Asymmetric synergism and heteroencapsidation between two bipartite begomoviruses, tomato leaf curl New Delhi virus and tomato leaf curl Palampur virus

Genomic distinctiveness and recombination in tomato leaf curl New Delhi virus (ToLCNDV-BG) isolates infecting bitter gourd

There are two begomoviruses, tomato leaf curl New Delhi virus (ToLCNDV) and bitter gourd yellow mosaic virus (BgYMV) infecting bitter gourd in India. An extensive survey conducted from 2019 to 2022 clearly established that infection by ToLCNDV is more predominant (92.43%) than BgYMV (44%). The ToLCNDV isolates infecting bitter gourd shared only 88% identity in the DNA-A component with other ToLCNDV isolates and were found to be a distinct variant. The predicted amino acid sequence of the viral proteins, replication initiation protein, coat protein, and the symptom determinant protein in the study isolates are markedly different. Especially the RCR motif I and RCR motif III are different from other geminiviruses. Infectivity of cloned components of one of the isolates ToLCNDV-BG NP was demonstrated in bitter gourd. Recombination analysis clearly revealed that the study isolates are recombinants with the major parent predicted as squash leaf curl Yunnan virus (GenBank Accession Number: MK064241) and the minor parent as ToLCNDV from Pakistan (GenBank Accession Number: AM747291). Due to distinct genomic features and less than 90% identity with the majority of ToLCNDV isolates, the study isolates deserve to be raised to the status of a distinct strain, designated as ToLCNDV-BG.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04009-3.

Bottle gourd IC-0262269, a super-susceptible genotype to tomato leaf curl Palampur virus

While conducting field trial of 82 genotypes of bottle gourd at Delhi during 2020-2021, a particular genotype, IC-0262269 was found to be affected by chlorotic curly stunt disease (CCSD). The affected plants were severely stunted and bearing very small chlorotic and crinkle leaves. The disease incidence in the said genotype was as high as 80% among different replicated trial blocks. The application of PCR using a generic primers specific to begomoviruses, as well as species-specific PCR diagnostics to six tomato-infecting begomoviruses: tomato leaf curl New Delhi virus (ToLCNDV), tomato leaf curl Palampur virus (ToLCPalV), tomato leaf curl Joydebpur virus (ToLCJoV), tomato leaf curl Gujrat virus (ToLCGuV), tomato leaf curl Bangalore virus (ToLCBV), and chilli leaf curl virus (ChiLCV) showed that, only ToLCPalV could be detected in the genotype IC-0262269. Following, rolling circle amplification, cloning and sequencing of full-length DNA-A and DNA-B genome of an isolate BoG1-ND from the genotype IC-0262269 revealed association of ToLCPalV with the disease. The successful agro-infection of the cloned genome of BoG1-ND (DNA-A and DNA-B) in the plants of Nicotiana benthamiana and bottle gourd demonstrated that ToLCPalV is the causal begomovirus of CCSD. The study provides the first evidence of the natural occurrence of ToLCPalV in bottle gourd crop and also showed that the bottle gourd genotype IC-0262269 is super-susceptible to ToLCPalV.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03838-y.

Virus-virus interactions alter the mechanical transmissibility and host range of begomoviruses

Introduction

Begomoviruses are mainly transmitted by whiteflies. However, a few begomoviruses can be transmitted mechanically. Mechanical transmissibility affects begomoviral distribution in the field.

Materials and methods

In this study, two mechanically transmissible begomoviruses, tomato leaf curl New Delhi virus-oriental melon isolate (ToLCNDV-OM) and tomato yellow leaf curl Thailand virus (TYLCTHV), and two nonmechanically transmissible begomoviruses, ToLCNDV-cucumber isolate (ToLCNDV-CB) and tomato leaf curl Taiwan virus (ToLCTV), were used to study the effects of virus-virus interactions on mechanical transmissibility.

