Geminiviruses: masters at redirecting and reprogramming plant processes

Phosphorylated viral protein evades plant immunity through interfering the function of RNA-binding protein

Successful pathogen infection in plant depends on a proper interaction between the invading pathogen and its host. Post-translational modification (PTM) plays critical role(s) in plant-pathogen interaction. However, how PTM of viral protein regulates plant immunity remains poorly understood. Here, we found that S162 and S165 of Chinese wheat mosaic virus (CWMV) cysteine-rich protein (CRP) are phosphorylated by SAPK7 and play key roles in CWMV infection. Furthermore, the phosphorylation-mimic mutant of CRP (CRP^S162/165D) but not the non-phosphorylatable mutant of CRP (CRP^S162/165A) interacts with RNA-binding protein UBP1-associated protein 2C (TaUBA2C). Silencing of TaUBA2C expression in wheat plants enhanced CWMV infection. In contrast, overexpression of TaUBA2C in wheat plants inhibited CWMV infection. TaUBA2C inhibits CWMV infection through recruiting the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to induce cell death and H₂O₂ production. This effect can be supressed by CRP^S162/165D through changing TaUBA2C chromatin-bound status and attenuating it’s the RNA- or DNA-binding activities. Taken together, our findings provide new knowledge on how CRP phosphorylation affects CWMV infection as well as the arms race between virus and wheat plants.

Chinese wheat mosaic virus (CWMV) causes a damaging disease in cereal plants. However, CWMV interacts with host factors to facilitate virus infection is not clear yet. Here, we found that S162 and S165 of CWMV cysteine-rich protein (CRP) are phosphorylated by SAPK7 in vivo and in vitro. Mutational analyses have indicated that these two phosphorylation sites of CRP (CRP^S162/165D) promoting CWMV infection in plants, due to the supressed cell death and H₂O₂ production. Further investigations found the CRP^S162/165D can interact with TaUBA2C, while the non-phosphorylatable mutant of CRP (CRP^S162/165A) does not. Futhermore, we have determined that CRP^S162/165D and TaUBA2C interaction inhibited the formation of TaUBA2C speckles in nucleus to attenuate its RNA- and DNA-binding activity. We also showed that TaUBA2C recruit the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to up-regulated these genes expressions and then induce cell death and H₂O₂ production in plant. This effect can be supressed by the expression of CRP^S162/165D, in a dose-dependent manner. Taken together, our discovery may provide a new sight for the arms race between virus and its host plants.

Gene Overlapping as a Modulator of Begomovirus Evolution

In RNA viruses, which have high mutation—and fast evolutionary— rates, gene overlapping (i.e., genomic regions that encode more than one protein) is a major factor controlling mutational load and therefore the virus evolvability. Although DNA viruses use host high-fidelity polymerases for their replication, and therefore should have lower mutation rates, it has been shown that some of them have evolutionary rates comparable to those of RNA viruses. Notably, these viruses have large proportions of their genes with at least one overlapping instance. Hence, gene overlapping could be a modulator of virus evolution beyond the RNA world. To test this hypothesis, we use the genus Begomovirus of plant viruses as a model. Through comparative genomic approaches, we show that terminal gene overlapping decreases the rate of virus evolution, which is associated with lower frequency of both synonymous and nonsynonymous mutations. In contrast, terminal overlapping has little effect on the pace of virus evolution. Overall, our analyses support a role for gene overlapping in the evolution of begomoviruses and provide novel information on the factors that shape their genetic diversity.

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.

Synergy between an emerging monopartite begomovirus and a DNA-B component

In recent decades, a legion of monopartite begomoviruses transmitted by the whitefly Bemisia tabaci has emerged as serious threats to vegetable crops in Africa. Recent studies in Burkina Faso (West Africa) reported the predominance of pepper yellow vein Mali virus (PepYVMLV) and its frequent association with a previously unknown DNA-B component. To understand the role of this DNA-B component in the emergence of PepYVMLV, we assessed biological traits related to virulence, virus accumulation, location in the tissue and transmission. We demonstrate that the DNA-B component is not required for systemic movement and symptom development of PepYVMLV (non-strict association), but that its association produces more severe symptoms including growth arrest and plant death. The increased virulence is associated with a higher viral DNA accumulation in plant tissues, an increase in the number of contaminated nuclei of the phloem parenchyma and in the transmission rate by B. tabaci. Our results suggest that the association of a DNA-B component with the otherwise monopartite PepYVMLV is a key factor of its emergence.

Identification and Functional Analysis of Four RNA Silencing Suppressors in Begomovirus Croton Yellow Vein Mosaic Virus

Croton yellow vein mosaic virus (CYVMV), a species in the genus Begomovirus, is a prolific monopartite begomovirus in the Indian sub-continent. CYVMV infects multiple crop plants to cause leaf curl disease. Plants have developed host RNA silencing mechanisms to defend the threat of viruses, including CYVMV. We characterized four RNA silencing suppressors, namely, V2, C2, and C4 encoded by CYVMV and betasatellite-encoded C1 protein (βC1) encoded by the cognate betasatellite, croton yellow vein betasatellite (CroYVMB). Their silencing suppressor functions were verified by the ability of restoring the β-glucuronidase (GUS) activity suppressed by RNA silencing. We showed here for the first time that V2 was capable of self-interacting, as well as interacting with the V1 protein, and could be translocalized to the plasmodesmata in the presence of CYVMV. The knockout of either V2 or V1 impaired the intercellular mobility of CYVMV, indicating their novel coordinated roles in the cell-to-cell movement of the virus. As pathogenicity determinants, each of V2, C2, and C4 could induce typical leaf curl symptoms in Nicotiana benthamiana plants even under transient expression. Interestingly, the transcripts and proteins of all four suppressors could be detected in the systemically infected leaves with no correlation to symptom induction. Overall, our work identifies four silencing suppressors encoded by CYVMV and its cognate betasatellite and reveals their subcellular localizations, interaction behavior, and roles in symptom induction and intercellular virus movement.

