Frequent Occurrence of Tomato Leaf Curl New Delhi Virus in Cotton Leaf Curl Disease Affected Cotton in Pakistan

Dominance of Cotton leaf curl Multan virus-Rajasthan strain associated with third epidemic of cotton leaf curl disease in Pakistan

Cotton (Gossypium hirsutum) is an economically potent crop in many countries including Pakistan, India, and China. For the last three decades, cotton production is under the constant stress of cotton leaf curl disease (CLCuD) caused by begomoviruses/satellites complex that is transmitted through the insect pest, whitefly (Bemisia tabaci). In 2018, we identified a highly recombinant strain; Cotton leaf curl Multan virus-Rajasthan (CLCuMuV-Raj), associated with the Cotton leaf curl Multan betasatellite-Vehari (CLCuMuBVeh). This strain is dominant in cotton-growing hub areas of central Punjab, Pakistan, causing the third epidemic of CLCuD. In the present study, we have explored the CLCuD diversity from central to southern districts of Punjab (Faisalabad, Lodhran, Bahawalpur, Rahimyar Khan) and the major cotton-growing region of Sindh (Tandojam), Pakistan for 2 years (2020-2021). Interestingly, we found same virus (CLCuMuV-Raj) and associated betasatellite (CLCuMuBVeh) strain that was previously reported with the third epidemic in the central Punjab region. Furthermore, we found minor mutations in two genes of CLCuMuV-Raj C4 and C1 in 2020 and 2021 respectively as compared to its isolates in 2018, which exhibited virus evolution. Surprisingly, we did not find these mutations in CLCuMuV-Raj isolates identified from Sindh province. The findings of the current study represent the stability of CLCuMuV-Raj and its spread toward the Sindh province where previously Cotton leaf curl Kokhran virus (CLCuKoV) and Cotton leaf curl Shahdadpur virus (CLCuShV) have been reported. The findings of the current study demand future research on CLCuD complex to explore the possible reasons for prevalence in the field and how the virus-host-vector compatible interaction can be broken to develop resistant cultivars.

Phylogenetic and recombination analysis of Begomoviruses associated with Cotton leaf curl disease and in silico analysis of viral-host protein interactions

Cotton leaf curl disease (CLCuD), caused by numerous begomoviruses (BGVs), is a highly disastrous disease in cotton crops worldwide. To date, several efforts have shown limited success in controlling this disease. CLCuD-associated BGVs (CABs) are known for their high rate of intra and interspecific recombinations, which raises an urgent need to find an efficient and conserved target region to combat disease. In the present study, phylogenetic analysis of selected 11 CABs, along with associated alphasatellites, and betasatellites revealed a close evolutionary relationship among them. Recombination analysis of 1374 isolates of CABs revealed 54 recombination events for the major players of CLCuD in cotton and the Cotton leaf curl Multan virus (CLCuMuV) as the most recombinant CAB. Recombination breakpoints were frequent in all regions except C2 and C3. C3-encoded protein, known as viral replication enhancer (REn), promotes viral replication by enhancing the activity of replicase (Rep) protein. Both proteins were found to contain significantly conserved domains and motifs. The identified motifs were found crucial for their interaction with host protein PCNA (Proliferating cell nuclear antigen), facilitating viral replication. Interruption at the REn-PCNA and Rep-PCNA interactions by targeting the identified conserved motifs is proposed as a prospect to halt viral replication, after suitable experimental validation.

Molecular characterization and pathogenicity of an infectious clone of tomato leaf curl New Delhi virus isolate infecting Cucumis melo

Tomato leaf curl New Delhi virus (ToLCNDV) is a member of the genus Begomovirus, and causes devastating disease in the world. In recent years, ToLCNDV was rapidly spreading in China and induces severe economic losses in agriculture. In this study, we sequenced and characterized the complete genome of ToLCNDV isolates from melon plants showing leaf curling and stunting symptoms in Jiangsu Province of China. We constructed a full-length infectious cDNA clone of ToLCNDV, which could induce systemic infection with typical symptoms in Nicotiana benthamiana, Citrullus melo, and Citrullus lanatus plants through agrobacterium-mediated inoculation. Further experimental evidence demonstrated that the virions produced in plants infected with the infectious clone of ToLCNDV are biologically active and sap-transmissible. We also evaluated the resistance of commercial melon cultivars to ToLCNDV and found all testing melon cultivars were susceptible to ToLCNDV. Collectively, the reverse genetic system developed herein will facilitate further research on biological functions of proteins encoded by ToLCNDV and plant-ToLCNDV interactions, which might provide new insights into breeding resistance germplasm in crops.

Antioxidant defense in cotton under environmental stresses: Unraveling the crucial role of a universal defense regulator for enhanced cotton sustainability

Cotton (Gossypium spp.) is a globally significant crop that provides natural fibers for the textile industry and also an important oil and biopharmaceutical resources. However, the production of cotton faces substantial challenges due to various biotic and abiotic stress factors that can negatively impact cotton growth, yield, and fiber quality. This review offers a comprehensive overview of the effects of biotic stress factors, such as insect pests, bacterial, fungal, and viral pathogens, and nematodes, as well as abiotic stress factors, including extreme hot and cold temperature, drought, toxicity induced by heavy metal and salinity, on the antioxidant systems in cotton. We discuss the crucial antioxidants, such as glutathione, proline, and phenolics, and highlight major antioxidant enzymes, including ascorbate peroxidase (APX), superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione reductase (GR), and their roles in cotton's response to these stress factors. Furthermore, we explore the potential mechanisms and the crosstalk between different stress factors signaling pathways. We also examine the implications of stress-induced changes in antioxidant levels and enzyme activities for cotton productivity and breeding strategies. Additionally, we shed light on the unanswered questions, research gaps, and future perspectives in this field, paving the way for further investigations to enhance our understanding of cotton's antioxidant defenses and develop novel strategies for improving cotton stress tolerance and yield stability.

