Multifaceted role of geminivirus associated betasatellite in pathogenesis

Deciphering intricate plant-virus interactions: Potyvirids orchestrate protein posttranslational modifications to regulate pathogenicity

In a molecular-arm-race between viruses and their hosts, viruses have evolved to harness their host's post-translational modifications (PTMs) machinery to gain a competitive edge. These modifications are the most reliable target of plant viruses to overcome the host defence for successful infection. Relatively fewer PTMs i.e., phosphorylation, O-GlcNAcylation, Ubiquitination, and SUMOylation have been studied regulating the potyvirus-plant interaction. Therefore, it is worth drawing attention towards the importance and potential of this undermined but key strategy of potyvirids (members of family Potyviridae) to abduct their host defence line, suggesting to review in detail the existing knowledge of these PTMs and highlight the unexplored modifications that might have played their part in establishing successful infection. The current review provides an understanding of how PTMs execute viral replication and infection dynamics during plant-potyvirid interactions. We highlighted that PTMs linked to CP, NIa-pro, NIb, and VPg are important to specify their host, virulence, overcoming host innate immunity, and most importantly disarming the host of RNA silencing tool of nailing any intruder. The limitations and potential improvements in studying undermined PTMs, including acetylation, glycosylation, methylation, and neddylation, as well as challenges and future perspectives of this inevitable process are mechanistically deciphered in the course of plant-virus interactions. This communication opens new avenues for investigating the fundamental mechanisms of virus infection and the development of new antiviral strategies for sustainable disease managements.

Predicting candidate miRNAs for targeting begomovirus to induce sequence-specific gene silencing in chilli plants

The begomoviruses are the most economically damaging pathogens that pose a serious risk to India's chilli crop and have been associated with the chilli leaf curl disease (ChiLCD). Chilli cultivars infected with begomovirus have suffered significant decreases in biomass output, negatively impacting their economic characteristics. We used the C-mii tool to predict twenty plant miRNA families from SRA chilli transcriptome data (retrieved from the NCBI and GenBank databases). Five target prediction algorithms, i.e., C-mii, miRanda, psRNATarget, RNAhybrid, and RNA22, were applied to identify and evaluate chilli miRNAs (microRNAs) as potential therapeutic targets against ten begomoviruses that cause ChiLCD. In this study, the top five chilli miRNAs which were identified by all five algorithms were thoroughly examined. Moreover, we also noted strong complementarities between these miRNAs and the AC1 (REP), AC2 (TrAP) and betaC1 genes. Three computational approaches (miRanda, RNA22, and psRNATarget) identified the consensus hybridization site for CA-miR838 at locus 2052. The top predicted targets within ORFs were indicated by CA-miR2673 (a and b). Through Circos algorithm, we identified novel targets and create the miRNA-mRNA interaction network using the R program. Furthermore, free energy calculation of the miRNA-target duplex revealed that thermodynamic stability was optimal for miR838 and miR2673 (a and b). To the best of our knowledge, this was the first instance of miRNA being predicted from chilli transcriptome information that had not been reported in miRbase previously. Consequently, the anticipated biological results substantially assist in developing chilli plants resistant to ChiLCD.

Molecular characterization of begomovirus and DNA satellites associated with mosaic and leaf curl disease of Jamaica cherry (Muntingia calabura) in India: Uncovering a new host for chilli leaf curl India virus

Begomoviruses, member of the Geminiviridae family, are responsible for significant economic losses in crops worldwide. Chilli leaf curl India virus (ChiLCINV) is a well-known begomovirus that causes leaf curl disease, primarily affecting plants in the Solanaceae family. In this study, sample from a Jamaica cherry (Muntingia calabura) tree showing typical begomovirus symptoms of mosaic and leaf curling was collected from Nagavara village in the Bengaluru Rural district of Karnataka State, India. The collected sample was designated as the MUT-1 isolate. The association of the begomovirus (DNA-A) and betasatellites with the sample was confirmed by PCR using begomovirus-specific primers, resulting in the expected amplicons of approximately 1.2 kb and 1.3 kb, respectively. No amplification was obtained for DNA-B and alphasatellite specific primers. The complete genome sequence of DNA-A of begomovirus isolate MUT-1 was obtained through rolling circle amplification and compared with other begomoviruses using Sequence Demarcation Tool which revealed that, DNA-A of MUT-1 isolate, (Acc.No. PP475538) showed maximum nucleotide (nt) identity of 98.7-99.4% with chilli leaf curl India virus. Further, sequence of betasattelite (Acc.No. PP493212) of this isolate shared maximum nt identity of 86.5-100% with tomato leaf curl Bangladesh betasatellite (ToLCBDB). Recombination and GC plot analysis showed that the presence of two and three intraspecific recombination event in DNA-A and betasatellite genomic regions, respectively and are derived from the previously reported begomoviruses. This study presents one more evidence of expanding host range for begomoviruses and first record of begomovirus associated with mosaic and leaf curl disease of Jamaica cherry (M. calabura) from India.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-024-00891-w.

A transition from enemies to allies: how viruses improve drought resilience in plants

Global crop production is severely affected by environmental factors such as drought, salinity, cold, flood etc. Among these stresses, drought is one of the major abiotic stresses reducing crop productivity. It is expected that drought conditions will further increase because of the increasing global temperature. In general, viruses are seen as a pathogen affecting the crop productivity. However, several researches are showing that viruses can induce drought tolerance in plants. This review explores the mechanisms underlying the interplay between viral infections and the drought response mechanisms in plants. We tried to address the molecular pathways and physiological changes induced by viruses that confer drought tolerance, including alterations in hormone signaling, antioxidant defenses, scavenging the reactive oxygen species, role of RNA silencing and miRNA pathway, change in the expression of several genes including heat shock proteins, cellulose synthase etc. Furthermore, we discuss various viruses implicated in providing drought tolerance and examine the range of plant species exhibiting this phenomenon. By applying current knowledge and identifying gaps in understanding, this review aims to provide valuable insights into the complex dynamics of virus-induced drought tolerance in plants, paving the way for future research directions and practical applications in sustainable agriculture.

A geminivirus crosses the monocot-dicot boundary and acts as a viral vector for gene silencing and genome editing

INTRODUCTION

Members of the family Geminiviridae have been reported to infect either a monocot plant or a dicot plant, but not both. This study reports a geminivirus, Wheat Dwarf India Virus (WDIV), first identified in wheat, that is capable of infecting both monocot and dicot plants and acting as a viral vector.

OBJECTIVES

This study was aimed at developing a broad host range viral vector system for reverse genetics and genome editing.

METHODS

Here we used a wheat isolate of WDIV and Ageratum yellow leaf curl betasatellite (AYLCB) for infectivity assays and vector development. We performed Agrobacterium-mediated inoculation of WDIV and AYLCB in wheat, oat, barley, corn, soybean, and tobacco. To examine the potential of WDIV to act as a viral vector, we modified the WDIV genome and cloned DNA fragments of the phytoene desaturase (PDS) genes from wheat and tobacco, separately. For gene editing experiments, tobacco lines expressing Cas9 were infiltrated with a WDIV-based vector carrying gRNA targeting the PDS gene.

RESULTS

About 80 to 90% of plants inoculated with infectious clones of WDIV alone or WDIV together with AYLCB showed mild symptoms, whereas some plants showed more prominent symptoms. WDIV and AYLCB were detected in the systemically infected leaves of all the plant species. Furthermore, the inoculation of the WDIV vector carrying PDS fragments induced silencing of the PDS gene in both wheat and tobacco plants. We also observed high-efficiency genome editing in the Cas9-expressing tobacco plants that were inoculated with WDIV vector-carrying gRNA.

CONCLUSION

Detection of WDIV in naturally infected wheat, barley, and sugarcane in the field and its ability to systemically infect wheat, oat, barley, corn, soybean, and tobacco under laboratory conditions, provides compelling evidence that WDIV is the first geminivirus identified with the capability of infecting both monocot and dicot plant species. The wide host range of WDIV can be exploited for developing a single vector system for high-throughput genome editing in many plant species.

The Role of Satellites in the Evolution of Begomoviruses

Begomoviruses have emerged as destructive pathogens of crops, particularly in the tropics and subtropics, causing enormous economic losses and threatening food security. Epidemics caused by begomoviruses have even spread in regions and crops that were previously free from these viruses. The most seriously affected crops include cassava; cotton; grain legumes; and cucurbitaceous, malvaceous, and solanaceous vegetables. Alphasatellites, betasatellites, and deltasatellites are associated with the diseases caused by begomoviruses, but begomovirus-betasatellite complexes have played significant roles in the evolution of begomoviruses, causing widespread epidemics in many economically important crops throughout the world. This article provides an overview of the evolution, distribution, and approaches used by betasatellites in the suppression of host plant defense responses and increasing disease severity.