Achieving maximum efficiency of Mungbean yellow mosaic India virus infection in mungbean by agroinoculation

Mungbean is one of the important food legumes in the Indian-sub-continent. Yellow mosaic disease, caused by Mungbean yellow mosaic virus and Mungbean yellow mosaic India virus (MYMIV) poses a severe threat to its production. Agroinoculation has been the most preferred way to test the function of genomic components of these viruses. However, the available inoculation methods are not as efficient as whitefly transmission, thereby limiting their usage for screening and biological studies. We hereby report an efficient and reproducible agroinoculation method for achieving maximum (100%) efficiency using tandem repeat infectious agro-constructs of DNA A and DNA B of MYMIV. The present study targeted wounding of various meristematic tissues of root, shoot, parts of germinating seeds and also non-meristematic tissue of stem to test the suitable tissue types for maximum infection. Among the various tissues selected for, the inoculation on the epicotyl region showed maximum infectivity. Further, to enhance the infectivity of MYMIV, different concentrations of acetosyringone, incubation time and Agrobacterium cell density were also standardized. The incubation of wounded sprouted seeds in 1.0 OD of agroculture containing repeat construct of MYMIV for 2-4 h without acetosyringone followed by sowing in soil showed maximum infection of MYMIV within 10-12 days on the first trifoliate leaf. This standardized method is reproducible and has potential to screen germplasm lines and will be useful in mungbean biological/virological studies and breeding programmes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03088-w.

Selective REcruitmeNt of plant DNA polymerases by geminivirus

Geminiviruses reprogram host machineries to ensure their own propagation. They do not encode any DNA polymerase. Furthermore, the absence of direct evidence about the precise role of any host-encoded DNA polymerase has made geminivirus replication an enigma. Wu et al. recently resolved this puzzle by revealing that geminiviruses utilize plant DNA polymerase α and δ to drive their replication.

Effects of plant growth regulators on transient expression of foreign gene in Nicotiana benthamiana L. leaves

BACKGROUND

In the last decades, replicating expression vectors based on plant geminivirus have been widely used for enhancing the efficiency of plant transient expression. By using the replicating expression vector derived from bean yellow dwarf virus and green fluorescent protein as a reporter, we investigated the effects of α-naphthalene acetic acid, gibberellins3, and 6-benzyladenine, as three common plant growth regulators, on the plant biomass and efficiency of transient expression during the process of transient expression in Nicotiana benthamiana L. leaves.

RESULTS

With the increase of the concentration of α-naphthalene acetic acid, gibberellins3, and 6-benzyladenine (from 0.1 to 1.6 mg/L), the fresh weight, dry weight, and leaf area of the seedlings increased first and then returned to the levels similar to the controls (without chemical treatment). The treatment with α-naphthalene acetic acid at 0.2 and 0.4 mg/L can enhance the level of transient expression of green fluorescent protein, which peaked at 0.4 mg/L α-naphthalene acetic acid and was increased about by 19%, compared to the controls. Gibberellins3 at 0.1-0.4 mg/L can enhance the level of transient expression of green fluorescent protein, which peaked at 0.2 mg/L gibberellins3 and was increased by 25%. However, the application of 6-benzyladenine led to decrease in the level of transient expression of green fluorescent protein.

CONCLUSIONS

The appropriate plant growth regulators at moderate concentration could be beneficial to the expression of foreign genes from the Agrobacterium-mediated transient expression system in plants. Thus, appropriate plant growth regulators could be considered as exogenous components that are applied for the production of recombinant protein by plant-based transient expression systems.

An experimental strategy for preparing circular ssDNA virus genomes for next-generation sequencing

The ability of begomoviruses to evolve rapidly threatens many crops and underscores the importance of detecting these viruses quickly and to understand their genome diversity. This study presents an improved protocol for the enhanced amplification and enrichment of begomovirus DNA for use in next generation sequencing of the viral genomes. An enhanced rolling circle amplification (RCA) method using EquiPhi29 polymerase was combined with size selection to generate a cost-effective, short-read sequencing method. This improved short-read sequencing produced at least 50 % of the reads mapping to the target viral reference genomes, African cassava mosaic virus and East African cassava mosaic virus. This study provided other insights into common misconceptions about RCA and lessons that could be learned from the sequencing of single-stranded DNA virus genomes. This protocol can be used to examine the viral DNA as it moves from host to vector, thus producing valuable information for viral DNA population studies, and would likely work well with other circular Rep-encoding ssDNA viruses (CRESS) DNA viruses.

Targeted mutagenesis in plants using Beet curly top virus for efficient delivery of CRISPR/Cas12a components

Genome editing using CRISPR/Cas is rapidly being developed for gene targeting in eukaryotes including plants. However, gene targeting by homology-directed DNA recombination (HDR) is an infrequent event compared to the dominant DNA repair by non-homologous end-joining. Another bottleneck is the ineffective delivery of CRISPR/Cas components into plant cells. To overcome these constraints, here a geminiviral replicon from Beet curly top virus (BCTV) has been produced with a wide host range and high DNA accumulation capacity for efficient delivery of CRISPR/Cas12a components into plant cells. Initially, a BCTV replicon was prepared after removing the virion sense genes from an infectious full-length clone for agrobacterium mediated infection. This replicon expressed a green fluorescent protein (GFP) marker gene at a high level compared to T-DNA binary vector. In transient assay, the BCTV replicon produced a higher rate of mutagenesis and HDR in the GFP transgene in Nicotiana benthamiana through efficient delivery of CRISPR/Cas12a components compared to the cognate T-DNA control. This was through a range of complete or partial HDR for conversion of GFP into YFP after exchange of a single amino acid (Thr224Tyr) in the target gene. In addition, induced mutagenesis and HDR in the target gene were heritable. Thus, the BCTV replicon provides a new tool for efficient delivery of CRISPR/Cas12a components that could be used in a wide range of dicotyledonous plants. The established GFP to YFP system and the GFP mutant line produced also enable further optimization and understanding of HDR in plants via CRISPR/Cas12a system using geminiviral replicons.

Tomato Yellow Leaf Curl Virus Infection in a Monocotyledonous Weed (Eleusine indica)

Tomato yellow leaf curl virus (TYLCV) is one of the most important plant viruses belonging to the genus Begomovirus of the family Geminiviridae. To identify natural weed hosts that could act as reservoirs of TYLCV, 100 samples were collected at a TYLCV-affected tomato farm in Iksan from 2013 to 2014. The sample weeds were identified as belonging to 40 species from 18 families. TYLCV was detected in 57 samples belonging to 28 species through polymerase chain reaction using root samples including five species (Eleusine indica, Digitaria ciliaris, Echinochloa crus-galli, Panicum dichotomiflorum, and Setaria faberi) from the family Poaceae. Whitefly Bemisia tabaci-mediated TYLCV transmission from TYLCV-infected E. indica plants to healthy tomatoes was confirmed, and inoculated tomatoes showed typical symptoms, such as leaf curling and yellowing. In addition, TYLCV was detected in leaf and root samples of E. indica plants inoculated by both whitefly-mediated transmission using TYLCV-viruliferous whitefly and agro-inoculation using a TYLCV infectious clone. The majority of mastreviruses infect monocotyledonous plants, but there have also been reports of mastreviruses that can infect dicotyledonous plants, such as the chickpea chlorotic dwarf virus. No exception was reported among begomoviruses known as infecting dicots only. This is the first report of TYLCV as a member of the genus Begomovirus infecting monocotyledonous plants.