Results

Nicotiana benthamiana and host plants were coinoculated through mechanical transmission with inoculants derived from plants that were mix-infected or inoculants derived from individually infected plants, and the inoculants were mixed immediately before inoculation. Our results showed that ToLCNDV-CB was mechanically transmitted with ToLCNDV-OM to N. benthamiana, cucumber, and oriental melon, whereas ToLCTV was mechanically transmitted with TYLCTHV to N. benthamiana and tomato. For crossing host range inoculation, ToLCNDV-CB was mechanically transmitted with TYLCTHV to N. benthamiana and its nonhost tomato, while ToLCTV with ToLCNDV-OM was transmitted to N. benthamiana and its nonhost oriental melon. For sequential inoculation, ToLCNDV-CB and ToLCTV were mechanically transmitted to N. benthamiana plants that were either preinfected with ToLCNDV-OM or TYLCTHV. The results of fluorescence resonance energy transfer analyses showed that the nuclear shuttle protein of ToLCNDV-CB (CBNSP) and the coat protein of ToLCTV (TWCP) localized alone to the nucleus. When coexpressed with movement proteins of ToLCNDV-OM or TYLCTHV, CBNSP and TWCP relocalized to both the nucleus and the cellular periphery and interacted with movement proteins.

Discussion

Our findings indicated that virus-virus interactions in mixed infection circumstances could complement the mechanical transmissibility of nonmechanically transmissible begomoviruses and alter their host range. These findings provide new insight into complex virus-virus interactions and will help us to understand the begomoviral distribution and to reevaluate disease management strategies in the field.

Molecular characterization and infectivity analysis of tomato leaf curl New Delhi virus isolates infecting potato

Nucleotide sequence of complete genome of a new isolate (KAN-6) of tomato leaf curl New Delhi virus (ToLCNDV) from Kanpur, Uttar Pradesh, India was determined. Sequence analysis indicated that it shared maximum identity to ToLCNDV isolates from pumpkin and ashgourd. Infectious clones of isolate KAN-6 along with two other ToLCNDV isolates (MOD-21 & FAI-19) obtained from potato fields of Modipuram and Faizabad, India were produced and used in symptom expression studies in N. benthamiana and potato plants through agro-inoculation. These isolates produced different symptoms both in N. benthamiana and potato. Severe symptoms of yellow mottling, downward curling and stunted growth were observed in N. benthamiana plants inoculated with KAN-6. MOD-21-inoculated plants also showed downward curling, stunted growth, but yellow mottling was observed only in older leaves whereas FAI-19-inoculated plants produced only downward curling symptoms. In case of potato, typical symptoms of apical leaf curl disease were observed in cultivar Kufri Pukhraj inoculated with MOD-21 and KAN-6 that are similar to those produced by virus-infected plants in the field. However, MOD-21 produced more prominent yellow mosaic symptoms as compared to KAN-6. FAI-19 produced only restricted yellow spots in Kufri Pukhraj. Only mild symptoms appeared in KAN-6 and no symptoms were observed in MOD-21- and FAI-19-inoculated Kufri Bahar plants which is known to show lowest seed degeneration under field conditions. Analysis of genomic components indicated that these isolates had 94.8-94.9% and 87.9-97.3% identity among them in DNA A and DNA B, respectively. The results of the study indicate the association of ToLCNDV isolates of different symptomatology with apical leaf curl disease of potato. This is also a first experimental demonstration of Koch's postulate for a begomovirus associated with apical leaf curl disease of potato.Author names: Please confirm if the author names (Swarup Kumar Chakrabarti) are presented accurately and in the correct sequence (given name, middle name/initial, family name).Yes. It is correct.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02752-5.