An Interspecies Recombinant Sida Yellow Mosaic China Virus Isolate and Betasatellite Cause a Leaf Curl Disease in Tobacco in Hainan, China

Tobacco (Nicotiana tabacum) is an herbaceous crop. Cigar tobacco, a group of tobacco cultivars, has recently been planted in a few provinces in China. Since its introduction, symptoms such as leaf curling and vein thickening have appeared. Here we report a begomovirus, Sida yellow mosaic China virus-Hainan isolate (designated SiYMCNV-HN), associated with the betasatellite (designated SiYMCNB-HN) as the causal agent of a leaf curl disease in cigar tobacco (N. tabacum cv. Haiyan101) in Hainan Province, China. Phylogenetic and recombination analyses indicate that SiYMCNV-HN is an interspecies recombinant with a SiYMCNV isolate as the major parent and a Sida yellow vein Vietnam virus isolate as the minor parent. Full-length infectious clones of SiYMCNV-HN and SiYMCNB-HN were generated, which were highly infectious and induced high pathogenicity through agroinfiltration in Nicotiana benthamiana and N. tabacum. This newly reported recombinant begomovirus poses potential threats to tobacco plantations in the region.

A comprehensive review on Gossypium hirsutum resistance against cotton leaf curl virus

Cotton (Gossypium hirsutum L.) is a significant fiber crop. Being a major contributor to the textile industry requires continuous care and attention. Cotton is subjected to various biotic and abiotic constraints. Among these, biotic factors including cotton leaf curl virus (CLCuV) are dominant. CLCuV is a notorious disease of cotton and is acquired, carried, and transmitted by the whitefly (Bemisia tabaci). A cotton plant affected with CLCuV may show a wide range of symptoms such as yellowing of leaves, thickening of veins, upward or downward curling, formation of enations, and stunted growth. Though there are many efforts to protect the crop from CLCuV, long-term results are not yet obtained as CLCuV strains are capable of mutating and overcoming plant resistance. However, systemic-induced resistance using a gene-based approach remained effective until new virulent strains of CLCuV (like Cotton Leaf Curl Burewala Virus and others) came into existence. Disease control by biological means and the development of CLCuV-resistant cotton varieties are in progress. In this review, we first discussed in detail the evolution of cotton and CLCuV strains, the transmission mechanism of CLCuV, the genetic architecture of CLCuV vectors, and the use of pathogen and nonpathogen-based approaches to control CLCuD. Next, we delineate the uses of cutting-edge technologies like genome editing (with a special focus on CRISPR-Cas), next-generation technologies, and their application in cotton genomics and speed breeding to develop CLCuD resistant cotton germplasm in a short time. Finally, we delve into the current obstacles related to cotton genome editing and explore forthcoming pathways for enhancing precision in genome editing through the utilization of advanced genome editing technologies. These endeavors aim to enhance cotton's resilience against CLCuD.

Near-infrared light and PIF4 promote plant antiviral defense by enhancing RNA interference

The molecular mechanism underlying phototherapy and light treatment, which utilize various wavelength spectra of light, including near-infrared (NIR), to cure human and plant diseases, is obscure. Here we revealed that NIR light confers antiviral immunity by positively regulating PHYTOCHROME-INTERACTING FACTOR 4 (PIF4)-activated RNA interference (RNAi) in plants. PIF4, a central transcription factor involved in light signaling, accumulates to high levels under NIR light in plants. PIF4 directly induces the transcription of two essential components of RNAi, RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) and ARGONAUTE 1 (AGO1), which play important roles in resistance to both DNA and RNA viruses. Moreover, the pathogenic determinant βC1 protein, which is evolutionarily conserved and encoded by betasatellites, interacts with PIF4 and inhibits its positive regulation of RNAi by disrupting PIF4 dimerization. These findings shed light on the molecular mechanism of PIF4-mediated plant defense and provide a new perspective for the exploration of NIR antiviral treatment.

Molecular Signature of a Novel Alternanthera Yellow Vein Virus Variant Infecting the Ageratum conyzoides Weed in Oman

Alternanthera yellow vein virus (AlYVV), a monopartite begomovirus, has been identified infecting a diverse range of crops and native plants in Pakistan, India, and China. However, distinctive yellow vein symptoms, characteristic of begomovirus infection, were observed on the Ageratum conyzoides weed in Oman, prompting a thorough genomic characterization in this study. The results unveiled a complete genome sequence of 2745 base pairs and an associated betasatellite spanning 1345 base pairs. In addition, Sequence Demarcation Tool analyses indicated the highest nucleotide identity of 92.8% with a previously reported AlYVV-[IN_abalpur_A_17:LC316182] strain, whereas the betasatellite exhibited a 99.8% nucleotide identity with isolates of tomato leaf curl betasatellite. Thus, our findings propose a novel AlYVV Oman virus (AlYVV-OM) variant, emphasizing the need for additional epidemiological surveillance to understand its prevalence and significance in Oman and the broader region. To effectively manage the spread of AlYVV-OM and minimize its potential harm to (agro)ecosystems, future research should focus on elucidating the genetic diversity of AlYVV-OM and its interactions with other begomoviruses.

Transreplication Preference of the Tomato Leaf Curl Joydebpur Virus for a Noncognate Betasatellite through Iteron Resemblance on Nicotiana bethamiana

Pepper plants (Capsicum annuum) with severe leaf curl symptoms were collected in 2013 from Bangalore, Karnataka, India. The detection results showed a co-infection between the tomato leaf curl Joydebpur virus (ToLCJoV) and tomato leaf curl Bangladesh betasatellite (ToLCBDB) through the sequencing analysis of PCR amplicons. To pinpoint the molecular mechanism of this uncommon combination, infectious clones of ToLCJoV and two different betasatellites-ToLCBDB and tomato leaf curl Joydebpur betasatellite (ToLCJoB)-were constructed and tested for their infectivity in Nicotiana benthamiana. Together, we conducted various combined agroinoculation studies to compare the interaction of ToLCJoV with non-cognate and cognate betasatellites. The natural non-cognate interaction between ToLCJoV and ToLCBDB showed severe symptoms compared to the mild symptoms of a cognate combination (ToLCJoV × ToLCJoB) in infected plants. A sequence comparison among betasatellites and their helper virus wasperformed and the iteron resemblances in ToLCBDB as well as ToLCJoB clones were processed. Mutant betasatellites that comprised iteron modifications revealed that changes in iteron sequences could disturb the transreplication process between betasatellites and their helper virus. Our study might provide an important consideration for determining the efficiency of transreplication activity between betasatellites and their helper virus.

SEGS-1 a cassava genomic sequence increases the severity of African cassava mosaic virus infection in Arabidopsis thaliana

Cassava is a major crop in Sub-Saharan Africa, where it is grown primarily by smallholder farmers. Cassava production is constrained by Cassava mosaic disease (CMD), which is caused by a complex of cassava mosaic begomoviruses (CMBs). A previous study showed that SEGS-1 (sequences enhancing geminivirus symptoms), which occurs in the cassava genome and as episomes during viral infection, enhances CMD symptoms and breaks resistance in cassava. We report here that SEGS-1 also increases viral disease severity in Arabidopsis thaliana plants that are co-inoculated with African cassava mosaic virus (ACMV) and SEGS-1 sequences. Viral disease was also enhanced in Arabidopsis plants carrying a SEGS-1 transgene when inoculated with ACMV alone. Unlike cassava, no SEGS-1 episomal DNA was detected in the transgenic Arabidopsis plants during ACMV infection. Studies using Nicotiana tabacum suspension cells showed that co-transfection of SEGS-1 sequences with an ACMV replicon increases viral DNA accumulation in the absence of viral movement. Together, these results demonstrated that SEGS-1 can function in a heterologous host to increase disease severity. Moreover, SEGS-1 is active in a host genomic context, indicating that SEGS-1 episomes are not required for disease enhancement.

N-terminal acetylation of the βC1 protein encoded by the betasatellite of tomato yellow leaf curl China virus is critical for its viral pathogenicity

N-terminal acetylation (N-acetylation) is one of the most common protein modifications and plays crucial roles in viability and stress responses in animals and plants. However, very little is known about N-acetylation of viral proteins. Here, we identified the Thr residue at position 2 (Thr-2) in the βC1 protein encoded by the betasatellite of tomato yellow leaf curl China virus (TYLCCNB-βC1) as a novel N-acetylation site. Furthermore, the effects of TYLCCNB-βC1 N-acetylation on its function as a pathogenicity factor were determined via N-acetylation mutants in Nicotiana benthamiana plants. We found that N-acetylation of TYLCCNB-βC1 is critical for its self-interaction in the nucleus and viral pathogenesis, and that removal of N-acetylation of TYLCCNB-βC1 attenuated tomato yellow leaf curl China virus-induced symptoms and led to accelerated degradation of TYLCCNB-βC1 through the ubiquitin-proteasome system. Our data reveal a protective effect of N-acetylation of TYLCCNB-βC1 on its pathogenesis and demonstrate an antagonistic crosstalk between N-acetylation and ubiquitination in this geminiviral protein.