Genome-based identification of beet curly top Iran virus infecting sugar beet in Turkey and investigation of its pathogenicity by agroinfection

Beet curly top disease (BCTD) is a yield-limiting viral infection of sugar beet (Beta vulgaris) throughout the arid and semi-arid regions of the world. Two virus species, belonging to two different genera of the family Geminiviridae (Curtovirus and Becurtovirus) had been described as the disease's causative agents on sugar beet. Despite the detection of the BCTD in some sugar beet fields of Turkey sixty years ago, the genome based characterization of BCTD-associated viruses have not been studied previously. In this study, 628 sugar beet plants exhibiting BCTD symptoms were collected from fourteen cities in central Anatolia, the major sugar beet production areas in Turkey. PCR assays of these samples using the respective Curtovirus and Becurtovirus genus-specific primers indicated that the Turkish sugar beet samples' viral sequences belong only to the genus Becurtovirus. The results of sequencing and phylogenetic analysis of the partial genome of the virus obtained from fourteen cities confirmed that BCTD-associated virus in Turkish sugar beet fields is beet curly top Iran virus (BCTIV-Becurtovirus) species. The whole genome of the collected viruses from fourteen cities were amplified by the rolling circle amplification (RCA) and the five most phylogenetically diverse viruses obtained from Afyon, Ankara, Adapazarı, Yozgat and Aksaray were sequenced. The results of whole genome sequence analysis indicated >98 % sequence identities with that of a BCTIV variants reported from Urmia province (bordering Turkey) of Iran. A virus genome from Yozgat city had a genomic sequence identity of >97 % with those of BCTIV isolated from cowpea, tomato, pepper and sugar beet in the northern part of Iran. These results suggested that the spread of BCTIV through the region could create a significant threat to the production of sugar beet as well as other agricultural crops. A tandem dimer of a BCTIV-Turkish variant isolated from Ankara city was cloned into Agrobacterium plasmid to be used for agro-infection studies. Agroinoculation of this construct on sugar beet leaves generated severe BCTD symptoms (84 %) which were also confirmed by RCA and qPCR analysis. These results constituted the first genome based characterization of BCTIV Turkish variants and the first report of BCTIV spreading out of Iran.

Genetic Transformation of Apomictic Grasses: Progress and Constraints

The available methods for plant transformation and expansion beyond its limits remain especially critical for crop improvement. For grass species, this is even more critical, mainly due to drawbacks in in vitro regeneration. Despite the existence of many protocols in grasses to achieve genetic transformation through Agrobacterium or biolistic gene delivery, their efficiencies are genotype-dependent and still very low due to the recalcitrance of these species to in vitro regeneration. Many plant transformation facilities for cereals and other important crops may be found around the world in universities and enterprises, but this is not the case for apomictic species, many of which are C4 grasses. Moreover, apomixis (asexual reproduction by seeds) represents an additional constraint for breeding. However, the transformation of an apomictic clone is an attractive strategy, as the transgene is immediately fixed in a highly adapted genetic background, capable of large-scale clonal propagation. With the exception of some species like Brachiaria brizantha which is planted in approximately 100 M ha in Brazil, apomixis is almost non-present in economically important crops. However, as it is sometimes present in their wild relatives, the main goal is to transfer this trait to crops to fix heterosis. Until now this has been a difficult task, mainly because many aspects of apomixis are unknown. Over the last few years, many candidate genes have been identified and attempts have been made to characterize them functionally in Arabidopsis and rice. However, functional analysis in true apomictic species lags far behind, mainly due to the complexity of its genomes, of the trait itself, and the lack of efficient genetic transformation protocols. In this study, we review the current status of the in vitro culture and genetic transformation methods focusing on apomictic grasses, and the prospects for the application of new tools assayed in other related species, with two aims: to pave the way for discovering the molecular pathways involved in apomixis and to develop new capacities for breeding purposes because many of these grasses are important forage or biofuel resources.

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.

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.

Mastrevirus Rep and RepA Proteins Suppress de novo Transcriptional Gene Silencing

Transcriptional gene silencing (TGS) in plants is a defense mechanism against DNA virus infection. The genomes of viruses in the Geminiviridae family encode several TGS suppressors. In this study, we induced de novo TGS against the transgenic GFP gene encoding green fluorescent protein by expressing a hairpin-shaped self-complementary RNA corresponding to the enhancer region of the 35S promoter (hpE35S). In addition, we examined the TGS suppression activity of proteins encoded in the genome of Tobacco yellow dwarf virus (TYDV, genus Mastrevirus). The results show that the replication-associated protein (Rep) and RepA encoded by TYDV have TGS suppressor activity and lead to decreased accumulation of 24-nt siRNAs. These results suggest that Rep and RepA can block the steps before the loading of siRNAs into Argonaute (AGO) proteins. This is the first report of TGS suppressors in the genus Mastrevirus.

A new bipartite begomovirus naturally infecting Pyrenacantha sp. in Mozambique

A begomovirus was isolated from a Pyrenacantha sp. plant with yellow mosaic symptoms collected in a maize production field in Mozambique. The complete DNA-A and DNA-B components have a genomic organization typical of Old World, bipartite begomoviruses. Based on the current ICTV species demarcation criteria for the genus Begomovirus, the virus isolate, named Pyrenacantha yellow mosaic virus (PyYMV), is a member of a new species, for which the name "Begomovirus pyrenacanthae" is proposed. Alignment of their common regions (CR) indicated a 35-nt insertion in the DNA-A CR. The nt sequence identity between the CRs is only 83% but increases to 96% when the 35-nt insertion is removed from the alignment. This is the first report of a begomovirus naturally infecting Pyrenacantha spp.