Tomato leaf curl New Delhi virus: an emerging plant begomovirus threatening cucurbit production

Tomato leaf curl New Delhi virus (ToLCNDV), a bipartite begomovirus, was first reported to infect tomato and has recently spread rapidly as an emerging disease to Cucurbitaceae crops. To date, the virus has been reported to infect more than 11 cucurbit crops, in 16 countries and regions, causing severe yield losses. In autumn 2022, ToLCNDV was first isolated from cucurbit plants in Southeastern coastal areas of China. Phylogenetic analysis established that these isolates belong to the Asian ToLCNDV clade, and shared high nucleotide identity and closest genetic relationship with the DNA-A sequence from the Chinese tomato-infecting ToLCNDV isolate (Accession no. OP356207) and the tomato New Delhi ToLCNDV-Severe isolate (Accession no. HM159454). In this review, we summarize the occurrence and distribution, host range, detection and diagnosis, control strategies, and genetic resistance of ToLCNDV in the Cucurbitaceae. We then summarize pathways that could be undertaken to improve our understanding of this emerging disease, with the objective to develop ToLCNDV-resistant cucurbit cultivars.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00118-4.

Sentinel plot surveillance of cotton leaf curl disease in Pakistan- a case study at the cultivated cotton-wild host plant interface

A sentinel plot case study was carried out to identify and map the distribution of begomovirus-betasatellite complexes in sentinel plots and commercial cotton fields over a four-year period using molecular and high-throughput DNA 'discovery' sequencing approaches. Samples were collected from 15 study sites in the two major cotton-producing areas of Pakistan. Whitefly- and leafhopper-transmitted geminiviruses were detected in previously unreported host plant species and locations. The most prevalent begomovirus was cotton leaf curl Kokhran virus-Burewala (CLCuKoV-Bu). Unexpectedly, a recently recognized recombinant, cotton leaf curl Multan virus-Rajasthan (CLCuMuV-Ra) was prevalent in five of 15 sites. cotton leaf curl Alabad virus (CLCuAlV) and cotton leaf curl Kokhran virus-Kokhran, 'core' members of CLCuD-begomoviruses that co-occurred with CLCuMuV in the 'Multan' epidemic were detected in one of 15 sentinel plots. Also identified were chickpea chlorotic dwarf virus and 'non-core' CLCuD-begomoviruses, okra enation leaf curl virus, squash leaf curl virus, and tomato leaf curl New Delhi virus. Cotton leaf curl Multan betasatellite (CLCuMuB) was the most prevalent CLCuD-betasatellite, and less commonly, two 'non-core' betasatellites. Recombination analysis revealed previously uncharacterized recombinants among helper virus-betasatellite complexes consisting of CLCuKoV, CLCuMuV, CLCuAlV and CLCuMuB. Population analyses provided early evidence for CLCuMuV-Ra expansion and displacement of CLCuKoV-Bu in India and Pakistan from 2012-2017. Identification of 'core' and non-core CLCuD-species/strains in cotton and other potential reservoirs, and presence of the now predominant CLCuMuV-Ra strain are indicative of ongoing diversification. Investigating the phylodynamics of geminivirus emergence in cotton-vegetable cropping systems offers an opportunity to understand the driving forces underlying disease outbreaks and reconcile viral evolution with epidemiological relationships that also capture pathogen population shifts.

An Insight into Emerging Begomoviruses and their Satellite Complex causing Papaya Leaf Curl Disease

Papaya leaf curl disease (PaLCD) was primarily detected in India and causes major economic damage to agriculture crops grown globally, seriously threatening food security. Begomoviruses are communicated by the vector Bemisia tabaci, and their transmission efficiency and persistence in the vector are the highest, exhibiting the widest host range due to adaptation and evolution. Symptoms induced during PaLCD include leaf curl, leaf yellowing, interveinal chlorosis, and reduced fruit quality and yield. Consequently, plants have evolved several multi-layered defense mechanisms to resist Begomovirus infection and distribution. Subsequently, Begomovirus genomes organise circular ssDNA of size ~2.5-2.7 kb of overlapping viral transcripts and carry six-seven ORFs encoding multifunctional proteins, which are precisely evolved by the viruses to maintain the genome-constraint and develop complex but integrated interactions with a variety of host components to expand and facilitate successful infection cycles, i.e., suppression of host defense strategies. Geographical distribution is continuing to increase due to the advent and evolution of new Begomoviruses, and sweep to new regions is a future scenario. This review summarizes the current information on the biological functions of papaya-infecting Begomoviruses and their encoded proteins in transmission through vectors and modulating host-mediated responses, which may improve our understanding of how to challenge these significant plant viruses by revealing new information on the development of antiviral approaches against Begomoviruses associated with PaLCD.

Antagonism of tomato spotted wilt virus against tomato yellow leaf curl virus in Nicotiana benthamiana detected by transcriptome analysis

BACKGROUND

Tomato spot wilt virus (TSWV) and tomato yellow leaf curl virus (TYLCV) are highly harmful viruses in agricultural production, which can cause serious economic losses to crops and even devastating consequences for vegetable yield in some countries and regions. Although the two viruses belong to different families and have different transmission vectors, they share most hosts.