Betasatellite-encoded βC1 protein regulates helper virus accumulation by interfering with the ATP hydrolysis activity of geminivirus-encoded replication initiator protein

Geminivirus-betasatellite disease complexes are an epidemic threat to the majority of economically important crops across the world. Plant virus satellites including betasatellites are maintained by their associated helper virus. Geminivirus-betasatellites influence viral pathogenesis by substantially increasing or decreasing their helper virus accumulation. In the present study, we attempted to understand the mechanistic details of the geminivirus-betasatellite interaction. Here, we used tomato leaf curl Gujarat virus (ToLCGV) and tomato leaf curl Patna betasatellite (ToLCPaB) as a model system. This study reveals that ToLCGV can efficiently trans-replicate ToLCPaB in Nicotiana benthamiana plants, but ToLCPaB greatly reduced the accumulation of its helper virus DNA. For the first time, we have identified that the ToLCPaB-encoded βC1 protein is able to interact with ToLCGV-encoded replication initiator protein (Rep). In addition, we demonstrate that the C-terminal region of βC1 interacts with the C-terminus of Rep (RepC) protein. Our previous study had established that βC1 proteins encoded by diverse betasatellites possess a novel ATP hydrolysis activity and the conserved lysine/arginine residues at positions 49 and 91 are necessary for this function. Here, we show that mutating lysine at positions 49 to alanine of βC1 (βC1K49A) protein did not affect its ability to interact with RepC protein. Biochemical studies performed with ATP hydrolysis activity-deficient K49A mutated βC1 (βC1K49A) and RepC proteins revealed that Rep-βC1 interaction interferes with the ATP hydrolysis activity of Rep protein. Further, we demonstrate that βC1 protein is able to interact with D227A and D289A mutated RepC proteins but not with D262A, K272A or D286A mutated RepC proteins, suggesting that the βC1-interacting region of Rep protein encompasses its Walker-B and B' motifs. The results of docking studies supported that the βC1-interacting region of Rep protein encompasses its motifs associated with ATP binding and ATP hydrolysis activities. Docking studies also provided evidence that the Rep-βC1 interaction interferes with the ATP binding activity of Rep protein. Together, our findings suggest that βC1 protein regulates helper virus accumulation by interfering with the ATP hydrolysis activity of helper virus Rep protein.

Differences in the 3' intergenic region and the V2 protein of two sequence variants of tomato curly stunt virus play an important role in disease pathology in Nicotiana benthamiana

Tomato production in South Africa is threatened by the emergence of tomato curly stunt virus (ToCSV), a monopartite Begomovirus transmitted by the whitefly vector Bemisia tabaci (Genn.). We investigated the role of sequence differences present in the 3' intergenic region (IR) and the V2 coding region on the differing infectivity of ToCSV sequence variant isolates V30 and V22 in the model host Nicotiana benthamiana. Using virus mutant chimeras, we determined that the development of the upward leaf roll symptom phenotype is mediated by sequence differences present in the 3' IR containing the TATA-associated composite element. Sequence differences present in the V2 coding region are responsible for modulating disease severity and symptom recovery in V22-infected plants. Serine substitution of V22 V2 Val27 resulted in a significant increase in disease severity with reduced recovery, the first study to demonstrate the importance of this V2 residue in disease development. Two putative ORFs, C5 and C6, were identified using in silico analysis and detection of an RNA transcript spanning their coding region suggests that these ORFs may be transcribed during infection. Additional virus-derived RNA transcripts spanning multiple ORFs and crossing the boundaries of recognised polycistronic transcripts, as well as the origin of replication within the IR, were detected in ToCSV-infected plants providing evidence of bidirectional readthrough transcription. From our results, we conclude that the diverse responses of the model host to ToCSV infection is influenced by select sequence differences and our findings provide several avenues for further investigation into the mechanisms behind these responses to infection.

Recovery from virus infection: plant's armory in action

MAIN CONCLUSION

This review focuses on different factors involved in promoting symptom recovery in plants post-virus infection such as epigenetics, transcriptional reprogramming, phytohormones with an emphasis on RNA silencing as well as role of abiotic factors such as temperature on symptom recovery. Plants utilize several different strategies to defend themselves in the battle against invading viruses. Most of the viral proteins interact with plant proteins and interfere with molecular dynamics in a cell which eventually results in symptom development. This initial symptom development is countered by the plant utilizing various factors including the plant's adaptive immunity to develop a virus tolerant state. Infected plants can specifically target and impede the transcription of viral genes as well as degrade the viral transcripts to restrict their proliferation by the production of small-interfering RNA (siRNA) generated from the viral nucleic acid, known as virus-derived siRNA (vsiRNA). To further escalate the degradation of viral nucleic acid, secondary siRNAs are generated. The production of virus-activated siRNA (vasiRNA) from the host genome causes differential regulation of the host transcriptome which plays a major role in establishing a virus tolerant state within the infected plant. The systemic action of vsiRNAs, vasiRNA, and secondary siRNAs with the help of defense hormones like salicylic acid can curb viral proliferation, and thus the newly emerged leaves develop fewer symptoms, maintaining a state of tolerance.

Diverse begomovirus-betasatellite complexes cause tomato leaf curl disease in the western India

In the Indian sub-continent, tomato leaf curl disease (ToLCD) of tomato caused by begomoviruses has emerged as a major limiting factor for tomato cultivation. Despite the spread of this disease in the western India, a systematic study on the characterization of virus complexes with ToLCD is lacking. Here, we report the identification of a complex of begomoviruses including 19 DNA-A and 4 DNA-B as well as 15 betasatellites with ToLCD in the western part of the country. Additionally, a novel betasatellite and an alphasatellite were also identified. The recombination breakpoints were detected in the cloned begomoviruses and betasatellites. The cloned infectious DNA constructs cause disease on the tomato (a moderately virus-resistant cultivar) plants, thus fulfilling Koch's postulates for these virus complexes. Further, the role of non-cognate DNA B/betasatellite with ToLCD-associated begomoviruses on disease development was demonstrated. It also emphasizes the evolutionary potential of these virus complexes in breaking disease resistance and plausible expansion of its host range. This necessitates to investigate the mechanism of the interaction between resistance breaking virus complexes and the infected host.

Temporal Dynamic of the Ratio between Monopartite Begomoviruses and Their Associated Betasatellites in Plants, and Its Modulation by the Viral Gene βC1

The begomovirus-betasatellite complex constantly threatens crops in Asia. However, the quantitative relationship between begomoviruses and betasatellites remains largely unknown. The quantities of tobacco curly shoot virus (TbCSV) and its betasatellite (TbCSB) and their ratio varied significantly in initial infection, and thereafter, the ratio tended to become constant. The TbCSB/TbCSV ratio in agrobacteria inoculum significantly affected that in plants in the initial infection but not thereafter. Null-mutation of βC1 that encodes a multifunctional protein important for pathogenesis in TbCSB significantly reduced the TbCSB/TbCSV ratio in plants. Viral inoculum plants with higher TbCSB/TbCSV ratios promoted whitefly transmission of the virus. The expression of AV1 encoded by TbCSV, βC1 encoded by TbCSB and the βC1/AV1 ratio varied significantly in the initial infection and thereafter the ratio tended to become constant. Additionally, the temporal dynamics of the ratio between another begomovirus and its betasatellite was similar to that of TbCSV and was positively regulated by βC1. These results indicate that the ratio between monopartite begomoviruses and betasatellites tend to become constant as infection progresses, and is modulated by βC1, but a higher betasatellite/begomovirus ratio in virally inoculated plants promotes virus transmission by whiteflies. Our findings provide novel insights into the association between begomoviruses and betasatellites.