Twenty years of evolution and diversification of digitaria streak virus in Digitaria setigera

Within the family Geminiviridae, the emergence of new species results from their high mutation and recombination rates. In this study, we report the variability and evolution of digitaria streak virus (DSV), a mastrevirus isolated in 1986 from the grass Digitaria setigera in an island of the Vanuatu archipelago. Viral DNA of DSV samples was amplified from D. setigera specimens, derived from the naturally infected original plant, which were propagated in different laboratories in France and Italy for more than 20 years. From the consensus sequences, the nucleotide substitution rate was estimated for the period between a sample and the original sequence published in 1987, as well as for the period between samples. In addition, the intra-host genetic complexity and diversity of 8 DSV populations with a total of 165 sequenced haplotypes was characterized. The evolutionary rate of DSV was estimated to be between 1.13 × 10⁻⁴ and 9.87 × 10⁻⁴ substitutions/site/year, within the ranges observed in other single-stranded DNA viruses and RNA viruses. Bioinformatic analyses revealed high variability and heterogeneity in DSV populations, which confirmed that mutant spectra are continuously generated and are organized as quasispecies. The analysis of polymorphisms revealed nucleotide substitution biases in viral genomes towards deamination and oxidation of single-stranded DNA. The differences in variability in each of the genomic regions reflected a dynamic and modular evolution in the mutant spectra that was not reflected in the consensus sequences. Strikingly, the most variable region of the DSV genome, encoding the movement protein, showed rapid fixation of the mutations in the consensus sequence and a concomitant dN/dS ratio of 6.130, which suggests strong positive selection in this region. Phylogenetic analyses revealed a possible divergence in three genetic lineages from the original Vanuatu DSV isolate.

Genomic analysis finds no evidence of canonical eukaryotic DNA processing complexes in a free-living protist

Cells replicate and segregate their DNA with precision. Previous studies showed that these regulated cell-cycle processes were present in the last eukaryotic common ancestor and that their core molecular parts are conserved across eukaryotes. However, some metamonad parasites have secondarily lost components of the DNA processing and segregation apparatuses. To clarify the evolutionary history of these systems in these unusual eukaryotes, we generated a genome assembly for the free-living metamonad Carpediemonas membranifera and carried out a comparative genomics analysis. Here, we show that parasitic and free-living metamonads harbor an incomplete set of proteins for processing and segregating DNA. Unexpectedly, Carpediemonas species are further streamlined, lacking the origin recognition complex, Cdc6 and most structural kinetochore subunits. Carpediemonas species are thus the first known eukaryotes that appear to lack this suite of conserved complexes, suggesting that they likely rely on yet-to-be-discovered or alternative mechanisms to carry out these fundamental processes.

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.

Turning Waste into Beneficial Resource: Implication of Ageratum conyzoides L. in Sustainable Agriculture, Environment and Biopharma Sectors

The annual herb, Ageratum conyzoides L. (Asteraceae), is distributed throughout the world. Although invasive, it can be very useful as a source of essential oils, pharmaceuticals, biopesticides, and bioenergy. However, very limited information exists on the molecular basis of its different utility as previous investigations were mainly focused on phytochemical/biological activity profiling. Here we have explored various properties of A. conyzoides that may offer environmental, ecological, agricultural, and health benefits. As this aromatic plant harbors many important secondary metabolites that may have various implications, biotechnological interventions such as genomics, metabolomics and tissue-culture can be indispensable tools for their mass-production. Further, A. conyzoides acts as a natural reservoir of begomoviruses affecting a wide range of plant species. As the mechanisms of disease spreading and crop infection are not fully clear, whole-genome sequencing and various advanced molecular technologies including RNAi, CRISPER/Cas9, multi-omics approaches, etc., may aid to decipher the molecular mechanism of such disease development and thus, can be useful in crop protection. Overall, improved knowledge of A. conyzoides is not only essential for developing sustainable weed control strategy but can also offer potential ways for biomedicinal, environment, safe and clean agriculture applications.

A Bipartite Geminivirus with a Highly Divergent Genomic Organization Identified in Olive Trees May Represent a Novel Evolutionary Direction in the Family Geminiviridae

Olea europaea Geminivirus (OEGV) was recently identified in olive in Italy through HTS. In this work, we used HTS to show the presence of an OEGV isolate in Portuguese olive trees and suggest the evolution direction of OEGV. The bipartite genome (DNA-A and DNA-B) of the OEGV-PT is similar to Old World begomoviruses in length, but it lacks a pre-coat protein (AV2), which is a typical feature of New World begomoviruses (NW). DNA-A genome organization is closer to NW, containing four ORFs; three in complementary-sense AC1/Rep, AC2/TrAP, AC3/REn and one in virion-sense AV1/CP, but no AC4, typical of begomoviruses. DNA-B comprises two ORFs; MP in virion sense with higher similarity to the tyrosine phosphorylation site of NW, but in opposite sense to begomoviruses; BC1, with no known conserved domains in the complementary sense and no NSP typical of bipartite begomoviruses. Our results show that OEGV presents the longest common region among the begomoviruses, and the TATA box and four replication-associated iterons in a completely new arrangement. We propose two new putative conserved regions for the geminiviruses CP. Lastly, we highlight unique features that may represent a new evolutionary direction for geminiviruses and suggest that OEGV-PT evolution may have occurred from an ancient OW monopartite Begomovirus that lost V2 and C4, gaining functions on cell-to-cell movement by acquiring a DNA-B component.

Small but mighty: Functional landscape of the versatile geminivirus-encoded C4 protein

The fast-paced evolution of viruses enables them to quickly adapt to the organisms they infect by constantly exploring the potential functional landscape of the proteins encoded in their genomes. Geminiviruses, DNA viruses infecting plants and causing devastating crop diseases worldwide, produce a limited number of multifunctional proteins that mediate the manipulation of the cellular environment to the virus’ advantage. Among the proteins produced by the members of this family, C4, the smallest one described to date, is emerging as a powerful viral effector with unexpected versatility. C4 is the only geminiviral protein consistently subjected to positive selection and displays a number of dynamic subcellular localizations, interacting partners, and functions, which can vary between viral species. In this review, we aim to summarize our current knowledge on this remarkable viral protein, encompassing the different aspects of its multilayered diversity, and discuss what it can teach us about geminivirus evolution, invasion requirements, and virulence strategies.

