BBTD) is one of the world's most destructive viral diseases of banana and plantain, causing up to 100% yield loss in severe cases. The disease is vectored by banana aphids (Pentalonia nigronervosa) and carried long distances through the movement of infected plant materials. The banana aphids harboring banana bunchy top virus (BBTV) present in banana producing regions are the sole vector and the most efficient method of transmitting the virus to the healthy plants. Controlling the spread of BBTD has been very challenging since no known banana germplasm is immune to BBTV. The disease can be managed with the use of virus-free planting material and roguing. However, once BBTD is established in the field, it is very difficult to eradicate or manage it. Therefore, a more sustainable way of controlling the disease is developing host plant resistance against the virus and the vector. Biotechnological strategies via RNA interference (RNAi) could be used to target the banana aphid as well as BBTV to reduce virus-associated yield losses of banana and plantain, which feed over 500 million people around the world. This review discusses the status of BBTD and perspectives on effective RNAi technologies for controlling BBTV and the vector, banana aphid, transmitting the virus as sustainable management of the disease.

A New Method to Obtain the Complete Genome Sequence of Multiple-Component Circular ssDNA Viruses by Transcriptome Analysis

Circular single-stranded DNA (ssDNA) viruses are widely distributed globally, infecting diverse hosts ranging from bacteria, archaea, and eukaryotes. Among these, the genome of Banana bunchy top virus (BBTV) comprises at least six circular, ssDNA components that are ∼1 kb in length. Its genome is usually amplified and obtained at the DNA level. However, RNA-based techniques to obtain the genome sequence of such multi-component viruses have not been reported. In this study, transcriptome sequencing analysis showed that the full-length of BBTV each genomic component was transcribed into viral mRNA (vmRNA). Accordingly, the near-complete genome of BBTV B2 isolate was assembled using transcriptome sequencing data from virus-infected banana leaves. Assembly analysis of BBTV-derived reads indicated that the full-length sequences were obtained for DNA-R, DNA-U3, DNA-S, DNA-M, DNA-N, NewS2, and Sat4 components, while two gaps (73 and 25 nt) missing in the DNA-C component which was further filled by reverse transcription-PCR (RT-PCR). The RT-qPCR analysis indicated that the vmRNA levels of coding regions were 3.19–103.53 folds higher than those of non-coding regions, implying that the integrity of genome assembly depended on the transcription level of non-coding region. In conclusion, this study proposes a new approach to obtain the genome of nanovirids, and provides insights for studying the transcriptional mechanism of the family Nanoviridae, Genomoviridae, and Geminiviridae.

Aphid transmission of nanoviruses

The genus Nanovirus consists of plant viruses that predominantly infect legumes leading to devastating crop losses. Nanoviruses are transmitted by various aphid species. The transmission occurs in a circulative nonpropagative manner. It was long suspected that a virus-encoded helper factor would be needed for successful transmission by aphids. Recently, a helper factor was identified as the nanovirus-encoded nuclear shuttle protein (NSP). The mode of action of NSP is currently unknown in contrast to helper factors from other plant viruses that, for example, facilitate binding of virus particles to receptors within the aphids' stylets. In this review, we are summarizing the current knowledge about nanovirus-aphid vector interactions.