The captivating role of calcium in plant-microbe interaction

Plant immune response is fascinating due to the complete absence of a humoral system. The adaptive immune response in plants relies on the intracellular orchestration of signalling molecules or intermediates associated with transcriptional reprogramming. Plant disease response phenomena largely depend on pathogen recognition, signal perception, and intracellular signal transduction. The pathogens possess specific pathogen-associated molecular patterns (PAMP) or microbe-associated molecular patterns (MAMP), which are first identified by pattern recognition receptors (PRRs) of host plants for successful infection. After successful pathogen recognition, the defence response is initiated within plants. The first line of non-specific defence response is called PAMP-triggered immunity (PTI), followed by the specific robust signalling is called effector-triggered immunity (ETI). Calcium plays a crucial role in both PTI and ETI. The biphasic induction of reactive oxygen species (ROS) is inevitable in any plant-microbe interaction. Calcium ions play crucial roles in the initial oxidative burst and ROS induction. Different pathogens can induce calcium accumulation in the cytosol ([Ca²⁺]_Cyt), called calcium signatures. These calcium signatures further control the diverse defence-responsive proteins in the intracellular milieu. These calcium signatures then activate calcium-dependent protein kinases (CDPKs), calcium calmodulins (CaMs), calcineurin B-like proteins (CBLs), etc., to impart intricate defence signalling within the cell. Decoding this calcium ionic map is imperative to unveil any plant microbe interplay and modulate defence-responsive pathways. Hence, the present review is unique in developing concepts of calcium signature in plants and their subsequent decoding mechanism. This review also intends to articulate early sensing of calcium oscillation, signalling events, and comprehensive mechanistic roles of calcium within plants during pathogenic ingression. This will accumulate and summarize the exciting roles of calcium ions in plant immunity and provide the foundation for future research.

The rising threat of geminiviruses: molecular insights into the disease mechanism and mitigation strategies

BACKGROUND

Geminiviruses are among the most threatening emerging plant viruses, accountable for a huge loss to agricultural production worldwide. These viruses have been responsible for some serious outbreaks during the last few decades across different parts of the world. Sincere efforts have been made to regulate the disease incidence by incorporating a multi-dimensional approach, and this process has been facilitated greatly by the advent of molecular techniques. But, the mixed infection due to the polyphagous nature of vectors results in viral recombination followed by the emergence of novel viral strains which thus renders the existing mitigation strategies ineffective. Hence, a multifaceted insight into the molecular mechanism of the disease is really needed to understand the regulatory points; much has been done in this direction during the last few years. The present review aims to explore all the latest developments made so far and to organize the information in a comprehensive manner so that some novel hypotheses for controlling the disease may be generated.

METHODS AND RESULTS

Starting with the background information, diverse genera of geminiviruses are listed along with their pathological and economic impacts. A comprehensive and detailed mechanism of infection is elaborated to study the interactions between vector, host, and virus at different stages in the life cycle of geminiviruses. Finally, an effort isalso made to analyze the progress made at the molecular level for the development of various mitigation strategies and suggest more effective and better approaches for controlling the disease.

CONCLUSION

The study has provided a thorough understanding of molecular mechanism of geminivirus infection.

Genetic Diversity and Geographic Distribution of Cucurbit-Infecting Begomoviruses in the Philippines

Cucurbits are important economic crops worldwide. However, the cucurbit leaf curl disease (CuLCD), caused by whitefly-transmitted begomoviruses constrains their production. In Southeast Asia, three major begomoviruses, Tomato leaf curl New Delhi virus (ToLCNDV), Squash leaf curl China virus (SLCCNV) and Squash leaf curl Philippines virus (SLCuPV) are associated with CuLCD. SLCuPV and SLCCNV were identified in Luzon, the Philippines. Here, the genetic diversity and geographic distribution of CuLCD-associated begomoviruses in the Philippines were studied based on 103 begomovirus detected out of 249 cucurbit samples collected from 60 locations throughout the country in 2018 and 2019. The presence of SLCCNV and SLCuPV throughout the Philippines were confirmed by begomovirus PCR detection and viral DNA sequence analysis. SLCuPV was determined as a predominant CuLCD-associated begomovirus and grouped into two strains. Interestingly, SLCCNV was detected in pumpkin and bottle gourd without associated viral DNA-B and mixed-infected with SLCuPV. Furthermore, the pathogenicity of selected isolates of SLCCNV and SLCuPV was confirmed. The results provide virus genetic diversity associated with CuLCD for further disease management, especially in developing the disease-resistant cultivars in the Philippines as well as Southeast Asia.

Diverse Begomoviruses Evolutionarily Hijack Plant Terpenoid-Based Defense to Promote Whitefly Performance

Arthropod-borne pathogens and parasites are major threats to human health and global agriculture. They may directly or indirectly manipulate behaviors of arthropod vector for rapid transmission between hosts. The largest genus of plant viruses, Begomovirus, is transmitted exclusively by whitefly (Bemisia tabaci), a complex of at least 34 morphologically indistinguishable species. We have previously shown that plants infected with the tomato yellowleaf curl China virus (TYLCCNV) and its associated betasatellite (TYLCCNB) attract their whitefly vectors by subverting plant MYC2-regulated terpenoid biosynthesis, therefore forming an indirect mutualism between virus and vector via plant. However, the evolutionary mechanism of interactions between begomoviruses and their whitefly vectors is still poorly understood. Here we present evidence to suggest that indirect mutualism may happen over a millennium ago and at present extensively prevails. Detailed bioinformatics and functional analysis identified the serine-33 as an evolutionary conserved phosphorylation site in 105 of 119 Betasatellite species-encoded βC1 proteins, which are responsible for suppressing plant terpenoid-based defense by interfering with MYC2 dimerization and are essential to promote whitefly performance. The substitution of serine-33 of βC1 proteins with either aspartate (phosphorylation mimic mutants) or cysteine, the amino acid in the non-functional sβC1 encoded by Siegesbeckia yellow vein betasatellite SiYVB) impaired the ability of βC1 functions on suppression of MYC2 dimerization, whitefly attraction and fitness. Moreover the gain of function mutation of cysteine-31 to serine in sβC1 protein of SiYVB restored these functions of βC1 protein. Thus, the dynamic phosphorylation of serine-33 in βC1 proteins helps the virus to evade host defense against insect vectors with an evolutionarily conserved manner. Our data provide a mechanistic explanation of how arboviruses evolutionarily modulate host defenses for rapid transmission.

Soaud S, Huang Q, M. Saleh A, A. S. Abourehab M, Wan L, Cheng GT, Liu J, Ihtisham M, Noor Z, Rouf Mir R, Zhao X, Yan K, Abbas M, Li J. Natural resistance of tomato plants to Tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) is one of the most harmful afflictions in the world that affects tomato growth and production. Six regular antagonistic genes (Ty-1, Ty-2, Ty-3, Ty-4, ty-5, and Ty-6) have been transferred from wild germplasms to commercial cultivars as TYLCV protections. With Ty-1 serving as an appropriate source of TYLCV resistance, only Ty-1, Ty-2, and Ty-3 displayed substantial levels of opposition in a few strains. It has been possible to clone three TYLCV opposition genes (Ty-1/Ty-3, Ty-2, and ty-5) that target three antiviral safety mechanisms. However, it significantly impacts obtaining permanent resistance to TYLCV, trying to maintain opposition whenever possible, and spreading opposition globally. Utilizing novel methods, such as using resistance genes and identifying new resistance resources, protects against TYLCV in tomato production. To facilitate the breeders make an informed decision and testing methods for TYLCV blockage, this study highlights the portrayal of typical obstruction genes, common opposition sources, and subatomic indicators. The main goal is to provide a fictitious starting point for the identification and application of resistance genes as well as the maturation of tomato varieties that are TYLCV-resistant.

Interaction estimation of pathogenicity determinant protein βC1 encoded by Cotton leaf curl Multan Betasatellite with Nicotiana benthamiana Nuclear Transport Factor 2

Background

Begomovirus is one of the most devastating pathogens that can cause more than 90% yield loss in various crop plants. The pathogenicity determinant βC1, located on the betasatellite associated with monopartite begomoviruses, alters the host signaling mechanism to enhance the viral disease phenotype by undermining the host immunity. The understanding of its interacting proteins in host plants to develop disease symptoms such as curly leaves, enations, vein swelling, and chlorosis is crucial to enhance the disease resistance in crop plants. The current study was designed to reveal the contribution of βC1 in disease pathogenicity and to unveil potential interacting partners of βC1 protein in the model plant Nicotiana benthamiana.

Methods

The βC1 gene was cloned in pGKBT7 and used as bait against the cDNA library of N. benthamiana and its pathogenesis was tested against the healthy plant and the plants infiltrated with empty vectors. The yeast two-hybrid-based screening was performed to find the interacting factors. Successful interacting proteins were screened and evaluated in various steps and confirmed by sequence analysis. The three-dimensional structure of the Nuclear Transport Factor 2 (NTF2) protein was predicted, and in-silico protein-protein interaction was evaluated. Furthermore, protein sequence alignment and molecular phylogenetic analysis were carried out to identify its homologues in other related families. In-silico analyses were performed to validate the binding affinity of βC1 protein with NTF2. The 3D model was predicted by using I-TASSER and then analyzed by SWISS MODEL-Workspace, RAMPAGE, and Verify 3D. The interacting amino acid residues of βC1 protein with NTF2 were identified by using PyMOL and Chimera.