Molecular analysis, infectivity and host range of Tomato leaf curl Karnataka virus associated with Corchorus yellow vein mosaic betasatellite

Severe leaf curl disease of tomato (ToLCD) was noticed recently in the central parts of India and is an emerging threat to the cultivation of tomato. The genomic components of the begomovirus isolate, DNA A and betasatellite associated with ToLCD were cloned by rolling circle amplification method and sequenced. The sequence analysis revealed that the DNA A (2766 nt) of this isolate had the nucleotide identity of >91% with other strains of Tomato leaf curl Karnataka virus (ToLCKV), hence this isolate is proposed as a strain of ToLCKV, named as ToLCKV-Raipur. Similarly, the betasatellite molecule (1355 nt) had the highest identity of 91.1% with Corchorus yellow vein mosaic betasatellite (CoYVMB) and named as CoYVMB-Raipur. The full-length dimerized clones of these two genomic components were agroinoculated on natural (tomato), experimental (Nicotiana benthamiana) hosts and other 20 plant species belong to six different families. The severe leaf curl symptoms appeared only in the hosts, N. benthamiana, and in tomato inoculated with ToLCKV-Raipur alone and ToLCKV-Raipur with CoYVMB-Raipur after 8 and 16-18 days inoculation, respectively. This isolate was also transmissible to healthy tomato plants by whitefly from the tomato plant agroinoculated with ToLCKV-Raipur alone and with CoYVMB-Raipur and produced symptoms within 14-16 days after inoculation. Interestingly, this isolate infects horse gram and chilli by whitefly transmission and both the hosts showed positive for DNA A alone but not for betasatellite. Quantification of the genomic components of this isolate with the agroinoculated N. benthamiana samples by qRT-PCR results showed that the quantity of ToLCKV-Raipur was enhanced by three-fold while inoculated with CoYVMB-Raipur compared to ToLCKV-Raipur alone inoculated plants. However, CoYVMB-Raipur did not enhance the levels of ToLCKV-Raipur in the agroinoculated tomato plants. This is the first evidence of the natural co-occurrence of ToLCKV with betasatellite, CoYVMB causing ToLCD.

The Begomovirus Species Melon Chlorotic Leaf Curl Virus is Composed of Two Highly Divergent Strains that Differ in Their Genetic and Biological Properties

Since the early 1990s, squash production in Costa Rica has been affected by a whitefly-transmitted disease characterized by stunting and yellow mottling of leaves. The squash yellow mottle disease (SYMoD) was shown to be associated with a bipartite begomovirus, originally named squash yellow mild mottle virus (SYMMoV). It was subsequently established that SYMMoV is a strain of melon chlorotic leaf curl virus (MCLCuV), a bipartite begomovirus that causes a chlorotic leaf curl disease of melons in Guatemala. In the present study, the complete sequences of the DNA-A and DNA-B components of a new isolate of the strain MCLCuV-Costa Rica (MCLCuV-CR) were determined. Comparisons of full-length DNA-A sequences revealed 97% identity with a previously characterized isolate of MCLCuV-CR and identities of 90 to 91% with those of isolates of the strain MCLCuV-Guatemala (MCLCuV-GT), which is below or at the current begomovirus species demarcation threshold of 91%. A more extensive analysis of the MCLCuV-CR and -GT sequences revealed substantial divergence in both components and different histories of recombination for the DNA-A components. The cloned full-length DNA-A and DNA-B components of this new MCLCuV-CR isolate were infectious and induced SYMoD in a range of squashes and in pumpkin, thereby fulfilling Koch's postulates for this disease. However, in contrast to MCLCuV-GT, MCLCuV-CR induced mild symptoms in watermelon and no symptoms in melon and cucumber. Taken together, our results indicate that MCLCuV-CR and -GT have substantially diverged, genetically and biologically, and have evolved to cause distinct diseases of different cucurbit crops. Taxonomically, these viruses are at the strain/species boundary, but retain the designation as strains of Melon chlorotic leaf curl virus under current International Committee on Taxonomy guidelines.

The Association between New World Alphasatellites and Bipartite Begomoviruses: Effects on Infection and Vector Transmission

Begomoviruses can be found in association with alphasatellites, which are capable of autonomous replication but are dependent on the helper begomovirus for systemic infection, encapsidation and vector transmission. Previous studies suggest that the presence of NW alphasatellites (genus Clecrusatellite) is associated with more severe symptoms. To better understand this interaction, we investigated the effects of two alphasatellites on infectivity, symptom development, viral DNA accumulation and vector transmission of three begomoviruses in three hosts. In tomato and Nicotiana benthamiana, all combinations were infectious. In Leonurus sibiricus, only the ToYSV/ToYSA combination was infectious. The presence of EuYMA increased symptom severity of EuYMV and ToYSV in N. benthamiana, and the presence of ToYSA was associated with more severe symptoms of ToYSV in N. benthamiana and L. sibiricus. EuYMA increased the accumulation of ToYSV in N. benthamiana but reduced the accumulation of EuYMV in tomato and of ToSRV in N. benthamiana. The presence of ToYSA decreased the accumulation of ToYSV in N. benthamiana and L. sibiricus. ToYSA negatively affected transmission of ToSRV by Bemisia tabaci MEAM1. Together, our results indicate that NW alphasatellites can interact with different begomoviruses, increasing symptom severity and interfering in the transmission of the helper begomovirus. Understanding this interaction is important as it may affect the emergence of diseases caused by begomovirus-alphasatellite complexes in the field.

Conserved Structural Motif Identified in Peptides That Bind to Geminivirus Replication Protein Rep

The geminivirus replication protein, Rep, has long been recognized as a high-value target for control of geminivirus infections as this protein is highly conserved and essential for viral replication and proliferation. In addition, inhibition of viral replication has been pursued through various antiviral strategies with varying degrees of success, including inhibitory peptides that target Rep. While much effort has centered around sequence characterization of the Rep protein and inhibitory peptides, detailed structural analysis has been missing. This study computationally investigated the presence of common structural features within these inhibitory peptides and if these features could inform if a particular peptide will bind Rep and/or interfere with viral replication. Molecular dynamics simulations of the inhibitory peptide library showed that simply possessing stable structural features does not inform interference of viral replication regardless of the binding of Rep. Additionally, nearly all known Rep inhibitory peptides sample a conserved β-sheet structural motif, possibly informing structure-function relationships in binding Rep. In particular, two peptides (A22 and A64) characterized by this structural motif were computationally docked against a wide variety of geminivirus Rep proteins to determine a mechanism of action. Computational docking revealed these peptides utilize a common Rep protein sequence motif for binding, HHN-x_1/2-Q. The results identified residues in both Rep and the inhibitory peptides that play a significant role in the interaction, establishing the foundation for a rational structure-based design approach for the construction of both broadly reactive and geminivirus species-specific inhibitors.