OBJECTIVE

This study aimed to examine the transcriptomic expression of single and mixed inoculations of TSWV and TYLCV, leading to antagonism using high-throughput RNA sequencing.

METHODS

We confirmed the single and mixed infections of these viruses in Nicotiana benthamiana (N. benthamiana) by artificial inoculation. And the expression changes of related genes and their biological functions and pathways during the mixed infection of TSWV and TYLCV were analyzed by comparative transcriptome.

RESULTS

Basically, similar symptoms were observed in the plants singly infected with TSWV and co-infected with TYLCV; the symptoms of TYLCV in the co-infected plants were not obvious compared with single TYLCV infections. When inoculated with TYLCV, the accumulation of the virus significantly reduced in single and mixed infections with TSWV; the TSWV accumulated slightly less in co-infection with TYLCV, whereas this reduction was much smaller than that of TYLCV. The results suggested that TSWV had an antagonistic effect on the accumulation of TYLCV in N. benthamiana. It mainly focused on the changes in unique differentially expressed genes (DEGs) caused by the co-infection of TSWV and TYLCV. The eight pathways enriched by upregulated DEGs mainly included amino acid biosynthesis, citrate cycle (or tricarboxylic acid cycle, TCA cycle), and so on. However, only pentose phosphate pathway (PPP) and peptidoglycan biosynthesis could be downregulated in the Kyoto Encyclopedia of Genes and Genomes pathway in which peptidoglycan biosynthesis was involved in upregulated and downregulated pathways.

CONCLUSIONS

The antagonistic effect of TSWV on TYLCV in N.benthamiana and the change trends and specific pathways of DEGs in this process were found. Our study provided new insights into the host regulation and competition between viruses in response to TSWV and TYLCV mixed infection.

In silico evaluation of molecular virus-virus interactions taking place between Cotton leaf curl Kokhran virus- Burewala strain and Tomato leaf curl New Delhi virus

Background

Cotton leaf curl disease (CLCuD) is a disease of cotton caused by begomoviruses, leading to a drastic loss in the annual yield of the crop. Pakistan has suffered two epidemics of this disease leading to the loss of billions in annual exports. The speculation that a third epidemic of CLCuD may result as consequence of the frequent occurrence of Tomato leaf curl New Delhi virus (ToLCNDV) and Cotton leaf curl Kokhran Virus-Burewala Strain (CLCuKoV-Bu) in CLCuD infected samples, demand that the interactions taking between the two viruses be properly evaluated. This study is designed to assess virus-virus interactions at the molecular level and determine the type of co-infection taking place.

Methods

Based on the amino acid sequences of the gene products of both CLCuKoV-Bu and ToLCNDV, protein structures were generated using different software, i.e., MODELLER, I-TASSER, QUARKS, LOMETS and RAPTORX. A consensus model for each protein was selected after model quality assessment using ERRAT, QMEANDisCo, PROCHECK Z-Score and Ramachandran plot analysis. The active and passive residues in the protein structures were identified using the CPORT server. Protein–Protein Docking was done using the HADDOCK webserver, and 169 Protein–Protein Interaction (PPIs) were performed between the proteins of the two viruses. The docked complexes were submitted to the PRODIGY server to identify the interacting residues between the complexes. The strongest interactions were determined based on the HADDOCK Score, Desolvation energy, Van der Waals Energy, Restraint Violation Energy, Electrostatic Energy, Buried Surface Area and Restraint Violation Energy, Binding Affinity and Dissociation constant (K_d). A total of 50 ns Molecular Dynamic simulations were performed on complexes that exhibited the strongest affinity in order to validate the stability of the complexes, and to remove any steric hindrances that may exist within the structures.

Results

Our results indicate significant interactions taking place between the proteins of the two viruses. Out of all the interactions, the strongest were observed between the Replication Initiation protein (Rep) of CLCuKoV-Bu with the Movement protein (MP), Nuclear Shuttle Protein (NSP) of ToLCNDV (DNA-B), while the weakest were seen between the Replication Enhancer protein (REn) of CLCuKoV-Bu with the REn protein of ToLCNDV. The residues identified to be taking a part in interaction belonged to domains having a pivotal role in the viral life cycle and pathogenicity. It maybe deduced that the two viruses exhibit antagonistic behavior towards each other, and the type of infection may be categorised as a type of Super Infection Exclusion (SIE) or homologous interference. However, further experimentation, in the form of transient expression analysis, is needed to confirm the nature of these interactions and increase our understanding of the direct interactions taking place between two viruses.

Circular DNA enrichment sequencing reveals the viral/satellites genetic diversity associated with the third epidemic of cotton leaf curl disease

Cotton leaf curl disease (CLCuD) is the most important limiting factor for cotton production in Pakistan. The CLCuD passed through two major epidemics in this region with distinct begomoviruses/satellites complexes. Since 2015 the disease has again started to appear in epidemic form, causing heavy losses to cotton crop, which we termed as the “third epidemic”. We applied CIDER-seq (Circular DNA Enrichment Sequencing), a recently developed sequencing method for PCR-free virus enrichment to produce a full length read of a single circular viral genome coupled with Sanger sequencing to explore the genetic diversity of the disease complex. We identified a highly recombinant strain of Cotton leaf curl Multan virus and a recently evolved strain of Cotton leaf curl Multan betasatellite that are dominant in all major cotton growing regions in the country. Moreover, we also identified multiple species of alphasatellites with one distinct species, Mesta yellow vein mosaic alphasatellite (MeYVMA) for the first time in cotton. Relative abundance of virus and associated satellites was also determined by real-time quantitative PCR. To the best of our knowledge, this is the first study that determined the CLCuD complex associated with its third epidemic.