Route of a Multipartite Nanovirus across the Body of Its Aphid Vector

Vector transmission plays a primary role in the life cycle of viruses, and insects are the most common vectors. An important mode of vector transmission, reported only for plant viruses, is circulative nonpropagative transmission whereby the virus cycles within the body of its insect vector, from gut to salivary glands and saliva, without replicating. This mode of transmission has been extensively studied in the viral families Luteoviridae and Geminiviridae and is also reported for Nanoviridae The biology of viruses within these three families is different, and whether the viruses have evolved similar molecular/cellular virus-vector interactions is unclear. In particular, nanoviruses have a multipartite genome organization, and how the distinct genome segments encapsidated individually transit through the insect body is unknown. Here, using a combination of fluorescent in situ hybridization and immunofluorescence, we monitor distinct proteins and genome segments of the nanovirus Faba bean necrotic stunt virus (FBNSV) during transcytosis through the gut and salivary gland cells of its aphid vector Acyrthosiphon pisum FBNSV specifically transits through cells of the anterior midgut and principal salivary gland cells, a route similar to that of geminiviruses but distinct from that of luteoviruses. Our results further demonstrate that a large number of virus particles enter every single susceptible cell so that distinct genome segments always remain together. Finally, we confirm that the success of nanovirus-vector interaction depends on a nonstructural helper component, the viral protein nuclear shuttle protein (NSP), which is shown to be mandatory for viral accumulation within gut cells.IMPORTANCE An intriguing mode of vector transmission described only for plant viruses is circulative nonpropagative transmission, whereby the virus passes through the gut and salivary glands of the insect vector without replicating. Three plant virus families are transmitted this way, but details of the molecular/cellular mechanisms of the virus-vector interaction are missing. This is striking for nanoviruses that are believed to interact with aphid vectors in ways similar to those of luteoviruses or geminiviruses but for which empirical evidence is scarce. We here confirm that nanoviruses follow a within-vector route similar to that of geminiviruses but distinct from that of luteoviruses. We show that they produce a nonstructural protein mandatory for viral entry into gut cells, a unique phenomenon for this mode of transmission. Finally, noting that nanoviruses are multipartite viruses, we demonstrate that a large number of viral particles penetrate susceptible cells of the vector, allowing distinct genome segments to remain together.

Co-Acquired Nanovirus and Geminivirus Exhibit a Contrasted Localization within Their Common Aphid Vector

Single-stranded DNA (ssDNA) plant viruses belong to the families Geminiviridae and Nanoviridae. They are transmitted by Hemipteran insects in a circulative, mostly non-propagative, manner. While geminiviruses are transmitted by leafhoppers, treehoppers, whiteflies and aphids, nanoviruses are transmitted exclusively by aphids. Circulative transmission involves complex virus-vector interactions in which epithelial cells have to be crossed and defense mechanisms counteracted. Vector taxa are considered a relevant taxonomic criterion for virus classification, indicating that viruses can evolve specific interactions with their vectors. Thus, we predicted that, although nanoviruses and geminiviruses represent related viral families, they have evolved distinct interactions with their vector. This prediction is also supported by the non-structural Nuclear Shuttle Protein (NSP) that is involved in vector transmission in nanoviruses but has no similar function in geminiviruses. Thanks to the recent discovery of aphid-transmitted geminiviruses, this prediction could be tested for the geminivirus alfalfa leaf curl virus (ALCV) and the nanovirus faba bean necrotic stunt virus (FBNSV) in their common vector, Aphis craccivora. Estimations of viral load in midgut and head of aphids, precise localization of viral DNA in cells of insect vectors and host plants, and virus transmission tests revealed that the pathway of the two viruses across the body of their common vector differs both quantitatively and qualitatively.

RNA-seq data of banana bunchy top virus (BBTV) viruliferous and non-viruliferous banana aphid (Pentalonia nigronervosa)

Banana bunchy top disease (BBT) is one of the most economically serious viral diseases of banana caused by banana bunchy top virus (BBTV: Nanoviridae: Babuvirus). BBTV is a circular, ssDNA virus which is suitable in the phloem tissue and currently only being transmitted by the banana aphid (Pentalonia nigronervosa) in a persistent, non-propagative, circulative manner. Interaction of BBTV and banana aphid had been studied in several ways, such as transmission and translocation of BBTV inside the banana aphid body at cellular level. However, the molecular mechanism underlying the interaction between BBTV and banana aphid have been poorly understood. Therefore, this transcriptomic study was conducted to obtain the raw data for differential genes expression study in BBTV viruliferous (Vr) and non-viruliferous (NVr) banana aphid. Here, we present two data sets of RNA seq raw reads which is available in GenBank Sequence Read Archive (SRA) database with accession number of SRX6918251 and SRX6918252 for the Vr and NVr banana aphid respectively.