WRKY Transcription Factors in Cassava Contribute to Regulation of Tolerance and Susceptibility to Cassava Mosaic Disease through Stress Responses

Among the numerous biological constraints that hinder cassava (Manihot esculenta Crantz) production, foremost is cassava mosaic disease (CMD) caused by virus members of the family Geminiviridae, genus Begomovirus. The mechanisms of CMD tolerance and susceptibility are not fully understood; however, CMD susceptible T200 and tolerant TME3 cassava landraces have been shown to exhibit different large-scale transcriptional reprogramming in response to South African cassava mosaic virus (SACMV). Recent identification of 85 MeWRKY transcription factors in cassava demonstrated high orthology with those in Arabidopsis, however, little is known about their roles in virus responses in this non-model crop. Significant differences in MeWRKY expression and regulatory networks between the T200 and TME3 landraces were demonstrated. Overall, WRKY expression and associated hormone and enriched biological processes in both landraces reflect oxidative and other biotic stress responses to SACMV. Notably, MeWRKY11 and MeWRKY81 were uniquely up and downregulated at 12 and 67 days post infection (dpi) respectively in TME3, implicating a role in tolerance and symptom recovery. AtWRKY28 and AtWRKY40 homologs of MeWRKY81 and MeWRKY11, respectively, have been shown to be involved in regulation of jasmonic and salicylic acid signaling in Arabidopsis. AtWRKY28 is an interactor in the RPW8-NBS resistance (R) protein network and downregulation of its homolog MeWRKY81 at 67 dpi in TME3 suggests a negative role for this WRKY in SACMV tolerance. In contrast, in T200, nine MeWRKYs were differentially expressed from early (12 dpi), middle (32 dpi) to late (67 dpi) infection. MeWRKY27 (homolog AtWRKY33) and MeWRKY55 (homolog AtWRKY53) were uniquely up-regulated at 12, 32 and 67 dpi in T200. AtWRKY33 and AtWRKY53 are positive regulators of leaf senescence and oxidative stress in Arabidopsis, suggesting MeWRKY55 and 27 contribute to susceptibility in T200.

A plant-specific syntaxin-6 protein contributes to the intracytoplasmic route for the begomovirus CabLCV

Because of limited free diffusion in the cytoplasm, viruses must use active transport mechanisms to move intracellularly. Nevertheless, how the plant single-stranded DNA begomoviruses hijack the host intracytoplasmic transport machinery to move from the nucleus to the plasmodesmata remains enigmatic. Here, we identified nuclear shuttle protein (NSP)-interacting proteins from Arabidopsis (Arabidopsis thaliana) by probing a protein microarray and demonstrated that the cabbage leaf curl virus NSP, a facilitator of the nucleocytoplasmic trafficking of viral (v)DNA, interacts in planta with an endosomal vesicle-localized, plant-specific syntaxin-6 protein, designated NSP-interacting syntaxin domain-containing protein (NISP). NISP displays a proviral function, unlike the syntaxin-6 paralog AT2G18860 that failed to interact with NSP. Consistent with these findings, nisp-1 mutant plants were less susceptible to begomovirus infection, a phenotype reversed by NISP complementation. NISP-overexpressing lines accumulated higher levels of vDNA than wild-type. Furthermore, NISP interacted with an NSP-interacting GTPase (NIG) involved in NSP-vDNA nucleocytoplasmic translocation. The NISP-NIG interaction was enhanced by NSP. We also showed that endosomal NISP associates with vDNA. NISP may function as a docking site for recruiting NIG and NSP into endosomes, providing a mechanism for the intracytoplasmic translocation of the NSP-vDNA complex toward and from the cell periphery.

A recessive gene pepy-1 encoding Pelota confers resistance to begomovirus isolates of PepYLCIV and PepYLCAV in Capsicum annuum

A begomovirus resistance gene pepy-1, which encodes the messenger RNA surveillance factor Pelota, was identified in pepper (C. annuum) through map-based cloning and functional characterization. Pepper yellow leaf curl disease caused by begomoviruses seriously affects pepper (Capsicum spp.) production in a number of regions around the world. Ty genes of tomato, which confer resistance to the tomato yellow leaf curl virus, are the only begomovirus resistance genes cloned to date. In this study, we focused on the identification of begomovirus resistance genes in Capsicum annuum. BaPep-5 was identified as a novel source of resistance against pepper yellow leaf curl Indonesia virus (PepYLCIV) and pepper yellow leaf curl Aceh virus (PepYLCAV). A single recessive locus, which we named as pepper yellow leaf curl disease virus resistance 1 (pepy-1), responsible for PepYLCAV resistance in BaPep-5 was identified on chromosome 5 in an F₂ population derived from a cross between BaPep-5 and the begomovirus susceptible accession BaPep-4. In the target region spanning 34 kb, a single candidate gene, the messenger RNA surveillance factor Pelota, was identified. Whole-genome resequencing of BaPep-4 and BaPep-5 and comparison of their genomic DNA sequences revealed a single nucleotide polymorphism (A to G) located at the splice site of the 9th intron of CaPelota in BaPep-5, which caused the insertion of the 9th intron into the transcript, resulting in the addition of 28 amino acids to CaPelota protein without causing a frameshift. Virus-induced gene silencing of CaPelota in the begomovirus susceptible pepper No.218 resulted in the gain of resistance against PepYLCIV, a phenotype consistent with BaPep-5. The DNA marker developed in this study will greatly facilitate marker-assisted breeding of begomovirus resistance in peppers.

Selective autophagic receptor NbNBR1 prevents NbRFP1-mediated UPS-dependent degradation of βC1 to promote geminivirus infection

Autophagy is an evolutionarily conserved, lysosomal/vacuolar degradation mechanism that targets cell organelles and macromolecules. Autophagy and autophagy-related genes have been studied for their antiviral and pro-viral roles in virus-infected plants. Here, we demonstrate the pro-viral role of a selective autophagic receptor NbNBR1 in geminivirus-infected Nicotiana benthamiana plants. The βC1 protein encoded by tomato yellow leaf curl China betasatellite (TYLCCNB) that is associated with tomato yellow leaf curl China virus (TYLCCNV) enhanced the expression level of NbNBR1. Then NbNBR1 interacted with βC1 to form cytoplasmic granules. Interaction of NbNBR1 with βC1 could prevent degradation of βC1 by the NbRFP1, an E3 ligase. Overexpression of NbNBR1 in N. benthamiana plants increased βC1 accumulation and promoted virus infection. In contrast, silencing or knocking out NbNBR1 expression in N. benthamiana suppressed βC1 accumulation and inhibited virus infection. A single amino acid substitution in βC1 (βC1K4A) abolished its interaction with NbNBR1, leading to a reduced level of βC1K4A. The TYLCCNV/TYLCCNBK4A mutant virus caused milder disease symptoms and accumulated much less viral genomic DNAs in the infected plants. Collectively, the results presented here show how a viral satellite-encoded protein hijacks host autophagic receptor NbNBR1 to form cytoplasmic granules to protect itself from NbRFP1-mediated degradation and facilitate viral infection.