Pest categorisation of tomato leaf curl New Delhi virus

Following a request from the European Commission, the Panel on Plant Health performed a pest categorisation on tomato leaf curl New Delhi virus (ToLCNDV). ToLCNDV is a well-defined bipartite Begomovirus species, sometimes associated with satellite molecules. It is transmitted by Bemisia tabaci to a wide range of hosts. ToLCNDV is reported from Estonia, Greece, Italy, Portugal and Spain, with limited distribution. The prevalent strain (ToLCNDV-ES) in these countries is particularly adapted to cucurbits and is different from isolates reported outside the EU, which are better adapted to solanaceous crops and could therefore pose additional risk for EU agriculture. The virus is regulated under Commission Implementing Regulation (EU) 2019/2072. The main pathway of entry identified is plants for planting of susceptible hosts, even if entry could also occur via commodities carrying viruliferous B. tabaci and possibly by seeds. While establishment and local spread rely on B. tabaci, the virus can also be dispersed over long distances by movement of infected plants for planting. Establishment and spread are limited to regions with ecoclimatic conditions suitable for the establishment of vector populations (southern regions of Europe) or can occur as outbreaks wherever crops are grown under protected cultivation. The main uncertainties associated with this pest categorisation are the distribution and prevalence of ToLCNDV in the EU, the magnitude of the virus impact particularly on hosts different from Cucurbitaceae, and seed transmission. ToLCNDV meets all the criteria evaluated by EFSA to qualify as potential Union Quarantine Pest (QP); conversely, ToLCNDV does not meet the criterion of being widespread in the EU to qualify as a Regulated Non-Quarantine Pest (RNQP). Should new data show that ToLCNDV is widespread in the EU, the possibility would exist for non-EU isolates to qualify as QP, while ToLCNDV EU isolates (ToLCNDV-ES) could qualify as RNQP.

Dominance of recombinant cotton leaf curl Multan-Rajasthan virus associated with cotton leaf curl disease outbreak in northwest India

Cotton leaf curl disease (CLCuD), caused by whitefly (Bemisiatabaci) transmitted single-stranded DNA viruses belonging to the Genus, Begomovirus (family, Geminiviridae) in association with satellite molecules; is responsible for major economic losses in cotton in three northwest (NW) Indian states Haryana, Punjab, and Rajasthan. Annual CLCuD incidences during 2012 to 2014 were estimated to be 37.5%, 63.6%, and 38.8% respectively. Cotton leaves were collected from symptomatic plants annually for three years and subjected to DNA isolation, followed by rolling circle amplification (RCA), cloning, and DNA sequencing of apparently full-length begomoviral genomes and associated betasatellites and alphasatellites. Among the thirteen CLCuD-begomoviral genomes recovered, eight were identified as Cotton leaf curl Multan virus-Rajasthan (CLCuMuV-Ra), one as -Pakistan (PK) and another as -Faisalabad (Fai), whereas, three were as Cotton leaf curl Kokhran virus-Burewala (CLCuKoV-Bu), indicating that CLCuMuV-Ra was the most prevalent begomovirus species. Five of the eight CLCuMuV-Ra sequences were found to be recombinants. The CLCuMuV-Ra- associated satellites consisted of Cotton leaf curl Multan betasatellite (CLCuMB), and Gossypium darwinii symptomless alphasatellite (GDarSLA), and Croton yellow vein mosaic alphasatellite (CrYVMoA). The second most abundant helper virus species, CLCuKoV-Bu, was associated with CLCuMB and GDarSLA.

A Major QTL Located in Chromosome 8 of Cucurbita moschata Is Responsible for Resistance to Tomato Leaf Curl New Delhi Virus

Tomato leaf curl New Delhi virus (ToLCNDV) is a bipartite whitefly transmitted begomovirus, responsible since 2013 of severe damages in cucurbit crops in Southeastern Spain. Zucchini (Cucurbita pepo) is the most affected species, but melon (Cucumis melo) and cucumber (Cucumis sativus) are also highly damaged by the infection. The virus has spread across Mediterranean basin and European countries, and integrated control measures are not being enough to reduce economic losses. The identification of resistance genes is required to develop resistant cultivars. In this assay, we studied the inheritance of the resistance to ToLCNDV previously identified in two Cucurbita moschata accessions. We generated segregating populations crossing both resistant pumpkins, an American improved cultivar Large Cheese (PI 604506) and an Indian landrace (PI 381814), with a susceptible C. moschata genotype (PI 419083). The analysis of symptoms and viral titers of all populations established the same monogenic recessive genetic control in both resistant accessions, and the allelism tests suggest the occurrence of alleles of the same locus. By genotyping with a single nucleotide polymorphism (SNP) collection evenly distributed along the C. moschata genome, a major quantitative trait locus (QTL) was identified in chromosome 8 controlling resistance to ToLCNDV. This major QTL was also confirmed in the interspecific C. moschata × C. pepo segregating populations, although C. pepo genetic background affected the resistance level. Molecular markers here identified, linked to the ToLCNDV resistance locus, are highly valuable for zucchini breeding programs, allowing the selection of improved commercial materials. The duplication of the candidate region within the C. moschata genome was studied, and genes with paralogs or single-copy genes were identified. Its synteny with the region of chromosome 17 of the susceptible C. pepo revealed an INDEL including interesting candidate genes. The chromosome 8 candidate region of C. moschata was also syntenic to the region in chromosome 11 of melon, previously described as responsible of ToLCNDV resistance. Common genes in the candidate regions of both cucurbits, with high- or moderate-impact polymorphic SNPs between resistant and susceptible C. moschata accessions, are interesting to study the mechanisms involved in this recessive resistance.