Role of viral suppressors governing asymmetric synergism between tomato-infecting begomoviruses

Mixed viral infections are common in fields and frequently exacerbate disease severity via synergistic interactions among individual viral genomic components leading to major crop loss. Two predominant species of tomato-infecting begomoviruses, Tomato leaf curl New Delhi virus (ToLCNDV) and Tomato leaf curl Gujarat virus (ToLCGuV), are known to cause severe leaf curl disease of tomato in India. Previously, we have demonstrated asymmetric synergism between these two distinct begomovirus species during mixed infection in solanaceous hosts. In the present study, we have identified the underlying proteins that positively regulate asymmetric synergism and their effect on plant defense machinery. During co-infection, the AC2 and AV2 of ToLCGuV enhanced ToLCNDV DNA accumulation in Nicotiana benthamiana as well as in their natural host, tomato. Furthermore, we found that AC2 and AV2 of ToLCNDV and AV2 of ToLCGuV play a critical role in suppression of post transcriptional gene silencing (PTGS) machinery. Taken together, AC2 and AV2 encoded proteins of ToLCGuV are the crucial viral factors promoting asymmetric synergism with ToLCNDV. KEY POINTS: • Begomoviral suppressors play vital roles in viral synergism. • AC2 and AV2 of ToLCGuV asymmetrically enhance ToLCNDV accumulation. • AC2 and AV2 of ToLCNDV and ToLCGuV AV2 are major PTGS suppressors.

When Viruses Play Team Sports: Mixed Infections in Plants

The pathological importance of mixed viral infections in plants might be underestimated except for a few well-characterized synergistic combinations in certain crops. Considering that the host ranges of many viruses often overlap and that most plant species can be infected by several unrelated viruses, it is not surprising to find more than one virus simultaneously in the same plant. Furthermore, dispersal of the majority of plant viruses relies on efficient transmission mechanisms mediated by vector organisms, mainly but not exclusively insects, which can contribute to the occurrence of multiple infections in the same plant. Recent work using different experimental approaches has shown that mixed viral infections can be remarkably frequent, up to the point that they could be considered the rule more than the exception. The purpose of this review is to describe the impact of multiple infections not only on the participating viruses themselves but also on their vectors and on the common host. From this standpoint, mixed infections arise as complex events that involve several cross-interacting players, and they consequently require a more general perspective than the analysis of single-virus/single-host approaches for a full understanding of their relevance.

Gene pyramiding for elite tomato genotypes against ToLCV (Begomovirus spp.), late blight (Phytophthora infestans) and RKN (Meloidogyne spp.) for northern India farmers

Mega variety with multiple disease and pest tolerance is one of the major research objectives to protect the crop from biotic stresses in current scenario of climate change. Three tomato leaf curl virus resistance genes (Ty-1, Ty-2 and Ty-3), two late blight resistance genes (Ph-2 and Ph-3) and one root knot nematodes resistance gene (Mi-1.2) were pyramided from different donor parents with the help of marker-assisted backcrossing followed by phenotypic selection. During 2016-2017 (August-March), 100 lines of BC₁F₄ mapping population were evaluated against these diseases and pest screening. Punjab Chhuhara (a popular north Indian tomato cultivar) was used as a recipient parent that is susceptible for all these diseases and pest. Whereas the resistant parents included CLN3022F₂-154-11-11-0 for tomato leaf curl virus, LBR-15 for late blight and NR-5 for root knot nematodes. The study results evaluated, 13 advanced backcrossed lines which possessed multiple disease and pest resistance with better horticultural traits over the parents as well as checks. This investigation showed that the introgression of Ty-1, Ty-2, Ty-3, Ph-2, Ph-3 and Mi-1.2 genes significantly improved the resistance against leaf curl, late blight and root knot nematodes diseases in selected advanced backcrossed lines of tomato. These improved multiple diseases resistant lines may play an important role in future pre-breeding tomato improvement projects.