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.

Genomic Variation and Diversification in Begomovirus Genome in Implication to Host and Vector Adaptation

Begomoviruses (family Geminiviridae, genus Begomovirus) are DNA viruses transmitted in a circulative, persistent manner by the whitefly Bemisia tabaci (Gennadius). As revealed by their wide host range (more than 420 plant species), worldwide distribution, and effective vector transmission, begomoviruses are highly adaptive. Still, the genetic factors that facilitate their adaptation to a diverse array of hosts and vectors remain poorly understood. Mutations in the virus genome may confer a selective advantage for essential functions, such as transmission, replication, evading host responses, and movement within the host. Therefore, genetic variation is vital to virus evolution and, in response to selection pressure, is demonstrated as the emergence of new strains and species adapted to diverse hosts or with unique pathogenicity. The combination of variation and selection forms a genetic imprint on the genome. This review focuses on factors that contribute to the evolution of Begomovirus and their global spread, for which an unforeseen diversity and dispersal has been recognized and continues to expand.

Host GRXC6 restricts Tomato yellow leaf curl virus infection by inhibiting the nuclear export of the V2 protein

Geminiviruses cause serious symptoms and devastating losses in crop plants. With a circular, single-stranded DNA genome, geminiviruses multiply their genomic DNA in the nucleus, requiring the nuclear shuttling of viral proteins and viral genomic DNAs. Many host factors, acting as proviral or antiviral factors, play key roles in geminivirus infections. Here, we report the roles of a tomato glutaredoxin (GRX), SlGRXC6, in the infection of Tomato yellow leaf curl virus (TYLCV), a single-component geminivirus. The V2 protein of TYLCV specifically and preferentially interacts with SlGRXC6 among the 55-member tomato GRX family that are broadly involved in oxidative stress responses, plant development, and pathogen responses. We show that overexpressed SlGRXC6 increases the nuclear accumulation of V2 by inhibiting its nuclear export and, in turn, inhibits trafficking of the V1 protein and viral genomic DNA. Conversely, the silenced expression of SlGRXC6 leads to an enhanced susceptibility to TYLCV. SlGRXC6 is also involved in symptom development as we observed a positive correlation where overexpression of SlGRXC6 promotes while knockdown of SlGRXC6 expression inhibits plant growth. We further showed that SlGRXC6 works with SlNTRC80, a tomato NADPH-dependent thioredoxin reductase, to regulate plant growth. V2 didn’t interact with SlNTRC80 but competed with SlNTR80 for binding to SlGRXC6, suggesting that the V2-disrupted SlGRXC6-SlNTRC80 interaction is partially responsible for the virus-caused symptoms. These results suggest that SlGRXC6 functions as a host restriction factor that inhibits the nuclear trafficking of viral components and point out a new way to control TYLCV infection by targeting the V2-SlGRXC6 interaction.

Geminiviruses infect numerous crops, induce a wide range of symptoms, and cause tremendous crop losses annually. Tomato yellow leaf curl virus (TYLCV), a single-component geminivirus, is a causative agent leading to one of the most devastating tomato diseases in the world. As a single-stranded DNA virus, genomic replication occurs in the nucleus and therefore, the nuclear shuttling is a critical step of viral infection. The V2 protein of TYLCV is involved in symptom development and viral trafficking, among other steps, and hijacks host proteins for executing its functions. Nevertheless, host factors involved in the V2-mediated functions are not well addressed. We show that tomato GRXC6 (SlGRXC6) functions as a restriction factor of TYLCV infection by interacting with and preventing V2 from moving out of the nucleus, leading to the inhibited V2-mediated nuclear export of V1 and the V1-viral DNA complex. SlGRXC6 also contributes to symptom development via its interaction with SINTRC80. V2 sequesters SlGRXC6 from forming the SlGRXC6-SlNTRC80 complex and regulates plant growth. Our work, therefore, identified a new host partner of V2 and revealed the mechanisms whereby V2 functions as a pathogenicity determinant and can be targeted for virus control.

The Sw5a gene confers resistance to ToLCNDV and triggers an HR response after direct AC4 effector recognition

Several attempts have been made to identify antiviral genes against Tomato leaf curl New Delhi virus (ToLCNDV) and related viruses. This has led to the recognition of Ty genes (Ty1-Ty6), which have been successful in developing virus-resistant crops to some extent. Owing to the regular appearance of resistance-breaking strains of these viruses, it is important to identify genes related to resistance. In the present study, we identified a ToLCNDV resistance (R) gene, SlSw5a, in a ToLCNDV-resistant tomato cultivar, H-88-78-1, which lacks the known Ty genes. The expression of SlSw5a is controlled by the transcription factor SlMyb33, which in turn is regulated by microRNA159 (sly-miR159). Virus-induced gene silencing of either SlSw5a or SlMyb33 severely increases the disease symptoms and viral titer in leaves of resistant cultivar. Moreover, in SlMyb33-silenced plants, the relative messenger RNA level of SlSw5a was reduced, suggesting SlSw5a is downstream of the sly-miR159-SlMyb33 module. We also demonstrate that SlSw5a interacts physically with ToLCNDV-AC4 (viral suppressor of RNA silencing) to trigger a hypersensitive response (HR) and generate reactive oxygen species at infection sites to limit the spread of the virus. The "RTSK" motif in the AC4 C terminus is important for the interaction, and its mutation completely abolishes the interaction with Sw5a and HR elicitation. Overall, our research reports an R gene against ToLCNDV and establishes a connection between the upstream miR159-Myb33 module and its downstream target Sw5a to activate HR in the tomato, resulting in geminivirus resistance.

