Molecular Characterization of Geographically Different Banana bunchy top virus Isolates in India

Detection and in silico characterization of banana bunchy top virus in West Bengal, India: relevance to global genetic diversity and population structure

Banana bunchy top disease is one of the major prevailing virus diseases associated with banana cultivation, spreading rapidly within a small scale of time. Till date there are only few extensive reports of completely sequenced isolates in India. A study was conducted to detect BBTV infection across 12 districts in West Bengal (WB) where extensive prevalence of the disease was ascertained. In silico characterization of the six genome components were accomplished which showed 84.90-99.86% similarity with other BBTV isolates reported worldwide. The phylogenetic analysis based upon DNA R and DNA S suggested formation of monophyletic cluster of majority of the WB isolates and its close association with Tripura, Manipur, Australia and Africa isolates indicating diversion from geographical differentiation. Dynamics of evolutionary pattern such as genetic diversity including Tajima's D test and Fu Li's Fs test, average number of nucleotide differences (K), Polymorphic sites (S); Fst distance; Mismatch distribution plot; Haplotype network, and selection pressure were performed based upon geographical distribution of the virus. Population genetics analysis of both Pacific Indian Ocean group and South East Asian group of the global BBTV population revealed low nucleotide diversity, high haplotype diversity, high gene flow within the group, and negative or purifying selection constraint indicating recent population expansion. Hence, this study portrays Indian subcontinent as the possible hotspot for rapid demographic expansion from a small virus population size, contributing valuable addition to the currently available information on BBTV worldwide.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-023-00815-0.

Colorimetric detection platform for banana bunchy top virus (BBTV) based on closed-tube loop mediated isothermal amplification (LAMP) assay

A colorimetric closed-tube Loop mediated isothermal amplification (LAMP) assay was developed for rapid and sensitive detection of banana bunchy top virus (BBTV) from leaf and sucker tissues of infected banana plants. Six LAMP primers were designed targeting BBTV coat protein gene. Isothermal amplification was set at 65 °C and end point detection made by including hydroxy naphthol blue dye in the reaction where the positive samples showed colour change from violet to sky blue. Molecular characterization of LAMP amplicon was made with restriction digestion and sequencing. Restriction digestion with Sau3AI having single cut site within the LAMP internal primer flanking region yielded two fragments of expected size. Sequence analysis confirmed that amplification corresponded to BBTV coat protein gene. Comparison of LAMP assay with conventional PCR showed that LAMP assay was 1000 times more sensitive than conventional PCR in BBTV detection with a detection limit of 0.05 ng total DNA per reaction.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-022-00784-w.

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.

Restriction-free cloning for molecular manipulation and augmented expression of banana bunchy top viral coat protein

Banana bunchy top virus (BBTV) causing bunchy top disease, is one of the most devastating diseases of banana and plantain. All the six genomic components of isolates from different parts of the world have been well characterised, with most of the studies focusing on replicase gene and coat protein gene. Overexpression of coat protein (CP) in Escherichia coli system can contribute significantly in structural as well as immunological studies. In the present investigation, the full length BBTV CP was cloned to pGEX-4T-2 expression vector and overexpressed in various Escherichia coli strains to obtain high quality and quantity of the CP. An augmented overexpression and stability of recombinant coat protein was achieved by molecular manipulation of the clone by restriction-free (RF) cloning platform. The RF cloning was employed to replace the thrombin cleavage site in the vector backbone, which was also present in the protein of interest, and to incorporate TEV protease site to cleave fusion protein at this specific site, and separate the affinity tag. The RF method allows direct transformation of the PCR product to undergo ligation in vivo and obtain the transformants thereby avoiding the restriction digestion and ligation of the product to the linearized plasmid. From a litre culture, 1.084 mg/ml of fusion protein with GST tag was obtained after GSH sepharose affinity column chromatography. The fluorescence spectra indicated partial disordered tertiary structure of the fusion protein. Cleavage of tag was attempted using TEV protease overexpressed and purified in the laboratory.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03017-x.