Molecular insight into cotton leaf curl geminivirus disease resistance in cultivated cotton (Gossypium hirsutum)

Cultivated cotton (Gossypium hirsutum) is the most important fibre crop in the world. Cotton leaf curl disease (CLCuD) is the major limiting factor and a threat to textile industry in India and Pakistan. All the local cotton cultivars exhibit moderate to no resistance against CLCuD. In this study, we evaluated an exotic cotton accession Mac7 as a resistance source to CLCuD by challenging it with viruliferous whiteflies and performing qPCR to evaluate the presence/absence and relative titre of CLCuD-associated geminiviruses/betasatellites. The results indicated that replication of pathogenicity determinant betasatellite is significantly attenuated in Mac7 and probably responsible for resistance phenotype. Afterwards, to decipher the genetic basis of CLCuD resistance in Mac7, we performed RNA sequencing on CLCuD-infested Mac7 and validated RNA-Seq data with qPCR on 24 independent genes. We performed co-expression network and pathway analysis for regulation of geminivirus/betasatellite-interacting genes. We identified nine novel modules with 52 hubs of highly connected genes in network topology within the co-expression network. Analysis of these hubs indicated the differential regulation of auxin stimulus and cellular localization pathways in response to CLCuD. We also analysed the differential regulation of geminivirus/betasatellite-interacting genes in Mac7. We further performed the functional validation of selected candidate genes via virus-induced gene silencing (VIGS). Finally, we evaluated the genomic context of resistance responsive genes and found that these genes are not specific to A or D sub-genomes of G. hirsutum. These results have important implications in understanding CLCuD resistance mechanism and developing a durable resistance in cultivated cotton.

Frequent occurrence of Mungbean yellow mosaic India virus in tomato leaf curl disease affected tomato in Oman

Next generation sequencing (NGS) of DNAs amplified by rolling circle amplification from 6 tomato (Solanum lycopersicum) plants with leaf curl symptoms identified a number of monopartite begomoviruses, including Tomato yellow leaf curl virus (TYLCV), and a betasatellite (Tomato leaf curl betasatellite [ToLCB]). Both TYLCV and ToLCB have previously been identified infecting tomato in Oman. Surprisingly the NGS results also suggested the presence of the bipartite, legume-adapted begomovirus Mungbean yellow mosaic Indian virus (MYMIV). The presence of MYMIV was confirmed by cloning and Sanger sequencing from four of the six plants. A wider analysis by PCR showed MYMIV infection of tomato in Oman to be widespread. Inoculation of plants with full-length clones showed the host range of MYMIV not to extend to Nicotiana benthamiana or tomato. Inoculation to N. benthamiana showed TYLCV to be capable of maintaining MYMIV in both the presence and absence of the betasatellite. In tomato MYMIV was only maintained by TYLCV in the presence of the betasatellite and then only at low titre and efficiency. This is the first identification of TYLCV with ToLCB and the legume adapted bipartite begomovirus MYMIV co-infecting tomato. This finding has far reaching implications. TYLCV has spread around the World from its origins in the Mediterranean/Middle East, in some instances, in live tomato planting material. The results here may suggest that begomoviruses which do not commonly infect tomato, such as MYMIV, could be spread as a passenger of TYLCV in tomato.

Non-cultivated Cotton Species (Gossypium spp.) Act as a Reservoir for Cotton Leaf Curl Begomoviruses and Associated Satellites

A collection of cultivated and non-cultivated species of cotton (Gossypium spp.) has been maintained for the last four decades in Multan, Pakistan. This geographical location has been observed as a hotspot for the evolution of begomoviruses and satellites associated with cotton leaf curl disease (CLCuD). Recent studies showed that begomoviruses responsible for the CLCuD epidemic in the 1990s, and that almost disappeared from the CLCuD complex in 2000s, have been observed again in CLCuD-infected cotton fields. To identify host species that acted as probable reservoirs for these viruses, we characterized begomoviruses and satellites in non-cultivated cotton species G. raimondii, G. thurberi and G. mustelinum and identified several species of CLCuD associated begomoviruses and satellites. Further, phylogenetic analysis indicated that the identified begomoviruses and beta/alphasatellites are closely related to the ones associated with the most recent CLCuD complex. qPCR indicated that the comparative level of virus significantly decreased in the presence of alphasatellites. Our results indicated that non-cultivated cotton species have been continuously challenged by diverse begomoviruses and associated satellites and act as reservoirs for CLCuD associated begomoviruses. These results provide novel insights into understanding the spread of begomoviruses and associated satellites in New World cotton species introduced into the Old World.