Results

The agroinfiltrated leaf samples developed severe phenotypic symptoms of virus infection. The yeast-two-hybrid study identified the NTF2 as a strong interacting partner of the βC1. The NTF2 in Solanaceae and Nicotiana was found to be evolved from the Brassica and Gossypium species. The in-silico interaction studies showed a strong binding affinity with releasing energy value of -730.6 KJ/mol, and the involvement of 10 amino acids from the middle portion towards the C-terminus and five amino acid residues from the middle portion of βC1 to interact with six amino acids of NTF2. The study not only provided an insight into the molecular mechanism of pathogenicity but also put the foundation stone to develop the resistance genotypes for commercial purposes and food security.

Emergence of Asian endemic begomoviruses as a pandemic threat

Plant viruses are responsible for the most devastating and commercially significant plant diseases, especially in tropical and subtropical regions. The genus begomovirus is the largest one in the family Geminiviridae, with a single-stranded DNA genome, either monopartite or bipartite. Begomoviruses are transmitted by insect vectors, such as Bemisia tabaci. Begomoviruses are the major causative agents of diseases in agriculture globally. Because of their diversity and mode of evolution, they are thought to be geographic specific. The emerging begomoviruses are of serious concern due to their increasing host range and geographical expansion. Several begomoviruses of Asiatic origin have been reported in Europe, causing massive economic losses; insect-borne transmission of viruses is a critical factor in virus outbreaks in new geographical regions. This review highlights crucial information regarding Asia's four emerging and highly destructive begomoviruses. We also provided information regarding several less common but still potentially important pathogens of different crops. This information will aid possible direction of future studies in adopting preventive measures to combat these emerging viruses.

Functional characterization of a new ORF βV1 encoded by radish leaf curl betasatellite

Whitefly-transmitted begomoviruses infect and damage a wide range of food, feed, and fiber crops worldwide. Some of these viruses are associated with betasatellite molecules that are known to enhance viral pathogenesis. In this study, we investigated the function of a novel βV1 protein encoded by radish leaf curl betasatellite (RaLCB) by overexpressing the protein using potato virus X (PVX)-based virus vector in Nicotiana benthamiana. βV1 protein induced lesions on leaves, suggestive of hypersensitive response (HR), indicating cell death. The HR reaction induced by βV1 protein was accompanied by an increased accumulation of reactive oxygen species (ROS), free radicals, and HR-related transcripts. Subcellular localization through confocal microscopy revealed that βV1 protein localizes to the cellular periphery. βV1 was also found to interact with replication enhancer protein (AC3) of helper virus in the nucleus. The current findings suggest that βV1 functions as a protein elicitor and a pathogenicity determinant.

Role of Diversity and Recombination in the Emergence of Chilli Leaf Curl Virus

Chilli leaf curl virus (ChiLCV), (Genus Begomovirus, family Geminiviridae) and associated satellites pose a serious threat to chilli production, worldwide. This study highlights the factors accountable for genetic diversity, recombination, and evolution of ChiLCV, and associated chilli leaf curl alphasatellite (ChiLCA) and chilli leaf curl betasatellite (ChiLCB). Phylogenetic analysis of complete genome (DNA-A) sequences of 132 ChiLCV isolates from five countries downloaded from NCBI database clustered into three major clades and showed high population diversity. The dN/dS ratio and Tajima D value of all viral DNA-A and associated betasatellite showed selective control on evolutionary relationships. Negative values of neutrality tests indicated purified selection and an excess of low-frequency polymorphism. Nucleotide diversity (π) for C4 and Rep genes was higher than other genes of ChiLCV with an average value of π = 18.37 × 10⁻² and π = 17.52 × 10⁻² respectively. A high number of mutations were detected in TrAP and Rep genes, while ChiLCB has a greater number of mutations than ChiLCA. In addition, significant recombination breakpoints were detected in all regions of ChiLCV genome, ChiLCB and, ChiLCA. Our findings indicate that ChiLCV has the potential for rapid evolution and adaptation to a range of geographic conditions and could be adopted to infect a wide range of crops, including diverse chilli cultivars.

Effector-mediated plant-virus-vector interactions

Hemipterans (such as aphids, whiteflies, and leafhoppers) are some of the most devastating insect pests due to the numerous plant pathogens they transmit as vectors, which are primarily viral. Over the past decade, tremendous progress has been made in broadening our understanding of plant-virus-vector interactions, yet on the molecular level, viruses and vectors have typically been studied in isolation of each other until recently. From that work, it is clear that both hemipteran vectors and viruses use effectors to manipulate host physiology and successfully colonize a plant and that co-evolutionary dynamics have resulted in effective host immune responses, as well as diverse mechanisms of counterattack by both challengers. In this review, we focus on advances in effector-mediated plant-virus-vector interactions and the underlying mechanisms. We propose that molecular synergisms in vector-virus interactions occur in cases where both the virus and vector benefit from the interaction (mutualism). To support this view, we show that mutualisms are common in virus-vector interactions and that virus and vector effectors target conserved mechanisms of plant immunity, including plant transcription factors, and plant protein degradation pathways. Finally, we outline ways to identify true effector synergisms in the future and propose future research directions concerning the roles effectors play in plant-virus-vector interactions.

The novel C5 protein from tomato yellow leaf curl virus is a virulence factor and suppressor of gene silencing

Tomato yellow leaf curl virus (TYLCV) is known to encode 6 canonical viral proteins. Our recent study revealed that TYLCV also encodes some additional small proteins with potential virulence functions. The fifth ORF of TYLCV in the complementary sense, which we name C5, is evolutionarily conserved, but little is known about its expression and function during viral infection. Here, we confirmed the expression of the TYLCV C5 by analyzing the promoter activity of its upstream sequences and by detecting the C5 protein in infected cells by using a specific custom-made antibody. Ectopic expression of C5 using a potato virus X (PVX) vector resulted in severe mosaic symptoms and higher virus accumulation levels followed by a burst of reactive oxygen species (ROS) in Nicotiana benthamiana plants. C5 was able to effectively suppress local and systemic post-transcriptional gene silencing (PTGS) induced by single-stranded GFP but not double-stranded GFP, and reversed the transcriptional gene silencing (TGS) of GFP. Furthermore, the mutation of C5 in TYLCV inhibited viral replication and the development of disease symptoms in infected plants. Transgenic overexpression of C5 could complement the virulence of a TYLCV infectious clone encoding a dysfunctional C5. Collectively, this study reveals that TYLCV C5 is a pathogenicity determinant and RNA silencing suppressor, hence expanding our knowledge of the functional repertoire of the TYLCV proteome.

Geminivirus-Derived Vectors as Tools for Functional Genomics

A persistent issue in the agricultural sector worldwide is the intensive damage caused to crops by the geminivirus family of viruses. The diverse types of viruses, rapid virus evolution rate, and broad host range make this group of viruses one of the most devastating in nature, leading to millions of dollars' worth of crop damage. Geminiviruses have a small genome and can be either monopartite or bipartite, with or without satellites. Their ability to independently replicate within the plant without integration into the host genome and the relatively easy handling make them excellent candidates for plant bioengineering. This aspect is of great importance as geminiviruses can act as natural nanoparticles in plants which can be utilized for a plethora of functions ranging from vaccine development systems to geminivirus-induced gene silencing (GIGS), through deconstructed viral vectors. Thus, the investigation of these plant viruses is pertinent to understanding their crucial roles in nature and subsequently utilizing them as beneficial tools in functional genomics. This review, therefore, highlights some of the characteristics of these viruses that can be deemed significant and the subsequent successful case studies for exploitation of these potentially significant pathogens for role mining in functional biology.