Virus titre determines the efficiency of Pentalonia nigronervosa (Aphididae: Hemiptera) to transmit banana bunchy top virus

Banana bunchy top disease (BBTD) caused by banana bunchy top virus (BBTV) is one of the most serious viral diseases of banana and plantains. BBTV is transmitted by Pentalonia nigronervosa (Hemiptera, Aphididae) in a persistent circulative manner. Better knowledge of vector-virus-host relationship and the mechanism of transmission is essential for developing an effective control strategy. In this study, the viral copies in single to group of aphids with different acquisition access period (AAP) were quantified using SYBR green-based quantitative polymerase chain reaction (qPCR). The result indicated that a single aphid was able to acquire 861.04 copies of the virus after 24 h of AAP from the infected banana plant and transmitted the virus to 16.6% tissue culture plants, whereas 50 viruliferous aphids (15,066.94 viral copies) were necessary to achieve 100% transmission in a shortest time of 21.6 days. The number of viral copies acquired by the aphids were gradually increased with increased AAP. Hundred percent transmission was observed with 20 aphids in 48 h of AAP or 30-50 aphids in 24 h of AAP. The inoculated plants expressed typical bunchy top symptoms quickly when higher number of aphids (30 and above) were used with 24 h of AAP. Further, we report that the tissue culture banana plants are highly prone or vulnerable to BBTV infection compared to sucker grown plants. We conclude that higher the number of viral copies in the vector, higher the percent transmission and quicker the symptom expression and the results will contribute to a better understanding of vector-BBTV interactions and useful for epidemiological studies.

Distribution, molecular characterization and diversity of banana bunchy top virus in Tripura, India

Banana bunchy top virus (BBTV) is a serious threat to banana (Musa spp.) production in India. Generally, BBTV isolates within the country share very low genetic diversity. However, in India, relatively greater diversity has been observed between isolates from north-eastern (NE) region (Meghalaya) and rest of India. Tripura is situated in the south-west corner of NE India and shares international border with Bangladesh. During 2014-2015, diagnostic surveys were conducted in seven districts of Tripura and polymerase chain reaction based detection established that BBTD is widely prevalent in all parts of Tripura showing an average incidence of 22.02%. Among the cultivars, maximum BBTV infection (27.03%) was recorded in 'Chini Champa', followed by plantain (24.29%). A representative population (31 isolates) of BBTV from Tripura was characterized based on DNA R and DNA S. Phylogenetic analysis based on BBTV DNA R and DNA S generated two broad clusters of Pacific-Indian Oceans (PIO) and south-east Asian groups including all Tripura isolates within PIO cluster. The clustering pattern and genetic diversity of BBTV population from Tripura suggested monophyletic origin of majority of representative isolates from a common ancestor of PIO group. The exchange of vegetative propagules within and in between countries could have contributed to the geographical expansion of PIO isolates in Tripura. However, four variant BBTV isolates has been identified from North Tripura and Khowai districts possessing somewhat unique variability than that of distinct isolate (BBTV-Umiam) reported from NE India (Meghalaya).

Development of a gold-nano particle based novel dot immunobinding assay for rapid and sensitive detection of Banana bunchy top virus

An improved gold nanoparticle based Dot immunobinding assay (DIBA) was developed for the detection of Banana bunchy top virus (BBTV), that is more efficient, sensitive, rapid and simpler than conventional DIBA and ELISA. Instead of enzyme conjugates, gold nanoparticles were used as reporters owing to their unique optical properties. Antibody was raised against expressed recombinant coat protein of BBTV. The gold nanoparticles were conjugated to primary / detection antibody raised following immunization with recombinant coat protein, making it highly specific for the virus. Gold nanoparticle conjugated primary antibody (GCPab) based DIBA developed in this study has a detection efficiency comparable to ELISA. The results of using this assay format for detection of BBTV in banana plants from four geographical regions of India are also presented in this report. The test could detect the virus at sap dilution up-to 10^-2. Using this improved DIBA, any lab with basic amenities can perform indexing on large numbers of samples.