Natural Hosts and Genetic Diversity of the Emerging Tomato Leaf Curl New Delhi Virus in Spain

Knowledge about the host range and genetic structure of emerging plant viruses provides insights into fundamental ecological and evolutionary processes, and from an applied perspective, facilitates the design and implementation of sustainable disease control measures. Tomato leaf curl New Delhi virus (ToLCNDV) is an emerging whitefly transmitted begomovirus that is rapidly spreading and inciting economically important diseases in cucurbit crops of the Mediterranean basin. Genetic characterization of the ToLCNDV Mediterranean populations has shown that they are monophyletic in cucurbit plants. However, the extent to which other alternative (cultivated and wild) hosts may affect ToLCNDV genetic population structure and virus prevalence remains unknown. In this study a total of 683 samples from 13 cultivated species, and 203 samples from 24 wild species from three major cucurbit-producing areas of Spain (Murcia, Alicante and Castilla-La Mancha) from five cropping seasons (2012-2016) were analyzed for ToLCNDV infection. Except for watermelon, ToLCNDV was detected in all cultivated-cucurbit species as well as in tomato. Among weeds, Ecballium elaterium, Datura stramonium, Sonchus oleraceus, and Solanum nigrum were identified as alternative ToLCNDV plant hosts, which could act as new potential sources of virus inoculum. Furthermore, we performed full-genome deep-sequencing of 80 ToLCNDV isolates from different hosts, location and cropping year. Our phylogenetic analysis supports a Mediterranean virus population that is genetically very homogeneous, with no clustering pattern, and clearly different from Asian virus populations. Additionally, D. stramonium displayed higher levels of within-host genetic diversity than cultivated plants, and this variability appeared to increase with time. These results suggest that the potential ToLCNDV adaptive evolution occurring in wild plant hosts could serve as a source of virus genetic variability, thereby affecting the genetic structure and spatial-temporal dynamics of the viral population.

A Simplified Method to Engineer CRISPR/Cas9-Mediated Geminivirus Resistance in Plants

Throughout the world, geminiviruses cause devastating losses in economically important crops, including tomato, cotton, cassava, potato, chili, and cucumber; however, control mechanisms such as genetic resistance remain expensive and ineffective. CRISPR/Cas9 is an adaptive immunity mechanism used by prokaryotes to defend against invading nucleic acids of phages and plasmids. The CRISPR/Cas9 system has been harnessed for targeted genome editing in a variety of eukaryotic species, and in plants, CRISPR/Cas9 has been used to modify or introduce many traits, including virus resistance. Recently, we demonstrated that the CRISPR/Cas9 system could be used to engineer plant immunity against geminiviruses by directly targeting the viral genome for degradation. In this chapter, we describe a detailed method for engineering CRISPR/Cas9-mediated resistance against geminiviruses. This method may provide broad, durable viral resistance, as it can target conserved regions of the viral genome and can also be customized to emerging viral variants. Moreover, this method can be used in many crop species, as it requires little or no knowledge of the host plant's genome.

Cotton Leaf Curl Disease: Which Whitefly Is the Vector?

Cotton leaf curl disease is one of the most significant constraints to the production of cotton. In the past decades our understanding of the begomoviruses (family Geminiviridae) causing the disease has improved, but little is known regarding transmission of these viruses by the different species of whiteflies in the Bemisia tabaci complex. We compared transmission efficiency of cotton leaf curl Multan virus (CLCuMuV), one of the major begomoviruses associated with cotton leaf curl disease, by four whitefly species, of which two are indigenous to Asia and two are invasive worldwide. Only the indigenous Asia II 1 species was able to transmit this virus with high efficiency. By quantifying the virus and using immunoflorescence assays, we found that the differential transmission was associated with the varying efficiency of CLCuMuV to cross the midgut of various whitefly species. Further, we verified the role of coat protein in the whitefly transmission of CLCuMuV. Based on a phylogenetic analysis of the virus coat proteins, we found that most begomoviruses associated with cotton leaf curl disease might share similar whitefly transmission characteristics. These findings advance our understanding of the nature of cotton leaf curl disease and provide information for the development of control and preventive strategies against this disease.

Genetic diversity of begomoviruses in Pakistan captured through a vector based survey

Begomoviruses (Geminiviridea), transmitted by whiteflies, constitute one of the most dangerous groups of plant viruses posing a severe threat to economically important crops in tropical and sub-tropical areas. In this study, whiteflies were collected from various locations all over Pakistan. The begomoviruses carried by these whiteflies were detected by PCR with the degenerative primers pair AV94/Dep3. Analysis of the 177 sequences obtained in our study, revealed 14 distinct begomovirus species, including five which were not previously reported in this country. Putative novel strains of Corchorus yellow vein virus (CoYVV) and Chilli leaf curl virus (ChiLCV) showing less than 90% identity with the previously available taxa were also identified. The greatest number of begomoviruses per single site was detected in Sindh province, where up to five different begomovirus species were identified from the same cropping field. Moreover, Cotton leaf curl Multan virus - Rajasthan (CLCuMuV-Ra) was found prevalent in all the cotton growing areas. The data reported here may be useful in the development of control measures against begomoviruses.

Evaluation of carbon nanotube based copper nanoparticle composite for the efficient detection of agroviruses

We report a biosensor that combines the construction of a three-dimensional nanocomposite with electrochemical methods for the detection of viruses in plants. This is the first report, where carbon nanotubes are used as a conductive frame to anchor highly electrolytic agglomerates of copper nanoparticles to detect agroviruses. Morphological analysis of nanocomposite revealed the presence of carbon nanotubes having a diameter of 50-100nm with copper nanoparticles of 20-100nm, attached in the form of bunches. This material was applied to assess the infection caused by geminiviruses which are a major threat to the cotton plants in Asian and African countries. The hybridization events were studied by monitoring differential pulse voltammetry signals using methylene blue as a redox indicator. In the presence of target DNA, sensor signals decreased from 7×10^-4 to 1×10^-4Ampere. The probe exhibited 97.14% selectivity and the detection limit was found to be 0.01ngμL^-1. The developed biosensor is stable for at least four weeks, losing only 4.3% of the initial signal value. This sensor was able to detect the presence of viruses in sap extracted from cotton leaves, thus providing a promising platform to detect a range of other crops-infecting viruses.