Chilli leaf curl disease populations in India are highly recombinant, and rapidly segregated

Capsicum annuum, a valuable spice and vegetable crop belonging to the Solanaceae family, is extensively grown across the Indian subcontinent. Chilli production is restricted by a begomoviral infection named as chilli leaf curl disease (ChiLCD) mainly in tropical and subtropical regions which leads to considerable economic losses, thus affecting chilli cultivation. Here, we studied the genetic diversity with structural evaluation of chilli leaf curl disease and satellite molecules infecting Chilli in India. We retrieved 121 reference sequences of ChiLCD including DNA-A, DNA-B, beta-satellite and alpha-satellites from GenBank reported from India. The population diversity and genetic variation were estimated through various parameters which decipher the four major groups of phylogenetic divergence for DNA-A and five groups of beta-satellite showing percentage similarity with isolates within and across India. Further, transitional and transversional bias for ORFs were observed highest in C4 and REn genes, respectively, and for DNA-A and DNA-B, these values were 1.07 and 1.22, respectively. The recombination breakpoints for DNA-A were estimated 49 majorly in V1, C1,C2 and C4 genome region and highest 22 breakpoints were determined for Rep (AC1) of ORFs, similarly 9 events for beta-satellite were found less around βC1ORF. Moreover, the evolution and genetic variability were also contributed through parameters such as nucleotide substitution which were found within the range of RNA viruses for DNA-A, DNA-B, for all 6 ORFs (relaxed clock) and beta-satellite. Additionally, total numbers of mutations (η) for DNA-A, DNA-B, alpha-satellites and beta-satellites were 2505, 419, 807 and 1288 detected, respectively, while it was found 987 highest for Rep gene among all ORFs. Further, neutrality tests determine the dominant nature of population expansion and purifying selection for all the genes of begomovirus associated with ChiLCD and satellite molecules supporting conserved nature of gene. The combined Tajima's D and Fu and Li'S D* negative values in tests indicated that population are under purified selection and an excess of low-frequency polymorphism. Our analysis indicates the potential contribution of genetic mutations and recombination of ChiLCD which leads to rapid adaptation and evolution of begomovirus and its satellite molecules accelerating its host range and diversity within and across the Indian subcontinent.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-022-03139-w.

Insights into the multifunctional roles of geminivirus-encoded proteins in pathogenesis

Geminiviruses are a major threat to agriculture in tropical and subtropical regions of the world. Geminiviruses have small genome with limited coding capacity. Despite this limitation, these viruses have mastered hijacking the host cellular metabolism for their survival. To compensate for the small size of their genome, geminiviruses encode multifunctional proteins. In addition, geminiviruses associate themselves with satellite DNA molecules which also encode proteins that support the virus in establishing successful infection. Geminiviral proteins recruit multiple host factors, suppress the host defense, and manipulate host metabolism to establish infection. We have updated the knowledge accumulated about the proteins of geminiviruses and their satellites in the context of pathogenesis in a single review. We also discuss their interactions with host factors to provide a mechanistic understanding of the infection process.

The interplay of plant hormonal pathways and geminiviral proteins: partners in disease development

Viruses belonging to the family Geminiviridae infect plants and are responsible for a number of diseases of crops in the tropical and sub-tropical regions of the World. The innate immune response of the plant assists in its defense against such viral pathogens by the recognition of pathogen/microbe-associated molecular patterns through pattern-recognition receptors. Phytohormone signalling pathways play a vital role in plant defense responses against these devastating viruses. Geminiviruses, however, have developed counter-defense strategies that prevail over the above defense pathways. The proteins encoded by geminiviruses act as suppressors of plant immunity by interacting with the signalling components of several hormones. In this review we focus on the molecular interplay of phytohormone pathways and geminiviral infection and try to find interesting parallels with similar mechanisms known in other plant-infecting viruses and strengthen the argument that this interplay is necessary for disease development.

Novel circular DNA virus identified in Opuntia discolor (Cactaceae) that codes for proteins with similarity to those of geminiviruses

Viral metagenomic studies have enabled the discovery of many unknown viruses and revealed that viral communities are much more diverse and ubiquitous than previously thought. Some viruses have multiple genome components that are encapsidated either in separate virions (multipartite viruses) or in the same virion (segmented viruses). In this study, we identify what is possibly a novel bipartite plant-associated circular single-stranded DNA virus in a wild prickly pear cactus, Opuntia discolor, that is endemic to the Chaco ecoregion in South America. Two ~1.8 kb virus-like circular DNA components were recovered, one encoding a replication-associated protein (Rep) and the other a capsid protein (CP). Both of the inferred protein sequences of the Rep and CP are homologous to those encoded by members of the family Geminiviridae. These two putatively cognate components each have a nonanucleotide sequence within a likely hairpin structure that is homologous to the origins of rolling-circle replication (RCR), found in diverse circular single-stranded DNA viruses. In addition, the two components share similar putative replication-associated iterative sequences (iterons), which in circular single-stranded DNA viruses are important for Rep binding during the initiation of RCR. Such molecular features provide support for the possible bipartite nature of this virus, which we named utkilio virus (common name of the Opuntia discolor in South America) components A and B. In the infectivity assays conducted in Nicotiana benthamiana plants, only the A component of utkilio virus, which encodes the Rep protein, was found to move and replicate systemically in N. benthamiana. This was not true for component B, for which we did not detect replication, which may have been due to this being a defective molecule or because of the model plants (N. benthamiana) used for the infection assays. Future experiments need to be conducted with other plants, including O. discolor, to understand more about the biology of these viral components.

Insights into Emerging Begomovirus-Deltasatellite Complex Diversity: The First Deltasatellite Infecting Legumes

Begomoviruses and associated DNA satellites are involved in pathosystems that include many cultivated and wild dicot plants and the whitefly vector Bemisia tabaci. A survey of leguminous plants, both crops and wild species, was conducted in Venezuela, an understudied country, to determine the presence of begomoviruses. Molecular analysis identified the presence of bipartite begomoviruses in 37% of the collected plants. Four of the six begomoviruses identified constituted novel species, and two others had not been previously reported in Venezuela. In addition, a novel deltasatellite (cabbage leaf curl deltasatellite, CabLCD) was found to be associated with cabbage leaf curl virus (CabLCV) in several plant species. CabLCD was the first deltasatellite found to infect legumes and the first found in the New World to infect a crop plant. Agroinoculation experiments using Nicotiana benthamiana plants and infectious viral clones confirmed that CabLCV acts as a helper virus for CabLCD. The begomovirus-deltasatellite complex described here is also present in wild legume plants, suggesting the possible role of these plants in the emergence and establishment of begomoviral diseases in the main legume crops in the region. Pathological knowledge of these begomovirus-deltasatellite complexes is fundamental to develop control methods to protect leguminous crops from the diseases they cause.

Interaction between βC1 of satellite and coat protein of Chili leaf curl virus plays a crucial role in suppression of host RNA silencing

The monopartite Chili leaf curl virus (ChiLCV) and its β-satellite (ChiLCB) have been found to co-exist in infected plants. The ability of βC1 protein to suppress RNA silencing was investigated using an in-house developed in-planta reversal of silencing assay, using Nicotiana tabacum lines harboring green fluorescent protein (GFP) silenced by short hairpin GFP (ShGFP). Transient expression of recombinant βC1 complemented and increased the suppressor activity of ChiLCV coat protein (CP), and this was confirmed by molecular analysis. In silico analysis followed by a yeast two-hybrid screen-identified ChiLCV-CP as the interacting partner of the ChiLCB-βC1 protein. Subcellular localization through confocal analysis revealed that when βC1 and ChiLCV-CP were co-present, the fluorescence was localized in the cytoplasm indicating that nuclear localization of both proteins was obstructed. The cytoplasmic compartmentalization of the two viral suppressors of RNA silencing may be responsible for the enhanced suppression of the host gene silencing. This study presents evidence on the interaction of ChiLCV-CP and βC1 proteins and indicates that ChiLCB may support the ChiLCV in overcoming host gene silencing to cause Chili leaf curl disease. KEY POINTS: • CP of ChiLCV and βC1 of ChiLCB contain RNA silencing suppression activity • The RNA silencing suppression activity of ChiLCB-βC1 complements that of ChiLCV-CP • There is a direct interaction between ChiLCB-βC1 and ChiLCV-CP.

A New Type of Satellite Associated with Cassava Mosaic Begomoviruses

Cassava mosaic disease (CMD), which is caused by single-stranded DNA begomoviruses, severely limits cassava production across Africa. A previous study showed that CMD symptom severity and viral DNA accumulation increase in cassava in the presence of a DNA sequence designated SEGS-2 (sequence enhancing geminivirus symptoms). We report here that when SEGS-2 is coinoculated with African cassava mosaic virus (ACMV) onto Arabidopsis thaliana, viral symptoms increase. Transgenic Arabidopsis with an integrated copy of SEGS-2 inoculated with ACMV also display increased symptom severity and viral DNA levels. Moreover, SEGS-2 enables Cabbage leaf curl virus (CaLCuV) to infect a geminivirus-resistant Arabidopsis thaliana accession. Although SEGS-2 is related to cassava genomic sequences, an earlier study showed that it occurs as episomes and is packaged into virions in CMD-infected cassava and viruliferous whiteflies. We identified SEGS-2 episomes in SEGS-2 transgenic Arabidopsis. The episomes occur as both double-stranded and single-stranded DNA, with the single-stranded form packaged into virions. In addition, SEGS-2 episomes replicate in tobacco protoplasts in the presence, but not the absence, of ACMV DNA-A. SEGS-2 episomes contain a SEGS-2 derived promoter and an open reading frame with the potential to encode a 75-amino acid protein. An ATG mutation at the beginning of the SEGS-2 coding region does not enhance ACMV infection in A. thaliana. Together, the results established that SEGS-2 is a new type of begomovirus satellite that enhances viral disease through the action of an SEGS-2-encoded protein that may also be encoded by the cassava genome. IMPORTANCE Cassava is an important root crop in the developing world and a food and income crop for more than 300 million African farmers. Cassava is rising in global importance and trade as the demands for biofuels and commercial starch increase. More than half of the world's cassava is produced in Africa, where it is primarily grown by smallholder farmers, many of whom are from the poorest villages. Although cassava can grow under high temperature, drought, and poor soil conditions, its production is severely limited by viral diseases. Cassava mosaic disease (CMD) is one of the most important viral diseases of cassava and can cause up to 100% yield losses. We provide evidence that SEGS-2, which was originally isolated from cassava crops displaying severe and atypical CMD symptoms in Tanzanian fields, is a novel begomovirus satellite that can compromise the development of durable CMD resistance.