Absolute Quantification of Tomato leaf curl New Delhi virus Spain strain, ToLCNDV-ES: Virus Accumulation in a Host-Specific Manner

Tomato leaf curl New Delhi virus (ToLCNDV) (family Geminiviridae, genus Begomovirus) has recently been introduced in western Mediterranean countries. Isolates in Spain constitute a new strain, denominated ToLCNDV-ES, that is causing losses in commercial zucchini and melon crops; however, it is also, although less often, detected in commercial tomato crops. We developed a tissue-print hybridization test to detect the two genomic components of the virus and a TaqMan quantitative polymerase chain reaction (qPCR) test to estimate the number of genome copies in plants. qPCR was approximately 10⁴ to 10⁶ times more sensitive than tissue-print hybridization to detect viral genomic DNA-A and DNA-B, respectively. It also detected the virus in more experimentally and naturally ToLCNDV-ES-infected zucchini squash and tomato plants. ToLCNDV-ES DNA-A titers were significantly lower in tomato than in zucchini plants, often falling below the detection limits in the hybridization test. In addition, the DNA-B accumulation was impaired in tomato when compared with zucchini. According to the data obtained in this study, the differences in viral titers of both plant species contribute to explain the dissimilarities in symptom expression, capability of detection, and transmission of the virus.

Tomato Leaf Curl New Delhi Virus: An Emerging Virus Complex Threatening Vegetable and Fiber Crops

The tomato leaf curl New Delhi virus (ToLCNDV) (genus Begomovirus, family Geminiviridae) represents an important constraint to tomato production, as it causes the most predominant and economically important disease affecting tomato in the Indian sub-continent. However, in recent years, ToLCNDV has been fast extending its host range and spreading to new geographical regions, including the Middle East and the western Mediterranean Basin. Extensive research on the genome structure, protein functions, molecular biology, and plant-virus interactions of ToLCNDV has been conducted in the last decade. Special emphasis has been given to gene silencing suppression ability in order to counteract host plant defense responses. The importance of the interaction with DNA alphasatellites and betasatellites in the biology of the virus has been demonstrated. ToLCNDV genetic variability has been analyzed, providing new insights into the taxonomy, host adaptation, and evolution of this virus. Recombination and pseudorecombination have been shown as motors of diversification and adaptive evolution. Important progress has also been made in control strategies to reduce disease damage. This review highlights these various achievements in the context of the previous knowledge of begomoviruses and their interactions with plants.

Tomato leaf curl New Delhi virus: a widespread bipartite begomovirus in the territory of monopartite begomoviruses

Tomato leaf curl New Delhi virus (ToLCNDV) is an exceptional Old World bipartite begomovirus. On the Indian subcontinent, a region in which monopartite DNA satellite-associated begomoviruses with mostly narrow geographical ranges predominate, it is widespread, with a geographical range also including the Far East, Middle East, North Africa and Europe. The success of ToLCNDV probably derives from its broad host range and highly flexible genomic configuration: its DNA-A component is capable of productively interacting with, and trans-replicating, diverse DNA-B components and betasatellites. An understanding of the capacity of ToLCNDV to infect a variety of hosts and spread across a broad and ecologically variable geographical range could illuminate the potential economic threats associated with similar begomoviral invasions. Towards this end, we used available ToLCNDV sequences to reconstruct the history of ToLCNDV spread.

TAXONOMY

Family Geminiviridae, Genus Begomovirus. ToLCNDV is a bipartite begomovirus. Following the revised begomovirus taxonomic criteria of 91% and 94% nucleotide identity for species and strain demarcation, respectively, ToLCNDV is a distinct species with two strains: ToLCNDV and ToLCNDV-Spain.

HOST RANGE

The primary cultivated host of ToLCNDV is tomato (Solanum lycopersicum), but the virus is also known to infect 43 other plant species from a range of families, including Cucurbitaceae, Euphorbiaceae, Solanaceae, Malvaceae and Fabaceae.

DISEASE SYMPTOMS

Typical symptoms of ToLCNDV infection in its various hosts include leaf curling, vein thickening, puckering, purpling/darkening of leaf margins, leaf area reduction, internode shortening and severe stunting.