Two distinct begomoviruses associated with an alphasatellite coinfecting Emilia sonchifolia in Thailand

Emilia sonchifolia is a traditionally used medicinal plant that is widespread in tropical and subtropical regions of the world. Yellow vein symptoms were observed in E. sonchifolia plants in fields in the county of Koh Samui, Surat Thani Province, Thailand, in August 2015. Two distinct begomoviruses, designated TH4872-6 and TH4872-9, and an associated alphasatellite were obtained from an E. sonchifolia leaf sample (TH4872). Sequence analysis showed that the full-length sequence of TH4872-6 was most closely related to that of ageratum yellow vein China virus (AYVCNV), with 85.7% identity, suggesting that it is a novel begomovirus, while the TH4872-9 sequence closely resembled cotton leaf curl Multan virus (CLCuMuV) with 99.1% identity. The alphasatellite sequence showed the highest nucleotide sequence identity (92.8%) to an isolate of tobacco curly shoot alphasatellite (TbCSA) originating from China. Recombination analysis revealed that the isolate TH4872-6 is a potential recombinant begomovirus, derived from ageratum yellow vein virus (AYVV) and tobacco leaf curl Thailand virus (TbLCTHV). This study represents the first report of begomoviruses identified in E. sonchifolia in Thailand.

Maintenance of Cotton Leaf Curl Multan Betasatellite by Tomato Leaf Curl New Delhi Virus-Analysis by Mutation

Viruses of the genus Begomovirus (family Geminiviridae) are economically important phytopathogens that are transmitted plant-to-plant by the whitefly Bemisia tabaci. Most Old World (OW) begomoviruses are monopartite and many of these interact with symptoms and host range determining betasatellites. Tomato leaf curl New Delhi virus (ToLCNDV) is one of only a few OW begomoviruses with a bipartite genome (components known as DNA A and DNA B). Four genes [AV2, coat protein (CP), transcriptional-activator protein (TrAP), and AC4] of ToLCNDV were mutated and the effects of the mutations on infectivity, symptoms and the ability to maintain Cotton leaf curl Multan betasatellite (CLCuMuB) were investigated. Infectivity and virus/betasatellite DNA titer were assessed by Southern blot hybridization, PCR, and quantitative PCR. The results showed TrAP of ToLCNDV to be essential for maintenance of CLCuMuB and AV2 to be important only in the presence of the DNA B. AC4 was found to be important for the maintenance of CLCuMuB in the presence of, but indispensable in the absence of, the DNA B. Rather than being required for maintenance, the CP was shown to possibly interfere with maintenance of the betasatellite. The findings show that the interaction between a bipartite begomovirus and a betasatellite is more complex than just trans-replication. Clearly, multiple levels of interactions are present and such associations can cause additional significant losses to crops although the interaction may not be stable.

Transcriptomics reveals multiple resistance mechanisms against cotton leaf curl disease in a naturally immune cotton species, Gossypium arboreum

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl viruses (CLCuVs), is among the most devastating diseases in cotton. While the widely cultivated cotton species Gossypium hirsutum is generally susceptible, the diploid species G. arboreum is a natural source for resistance against CLCuD. However, the influence of CLCuD on the G. arboreum transcriptome and the interaction of CLCuD with G. arboreum remains to be elucidated. Here we have used an RNA-Seq based study to analyze differential gene expression in G. arboreum under CLCuD infestation. G. arboreum plants were infested by graft inoculation using a CLCuD infected scion of G. hirsutum. CLCuD infested asymptomatic and symptomatic plants were analyzed with RNA-seq using an Illumina HiSeq. 2500. Data analysis revealed 1062 differentially expressed genes (DEGs) in G. arboreum. We selected 17 genes for qPCR to validate RNA-Seq data. We identified several genes involved in disease resistance and pathogen defense. Furthermore, a weighted gene co-expression network was constructed from the RNA-Seq dataset that indicated 50 hub genes, most of which are involved in transport processes and might have a role in the defense response of G. arboreum against CLCuD. This fundamental study will improve the understanding of virus-host interaction and identification of important genes involved in G. arboreum tolerance against CLCuD.

An Insight into Cotton Leaf Curl Multan Betasatellite, the Most Important Component of Cotton Leaf Curl Disease Complex

Cotton leaf curl disease (CLCuD) is one of the most economically important diseases and is a constraint to cotton production in major producers, Pakistan and India. CLCuD is caused by monopartite plant viruses belonging to the family Geminiviridae (genus Begomovirus), in association with an essential, disease-specific satellite, Cotton leaf curl Multan betasatellite (CLCuMuB) belonging to a newly-established family Tolecusatellitidae (genus Betasatellite). CLCuMuB has a small genome (ca. 1350 nt) with a satellite conserved region, an adenine-rich region and a single gene that encodes for a multifunctional βC1 protein. CLCuMuB βC1 protein has a major role in pathogenicity and symptom determination, and alters several host cellular functions like autophagy, ubiquitination, and suppression of gene silencing, to assist CLCuD infectivity. Efficient trans-replication ability of CLCuMuB with several monopartite and bipartite begomoviruses, is also associated with the rapid evolution and spread of CLCuMuB. In this article we comprehensively reviewed the role of CLCuMuB in CLCuD, focusing on the βC1 functions and its interactions with host proteins.