Geminiviruses also encode small proteins with specific functions

Geminiviruses are small DNA viruses with single-stranded circular genomes that encode for few known proteins. Recently, Gong et al. reconsidered their protein identification criteria and identified open reading frames that encode small proteins with specific cellular localizations and virulence functions. Their data warrants exploration of such small proteins in other geminiviruses.

Bhendi Yellow Vein Mosaic Virus and Bhendi Yellow Vein Mosaic Betasatellite Cause Enation Leaf Curl Disease and Alter Host Phytochemical Contents in Okra

Whitefly (Bemisia tabaci)-transmitted begomoviruses cause severe diseases in numerous economically important dicotyledonous plants. Okra enation leaf curl disease (OELCuD) has emerged as a serious threat to okra (Abelmoschus esculentus L. Moench) cultivation in the Indian subcontinent. This study reports the association of a monopartite begomovirus (bhendi yellow vein mosaic virus; BYVMV) and betasatellite (bhendi yellow vein mosaic betasatellite; BYVB) with OELCuD in the Mau region of Uttar Pradesh, India. The BYVMV alone inoculated Nicotiana benthamiana and A. esculentus cv. Pusa Sawani plants developed mild symptoms. Co-inoculation of BYVMV and BYVB resulted in a reduced incubation period, an increased symptom severity, and an enhanced BYVMV accumulation by Southern hybridization and quantitative real-time PCR. This is the first study that satisfies Koch's postulates for OELCuD in its natural host. Activities of various antioxidative enzymes were significantly increased in the virus-inoculated okra plants. Differential responses in various biochemical components (such as photosynthetic pigments, phenol, proline, and sugar) in diseased okra plants were observed. This change in phytochemical responses is significant in understanding its impact on virus pathogenesis and disease development.

Impact of viral silencing suppressors on plant viral synergism: a global agro-economic concern

Plant viruses are known for their devastating impact on global agriculture. These intracellular biotrophic pathogens can infect a wide variety of plant hosts all over the world. The synergistic association of plant viruses makes the situation more alarming. It usually promotes the replication, movement, and transmission of either or both the coexisting synergistic viral partners. Although plants elicit a robust antiviral immune reaction, including gene silencing, to limit these infamous invaders, viruses counter it by encoding viral suppressors of RNA silencing (VSRs). Growing evidence also suggests that VSRs play a driving role in mediating the plant viral synergism. This review briefly discusses the evil impacts of mixed infections, especially synergism, and then comprehensively describes the emerging roles of VSRs in mediating the synergistic association of plant viruses. KEY POINTS: • Synergistic associations of plant viruses have devastating impacts on global agriculture. • Viral suppressors of RNA silencing (VSRs) play key roles in driving plant viral synergism.

Geminivirus Betasatellite-Encoded βC1 Protein Exhibits Novel ATP Hydrolysis Activity That Influences Its DNA-Binding Activity and Viral Pathogenesis

Plant virus satellites are maintained by their associated helper viruses, and satellites influence viral pathogenesis. Diseases caused by geminivirus-betasatellite complexes can become epidemics and therefore have become a threat to economically important crops across the world. Here, we identified a novel molecular function of the betasatellite-encoded pathogenicity determinant βC1. The tomato leaf curl Patna betasatellite (ToLCPaB)-encoded βC1 protein was found to exhibit novel ATPase activity in the presence of the divalent metal ion cofactor MgCl2. Moreover, ATPase activity was confirmed to be ubiquitously displayed by βC1 proteins encoded by diverse betasatellites. Mutational and sequence analysis showed that conserved lysine/arginine residues at positions 49/50 and 91 of βC1 proteins are essential for their ATPase activity. Biochemical studies revealed that the DNA-binding activity of the βC1 protein was interfered with by the binding of ATP to the protein. Mutating arginine 91 of βC1 to alanine reduced its DNA-binding activity. The results of docking studies provided evidence for an overlap of the ATP-binding and DNA-binding regions of βC1 and for the importance of arginine 91 for both ATP-binding and DNA-binding activities. A mutant betasatellite with a specifically βC1-ATPase dominant negative mutation was found to induce symptoms on Nicotiana benthamiana plants similar to those induced by wild-type betasatellite infection. The ATPase function of βC1 was found to be negatively associated with geminivirus-betasatellite DNA accumulation, despite the positive influence of this ATPase function on the accumulation of replication-associated protein (Rep) and βC1 transcripts. IMPORTANCE Most satellites influence the pathogenesis of their helper viruses. Here, we characterized the novel molecular function of βC1, a nonstructural pathogenicity determinant protein encoded by a betasatellite. We demonstrated the display of ATPase activity by this βC1 protein. Additionally, we confirmed the ubiquitous display of ATPase activity by βC1 proteins encoded by diverse betasatellites. The lysine/arginine residues conserved at positions 49 and 91 of βC1 were found to be crucial for its ATPase function. DNA-binding activity of βC1 was found to be reduced in the presence of ATP. Inhibition of ATPase activity of βC1 in the presence of an excess concentration of cold ATP, GTP, CTP, or UTP suggested that the purified βC1 can also hydrolyze other cellular nucleoside triphosphates (NTPs) besides ATP in vitro. These results established the importance of the ATPase and DNA-binding activities of the βC1 protein in regulating geminivirus-betasatellite DNA accumulation in the infected plant cell.

Plant responses to geminivirus infection: guardians of the plant immunity

BACKGROUND

Geminiviruses are circular, single-stranded viruses responsible for enormous crop loss worldwide. Rapid expansion of geminivirus diversity outweighs the continuous effort to control its spread. Geminiviruses channelize the host cell machinery in their favour by manipulating the gene expression, cell signalling, protein turnover, and metabolic reprogramming of plants. As a response to viral infection, plants have evolved to deploy various strategies to subvert the virus invasion and reinstate cellular homeostasis.

MAIN BODY

Numerous reports exploring various aspects of plant-geminivirus interaction portray the subtlety and flexibility of the host-pathogen dynamics. To leverage this pool of knowledge towards raising antiviral resistance in host plants, a comprehensive account of plant's defence response against geminiviruses is required. This review discusses the current knowledge of plant's antiviral responses exerted to geminivirus in the light of resistance mechanisms and the innate genetic factors contributing to the defence. We have revisited the defence pathways involving transcriptional and post-transcriptional gene silencing, ubiquitin-proteasomal degradation pathway, protein kinase signalling cascades, autophagy, and hypersensitive responses. In addition, geminivirus-induced phytohormonal fluctuations, the subsequent alterations in primary and secondary metabolites, and their impact on pathogenesis along with the recent advancements of CRISPR-Cas9 technique in generating the geminivirus resistance in plants have been discussed.

CONCLUSIONS

Considering the rapid development in the field of plant-virus interaction, this review provides a timely and comprehensive account of molecular nuances that define the course of geminivirus infection and can be exploited in generating virus-resistant plants to control global agricultural damage.

Capsicum-infecting begomoviruses as global pathogens: host-virus interplay, pathogenesis, and management

Whitefly-transmitted begomoviruses are among the major threats to the cultivation of Capsicum spp. (Family: Solanaceae) worldwide. Capsicum-infecting begomoviruses (CIBs) have a broad host range and are commonly found in mixed infections, which, in turn, fuels the emergence of better-adapted species through intraspecies and interspecies recombination. Virus-encoded proteins hijack host factors to breach the well-coordinated antiviral response of plants. Epigenetic modifications of histones associated with viral minichromosomes play a critical role in this molecular arms race. Moreover, the association of DNA satellites further enhances the virulence of CIBs as the subviral agents aid the helper viruses to circumvent plant antiviral defense and facilitate expansion of their host range and disease development. The objective of this review is to provide a comprehensive overview on various aspects of CIBs such as their emergence, epidemiology, mechanism of pathogenesis, and the management protocols being employed for combating them.