An insight into differentially regulated genes in resistant and susceptible genotypes of potato in response to tomato leaf curl New Delhi virus-[potato] infection

Apical leaf curl disease, caused by tomato leaf curl New Delhi virus-[potato] (ToLCNDV-[potato]) is one of the most important viral diseases of potato in India. Genetic resistance source for ToLCNDV in potato is not identified so far. However, the cultivar Kufri Bahar is known to show lowest seed degeneration even under high vector levels. Hence, microarray analysis was performed to identify differentially regulated genes during ToLCNDV-[potato] infection in a resistant (Kufri Bahar) and a susceptible cultivar (Kufri Pukhraj). Under artificial inoculation conditions, in Kufri Pukhraj, symptom expressions started at 15days after inoculation (DAI) and then progressed to severe symptoms, whereas no or only very mild symptoms were observed in Kufri Bahar up to 35 DAI. Correspondingly, qPCR assay indicated a high viral load in Kufri Pukhraj and a very low viral load in Kufri Bahar. Microarray analysis showed that a total of 1111 genes and 2588 genes were differentially regulated (|log2 (Fold Change)|>2) in Kufri Bahar and Kufri Pukhraj, respectively, following ToLCNDV-[potato] infection. Gene ontology and mapman analyses revealed that these altered transcripts were involved in various biological & metabolic processes. Several genes with unknown functions were 5 to 100 fold expressed after virus infection and further experiments are necessary to ascertain their role in disease resistance or susceptibility. This study gives an insight into differentially regulated genes in response to ToLCNDV-[potato] infection in resistant and susceptible cultivars and could serve as the basis for the development of new strategies for disease management.

Implication of the Whitefly Bemisia tabaci Cyclophilin B Protein in the Transmission of Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) is a single-stranded (ssDNA) begomoviruses that causes severe damage to tomato and several other crops worldwide. TYLCV is exclusively transmitted by the sweetpotato whitefly, Bemisia tabaci in a persistent circulative and propagative manner. Previous studies have shown that the transmission, retention, and circulation of TYLCV in its vector involves interaction with insect and endosymbiont proteins, which aid in the transmission of the virus, or have a protective role in response to the presence of the virus in the insect body. However, only a low number of such proteins have been identified. Here, the role of B. tabaci Cyclophilin B (CypB) in the transmission of TYLCV protein was investigated. Cyclophilins are a large family of cellular prolyl isomerases that have many molecular roles including facilitating protein-protein interactions in the cell. One cyclophilin protein has been implicated in aphid-luteovirus interactions. We demonstrate that the expression of CypB from B. tabaci is altered upon TYLCV acquisition and retention. Further experiments used immunocapture-PCR and co-immunolocalization and demonstrated a specific interaction and colocalization between CypB and TYLCV in the the midgut, eggs, and salivary glands. Membrane feeding of anti-CypB antibodies and TYLCV-infected plants showed a decrease in TYLCV transmission, suggesting a critical role that CypB plays in TYLCV transmission. Further experiments, which used membrane feeding with the CypB inhibitor Cyclosporin A showed decrease in CypB-TYLCV colocalization in the midgut and virus transmission. Altogether, our results indicate that CypB plays an important role in TYLCV transmission by B. tabaci.

Circulative, "nonpropagative" virus transmission: an orchestra of virus-, insect-, and plant-derived instruments

Species of plant viruses within the Luteoviridae, Geminiviridae, and Nanoviridae are transmitted by phloem-feeding insects in a circulative, nonpropagative manner. The precise route of virus movement through the vector can differ across and within virus families, but these viruses all share many biological, biochemical, and ecological features. All share temporal and spatial constraints with respect to transmission efficiency. The viruses also induce physiological changes in their plant hosts resulting in behavioral changes in the insects that optimize the transmission of virus to new hosts. Virus proteins interact with insect, endosymbiont, and plant proteins to orchestrate, directly and indirectly, virus movement in insects and plants to facilitate transmission. Knowledge of these complex interactions allows for the development of new tools to reduce or prevent transmission, to quickly identify important vector populations, and to improve the management of these economically important viruses affecting agricultural and natural plant populations.