Genome wide identification of cotton (Gossypium hirsutum)-encoded microRNA targets against Cotton leaf curl Burewala virus

Cotton leaf curl Burewala virus (CLCuBV, genus Begomovirus) causes devastating cotton leaf curl disease. Among various known virus controlling strategies, RNAi-mediated one has shown potential to protect host crop plants. Micro(mi) RNAs, are the endogenous small RNAs and play a key role in plant development and stress resistance. In the present study we have identified cotton (Gossypium hirsutum)-encoded miRNAs targeting the CLCuBV. Based on threshold free energy and maximum complementarity scores of host miRNA-viral mRNA target pairs, a number of potential miRNAs were annotated. Among them, ghr-miR168 was selected as the most potent candidate, capable of targeting several vital genes namely C1, C3, C4, V1 and V2 of CLCuBV genome. In addition, ghr-miR395a and ghr-miR395d were observed to target the overlapping transcripts of C1 and C4 genes. We have verified the efficacy of these miRNA targets against CLCuBV following suppression of RNAi-mediated virus control through translational inhibition or cleavage of viral mRNA.

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.

Engineering Molecular Immunity Against Plant Viruses

Genomic engineering has been used to precisely alter eukaryotic genomes at the single-base level for targeted gene editing, replacement, fusion, and mutagenesis, and plant viruses such as Tobacco rattle virus have been developed into efficient vectors for delivering genome-engineering reagents. In addition to altering the host genome, these methods can target pathogens to engineer molecular immunity. Indeed, recent studies have shown that clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) systems that target the genomes of DNA viruses can interfere with viral activity and limit viral symptoms in planta, demonstrating the utility of this system for engineering molecular immunity in plants. CRISPR/Cas9 can efficiently target single and multiple viral infections and confer plant immunity. Here, we discuss the use of site-specific nucleases to engineer molecular immunity against DNA and RNA viruses in plants. We also explore how to address the potential challenges encountered when producing plants with engineered resistance to single and mixed viral infections.

The Prediction of a New CLCuD Epidemic in the Old World

Cotton leaf curl disease (CLCuD), the most complex disease of cotton, is a major limiting biotic factor to worldwide cotton productivity. Several whitefly-transmitted monopartite begomoviruses causing CLCuD have been characterized and designated as CLCuD-associated begomoviruses. Despite of being reported over 100 years ago in Africa, CLCuD became economically pandemic causing massive losses to cotton production in Pakistan and India during past couple of decades. In Asia, cotton has faced two major epidemics during this period viz. "Multan epidemic" and "Burewala epidemic." The "Multan epidemic" era was 1988-1999 after which the virus remained calm until 2002 when "Burewala epidemic" broke into the cotton fields in Indo-Pak subcontinent, till 2013-2014. However, both the epidemics were caused by monopartite begomovirus complex. Similarly in Africa, Cotton leaf curl Gezira virus with associated DNA-satellites causes CLCuD. Quite recently, in the Old World (both Asia and Africa), bipartite begomoviruses have started appearing in the areas under cotton cultivation. Under such aggravated circumstances, it seems we are heading toward another epidemic of CLCuD in the Old World. Here we articulate the causes and potential emergence of the third epidemic of CLCuD in Asia. The current situation of CLCuD in Asia and Africa is also discussed.

Multiple begomoviruses found associated with cotton leaf curl disease in Pakistan in early 1990 are back in cultivated cotton

The first epidemic of cotton leaf curl disease (CLCuD) in early 1990's in the Indian subcontinent was associated with several distinct begomoviruses along with a disease-specific betasatellite. Resistant cotton varieties were introduced in late 1990's but soon resistance was broken and was associated with a single recombinant begomovirus named Burewala strain of Cotton leaf curl Kokhran virus that lacks a full complement of a gene encoding a transcription activator protein (TrAP). In order to understand the ongoing changes in CLCuD complex in Pakistan, CLCuD affected plants from cotton fields at Vehari were collected. Illumina sequencing was used to assess the diversity of CLCuD complex. At least three distinct begomoviruses characterized from the first epidemic; Cotton leaf curl Multan virus, Cotton leaf curl Kokhran virus and Cotton leaf curl Alabad virus, several distinct species of alphasatellites and cotton leaf curl Multan betasatellite were found associated with CLCuD. These viruses were also cloned and sequenced through Sanger sequencing to confirm the identity of the begomoviruses and that all clones possessed a full complement of the TrAP gene. A new strain of betasatellite was identified here and named CLCuMuB^Veh. The implications of these findings in efforts to control CLCuD are discussed.

Engineering Plant Immunity: Using CRISPR/Cas9 to Generate Virus Resistance

Plant viruses infect many economically important crops, including wheat, cotton, maize, cassava, and other vegetables. These viruses pose a serious threat to agriculture worldwide, as decreases in cropland area per capita may cause production to fall short of that required to feed the increasing world population. Under these circumstances, conventional strategies can fail to control rapidly evolving and emerging plant viruses. Genome-engineering strategies have recently emerged as promising tools to introduce desirable traits in many eukaryotic species, including plants. Among these genome engineering technologies, the CRISPR (clustered regularly interspaced palindromic repeats)/CRISPR-associated 9 (CRISPR/Cas9) system has received special interest because of its simplicity, efficiency, and reproducibility. Recent studies have used CRISPR/Cas9 to engineer virus resistance in plants, either by directly targeting and cleaving the viral genome, or by modifying the host plant genome to introduce viral immunity. Here, we briefly describe the biology of the CRISPR/Cas9 system and plant viruses, and how different genome engineering technologies have been used to target these viruses. We further describe the main findings from recent studies of CRISPR/Cas9-mediated viral interference and discuss how these findings can be applied to improve global agriculture. We conclude by pinpointing the gaps in our knowledge and the outstanding questions regarding CRISPR/Cas9-mediated viral immunity.