Molecular characterization of banana bunchy top virus isolate from Sri Lanka and its genetic relationship with other isolates

Bunchy top disease of banana caused by Banana bunchy top virus (BBTV, genus Babuvirus family Nanoviridae) is one of the most important constraints in production of banana in the different parts of the world. Six genomic DNA components of BBTV isolate from Kandy, Sri Lanka (BBTV-K) were amplified by polymerase chain reaction (PCR) with specific primers using total DNA extracted from banana tissues showing typical symptoms of bunchy top disease. The amplicons were of expected size of 1.0-1.1 kb, which were cloned and sequenced. Analysis of sequence data revealed the presence of six DNA components; DNA-R, DNA-U3, DNA-S, DNA-N, DNA-M and DNA-C for Sri Lanka isolate. Comparisons of sequence data of DNA components followed by the phylogenetic analysis, grouped Sri Lanka-(Kandy) isolate in the Pacific Indian Oceans (PIO) group. Sri Lanka-(Kandy) isolate of BBTV is classified a new member of PIO group based on analysis of six components of the virus.

Biology, etiology, and control of virus diseases of banana and plantain

Banana and plantain (Musa spp.), produced in 10.3 million ha in the tropics, are among the world's top 10 food crops. They are vegetatively propagated using suckers or tissue culture plants and grown almost as perennial plantations. These are prone to the accumulation of pests and pathogens, especially viruses which contribute to yield reduction and are also barriers to the international exchange of germplasm. The most economically important viruses of banana and plantain are Banana bunchy top virus (BBTV), a complex of banana streak viruses (BSVs) and Banana bract mosaic virus (BBrMV). BBTV is known to cause the most serious economic losses in the "Old World," contributing to a yield reduction of up to 100% and responsible for a dramatic reduction in cropping area. The BSVs exist as episomal and endogenous forms are known to be worldwide in distribution. In India and the Philippines, BBrMV is known to be economically important but recently the virus was discovered in Colombia and Costa Rica, thus signaling its spread into the "New World." Banana and plantain are also known to be susceptible to five other viruses of minor significance, such as Abaca mosaic virus, Abaca bunchy top virus, Banana mild mosaic virus, Banana virus X, and Cucumber mosaic virus. Studies over the past 100 years have contributed to important knowledge on disease biology, distribution, and spread. Research during the last 25 years have led to a better understanding of the virus-vector-host interactions, virus diversity, disease etiology, and epidemiology. In addition, new diagnostic tools were developed which were used for surveillance and the certification of planting material. Due to a lack of durable host resistance in the Musa spp., phytosanitary measures and the use of virus-free planting material are the major methods of virus control. The state of knowledge on BBTV, BBrMV, and BSVs, and other minor viruses, disease spread, and control are summarized in this review.

Identification and characterization of a distinct banana bunchy top virus isolate of Pacific-Indian Oceans group from North-East India

Banana bunch top virus (BBTV) is considered to be a serious threat to banana production. A new isolate of the virus (BBTV-Umiam) was identified and characterized from local banana mats growing in mid-hills of Meghalaya in North-East India. The complete nucleotide sequence analysis revealed the presence of six full-length ssDNA components (DNA R, DNA U3, DNA S, DNA M, DNA C and DNA N) sharing major common region (CR-M) and a stem-loop common region (CR-SL). BBTV-Umiam showed a unique deletion of 20 nucleotides in the intergenic region of DNA R, the absence of predicted open reading frame (ORF) in DNA U3 and probability for a small ORF in DNA U3 expecting functional evidence at transcriptional level. Phylogenetic analysis based on 88 complete nucleotide sequence of BBTV DNA R available in GenBank generated two broad clusters of Pacific-Indian Oceans (PIO) and South-East Asian (SEA) groups including BBTV-Umiam within PIO cluster. However, BBTV-Umiam was identified as the most distinct member of the PIO group with 100% bootstrap support. This was further supported by the phylogenetic grouping of each genomic component of BBTV-Umiam at the distant end of PIO group during clustering of 21 complete BBTV sequences. BBTV-Umiam shared relatively less nucleotide identity with PIO group for each genomic component (85.0-95.4%) and corresponding ORF (93.8-97.5%) than that of earlier PIO isolates (91.5-99.6% and 96.0-99.3%, respectively). Recombination analysis revealed two intra-component and five inter-component recombination events in BBTV-Umiam, but none of them was unique. Moreover, the isolate was identified as major parental sequence for intra-component recombination event spanning the replication-associated protein encoding region in Tongan BBTV DNA R. The current study indicated differential evolution of BBTV in North-East India (Meghalaya). The natural occurrence of hybrids of Musa balbisiana and M. acuminata in this geographically isolated region could be the contributing factor in accumulating genetic distinctiveness in BBTV-Umiam which need further characterization.