Integrating Viral Metagenomics into an Ecological Framework

Viral metagenomics has expanded our knowledge of the ecology of uncultured viruses, within both environmental (e.g., terrestrial and aquatic) and host-associated (e.g., plants and animals, including humans) contexts. Here, we emphasize the implementation of an ecological framework in viral metagenomic studies to address questions in virology rarely considered ecological, which can change our perception of viruses and how they interact with their surroundings. An ecological framework explicitly considers diverse variants of viruses in populations that make up communities of interacting viruses, with ecosystem-level effects. It provides a structure for the study of the diversity, distributions, dynamics, and interactions of viruses with one another, hosts, and the ecosystem, including interactions with abiotic factors. An ecological framework in viral metagenomics stands poised to broadly expand our knowledge in basic and applied virology. We highlight specific fundamental research needs to capitalize on its potential and advance the field. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Molecular interplay between phytohormones and geminiviruses: a saga of a never-ending arms race

Geminiviruses can infect a wide range of plant hosts worldwide and have hence become an emerging global agroeconomic threat. The association of these viruses with satellite molecules and highly efficient insect vectors such as whiteflies further prime their devastating impacts. Plants elicit a strong antiviral immune response to restrict the invasion of these destructive pathogens. Phytohormones help plants to mount this response and occupy a key position in combating these biotrophs. These defense hormones not only inhibit geminiviral propagation but also hamper viral transmission by compromising the performance of their insect vectors. Nonetheless, geminiviruses have co-evolved to have a few multitasking virulence factors that readily remodel host cellular machineries to circumvent the phytohormone-mediated manifestation of the immune response. Furthermore, these obligate parasites exploit plant growth hormones to produce a cellular environment permissive for virus replication. In this review, we outline the current understanding of the roles and regulation of phytohormones in geminiviral pathogenesis.

Two distinct monopartite begomovirus-betasatellite complexes in western India cause tomato leaf curl disease

In India, begomovirus infection causing tomato leaf curl disease (ToLCD) is a major constraint for tomato productivity. Here, we have identified two distinct monopartite begomovirus and betasatellite complexes causing ToLCD in the western part of India. A new monopartite begomovirus (Tomato leaf curl Mumbai virus, ToLCMumV) and betasatellite (Tomato leaf curl Mumbai betasatellite, ToLCMumB) were isolated from the Mumbai sample. A distinct Tomato leaf curl Gandhinagar virus (ToLCGanV) and Tomato leaf curl Gandhinagar betasatellite (ToLCGanB) were identified from the Gandhinagar sample. Both of the cloned begomoviruses were recombinants. The demonstration of systemic infection caused by begomovirus (ToLCGanV or ToLCMumV) alone in N. benthamiana and tomato (a virus resistant variety) emphasizes that they were monopartite begomoviruses. Co-inoculation of cognate begomovirus and betasatellite reduces the incubation period and increases symptom severity. Thus, Koch's postulates were satisfied for these virus complexes. Further, an enhanced accumulation of ToLCGanV was detected in the presence of cognate ToLCGanB, however ToLCMumB did not influence the level of ToLCMumV in the agro-inoculated tomato plants. Our results indicate that the cloned viruses form potential virus resistance breaking disease complexes in India. This necessitates to investigate the spread of these disease complexes to major tomato growing regions in the country.

Occurrence of Cotton leaf curl Multan virus and associated betasatellites with leaf curl disease of Bhut-Jolokia chillies (Capsicum chinense Jacq.) in India

Geminiviridae comprises the largest family of plant viruses which causes severe crop losses in India. The highest pungency chilli Bhut-Jolokia or ghost pepper (Capsicum chinense Jaqc.) hails from North-East region of India and is used in many dishes to add flavors and also for its medicinal value. However, this chilli variety is also affected by viruses leading to crop and economic losses. The present study reports the identification of begomoviruses in the infected chilli Bhut-Jolokia leaf samples collected from eight different places of North-East region (Manipur) of India. The infected leaf samples were screened for the presence of viral genome by rolling circle amplification (RCA) followed by PCR using degenerate primer pairs. The subsequent analyses using restriction fragment length polymorphism and sequencing revealed the presence of Cotton leaf curl Multan virus (CLCuMuV), and Tomato leaf curl Patna betasatellite (ToLCPaB). The findings focus on the phylogenetic relatedness, probable recombinational hot-spots and evolutionary divergence of the viral DNA sequences with the current reported begomoviral genome. To the best of our knowledge, this is the first report showing the presence of CLCuMuV, and associated non-cognate ToLCPaB with leaf curl disease of Bhut-Jolokia chillies. The study reveals potential recombination sites on both viral genome and betsatellite which, during the course of evolution, may have aided the virus to progress and successfully establish infection in chilli plants. Taken together, our results suggest a possible spread of CLCuMuV to the hitherto non-host crop in the North-East region of India.

SnRK1: a versatile plant protein kinase that limits geminivirus infection

Geminiviruses are a family of single-stranded DNA viruses that infect many plant species and cause serious diseases in important crops. The plant protein kinase, SnRK1, has been implicated in host defenses against geminiviruses. Overexpression of SnRK1 makes plants more resistant to geminivirus infection, and knock-down of SnRK1 increases susceptibility to geminivirus infection. GRIK, the SnRK1 activating kinase, is upregulated by geminivirus infection, while the viral C2 protein inhibits the SnRK1 activity. SnRK1 also directly phosphorylates geminivirus proteins to reduce infection. These data suggest that SnRK1 is involved in the co-evolution of plant hosts and geminiviruses.

A novel monopartite begomovirus and satellites associated with yellow mosaic disease of Sida spp. in India

Begomoviruses (family Geminiviridae) cause severe diseases in many economically important crops and non-cultivated plants in the warmer regions of the world. Non-cultivated weeds have been reported to act as natural virus reservoirs. In January 2016, Sida plants with yellow mosaic symptoms were found at the edge of an agricultural field in Gujarat, India. Sequence analysis of the viral genomic components cloned from a diseased Sida plant indicated the presence of a distinct monopartite begomovirus (proposed as sida yellow mosaic Gujarat virus) along with a betasatellite (ludwigia leaf distortion betasatellite) and an alphasatellite (malvastrum yellow mosaic alphasatellite). Our results emphasize that this weed may harbor a begomovirus-alphasatellite-betasatellite complex. This host serves as a potential source of virus inoculum, which can be transmitted by whiteflies to other cultivated crops.

Biology and Interaction of the Natural Occurrence of Distinct Monopartite Begomoviruses Associated With Satellites in Capsicum annum From India

Chili (Capsicum annuum L.) is an important vegetable and spice crop of tropical and sub-tropical regions. Chili plants showing upward leaf curling, leaf crinkling, and leaf yellowing symptoms, collected from Sikar district of Rajasthan, India, were found to be associated with begomovirus and satellite molecules. The presence of virus was confirmed by PCR using virus-specific primer. The full-length genomic DNA-A of three begomovirus (MM-1, CS-1 and RV-1) and two satellites (MM-2 and MM-3) were cloned which was identified from single symptomatic chili plant. The genome organization of isolated three viruses is similar to those of other Old World monopartite begomoviruses. The comparison of the sequences and closest phylogenetic relationships for the begomoviruses, betasatellite and alphasatellite DNAs revealed that MM-1 was designated as DNA-A of Chili leaf curl virus (ChiLCV), CS-1 is considered to be a new distinct species of Tomato leaf curl Gujrat virus (ToLCGV) whereas RV-1 as a new strain of Cotton leaf curl Multan virus (CLCuMuV). The DNA-A component of ChiLCV showed 8.6%, ToLCGV of 16.6% and CLCuMuV of 7.7% average evolutionary divergence, concomitantly, the betasatellite and alphasatellite molecule had 9.9% and 5.9% overall sequence divergence, respectively. Interestingly, most of the begomoviruses were found to be intra-species recombinants. The dN/dS ratio and Tajima D value of all viral DNA-A component and their associated betasatellite showed their selective control on evolutionary relationships. The nucleotide substitution rates were determined for the DNA-A genomes of ChiLCV (7.22 × 10^–4 substitutions site^–1 year^–1), CLCuMuV (1.49 × 10^–4 substitutions site^–1 year^–1), ToLCGV (7.47 × 10^–4 substitutions site^–1 year^–1), the genome of associated ChiLCB (4.20 × 10^–4 substitutions site^–1 year^–1) and CLCuMuA (1.49 × 10^–4 substitutions site^–1 year^–1). Agro-inoculation studies indicate that the presence of DNA betasatellite induce severe symptoms in N. benthamiana and chili, suggesting prerequisite association for typical disease development.