Interspecies Recombination Has Driven the Macroevolution of Cassava Mosaic Begomoviruses

Begomoviruses (family Geminiviridae, genus Begomovirus) significantly hamper crop production and threaten food security around the world. The frequent emergence of new begomovirus genotypes is facilitated by high mutation frequencies and the propensity to recombine and reassort. Homologous recombination has been especially implicated in the emergence of novel cassava mosaic begomovirus (CMB) genotypes, which cause cassava mosaic disease (CMD). Cassava (Manihot esculenta) is a staple food crop throughout Africa and an important industrial crop in Asia, two continents where production is severely constrained by CMD. The CMD species complex is comprised of 11 bipartite begomovirus species with ample distribution throughout Africa and the Indian subcontinent. While recombination is regarded as a frequent occurrence for CMBs, a revised, systematic assessment of recombination and its impact on CMB phylogeny is currently lacking. We assembled data sets of all publicly available, full-length DNA-A (n = 880) and DNA-B (n = 369) nucleotide sequences from the 11 recognized CMB species. Phylogenetic networks and complementary recombination detection methods revealed extensive recombination among the CMB sequences. Six out of the 11 species descended from unique interspecies recombination events. Estimates of recombination and mutation rates revealed that all species experience mutation more frequently than recombination, but measures of population divergence indicate that recombination is largely responsible for the genetic differences between species. Our results support that recombination has significantly impacted the CMB phylogeny and has driven speciation in the CMD species complex. IMPORTANCE Cassava mosaic disease (CMD) is a significant threat to cassava production throughout Africa and Asia. CMD is caused by a complex comprised of 11 recognized virus species exhibiting accelerated rates of evolution, driven by high frequencies of mutation and genetic exchange. Here, we present a systematic analysis of the contribution of genetic exchange to cassava mosaic virus species-level diversity. Most of these species emerged as a result of genetic exchange. This is the first study to report the significant impact of genetic exchange on speciation in a group of viruses.

Porcine circovirus 2 manipulates PERK-ERO1α axis of endoplasmic reticulum in favor of its replication by derepressing viral DNA from HMGB1 sequestration within nuclei

Porcine circovirus type 2 (PCV2) causes several disease syndromes in grower pigs. PCV2 infection triggers endoplasmic reticulum (ER) stress, autophagy and oxidative stress, all of which support PCV2 replication. We have recently reported that nuclear HMGB1 is an anti-PCV2 factor by binding to viral genomic DNA. However, how PCV2 manipulates host cell responses to favor its replication has not been explored. Here, we demonstrate that PCV2 infection increased expression of ERO1α, generation of ROS and nucleocytoplasmic migration of HMGB1 via PERK activation in PK-15 cells. Inhibition of PERK or ERO1α repressed ROS production in PCV2-infected cells and increased HMGB1 retention within nuclei. These findings indicate that PCV2-induced activation of the PERK-ERO1α axis would lead to enhanced generation of ROS sufficient to decrease HMGB1 retention in the nuclei, thus derepressing viral DNA from HMGB1 sequestration. The viral Rep and Cap proteins were able to induce PERK-ERO1α-mediated ROS accumulation. Cysteine residues 107 and 305 of Rep or 108 of Cap played important roles in PCV2-induced PERK activation and distribution of HMGB1. Of the mutant viruses, only the mutant PCV2 with substitution of all three cysteine residues failed to activate PERK with reduced ROS generation and decreased nucleocytoplasmic migration of HMGB1. Collectively, this study offers novel insight into the mechanism of enhanced viral replication in which PCV2 manipulates ER to perturb its redox homeostasis via the PERK-ERO1α axis and the ER-sourced ROS from oxidative folding is sufficient to reduce HMGB1 retention in the nuclei, hence the release of HMGB1-bound viral DNA for replication. IMPORTANCE Considering the fact that clinical PCVAD mostly results from activation of latent PCV2 infection by confounding factors such as co-infection or environmental stresses, we propose that such confounding factors might impose oxidative stress to the animals where PCV2 in infected cells might utilize the elevated ROS to promote HMGB1 migration out of nuclei in favor of its replication. An animal infection model with a particular stressor could be approached with or without antioxidant treatment to examine the relationship among the stressor, ROS level, HMGB1 distribution in target tissues, virus replication and severity of PCVAD. This will help decide the use of antioxidants in the feeding regime on pig farms that suffer from PCVAD. Further investigation could examine if similar strategies are employed by DNA viruses, such as PCV3 and BFDV and if there is cross-talk among ER stress, autophagy/mitophagy and mitochondria-sourced ROS in favor of PCV2 replication.

Geminiviruses encode additional small proteins with specific subcellular localizations and virulence function

Geminiviruses are plant viruses with limited coding capacity. Geminivirus-encoded proteins are traditionally identified by applying a 10-kDa arbitrary threshold; however, it is increasingly clear that small proteins play relevant roles in biological systems, which calls for the reconsideration of this criterion. Here, we show that geminiviral genomes contain additional ORFs. Using tomato yellow leaf curl virus, we demonstrate that some of these small ORFs are expressed during the infection, and that the encoded proteins display specific subcellular localizations. We prove that the largest of these additional ORFs, which we name V3, is required for full viral infection, and that the V3 protein localizes in the Golgi apparatus and functions as an RNA silencing suppressor. These results imply that the repertoire of geminiviral proteins can be expanded, and that getting a comprehensive overview of the molecular plant-geminivirus interactions will require the detailed study of small ORFs so far neglected.

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.

Population diversity of cassava mosaic begomoviruses increases over the course of serial vegetative propagation

Cassava mosaic disease (CMD) represents a serious threat to cassava, a major root crop for more than 300 million Africans. CMD is caused by single-stranded DNA begomoviruses that evolve rapidly, making it challenging to develop durable disease resistance. In addition to the evolutionary forces of mutation, recombination and reassortment, factors such as climate, agriculture practices and the presence of DNA satellites may impact viral diversity. To gain insight into the factors that alter and shape viral diversity in planta, we used high-throughput sequencing to characterize the accumulation of nucleotide diversity after inoculation of infectious clones corresponding to African cassava mosaic virus (ACMV) and East African cassava mosaic Cameroon virus (EACMCV) in the susceptible cassava landrace Kibandameno. We found that vegetative propagation had a significant effect on viral nucleotide diversity, while temperature and a satellite DNA did not have measurable impacts in our study. EACMCV diversity increased linearly with the number of vegetative propagation passages, while ACMV diversity increased for a time and then decreased in later passages. We observed a substitution bias toward C→T and G→A for mutations in the viral genomes consistent with field isolates. Non-coding regions excluding the promoter regions of genes showed the highest levels of nucleotide diversity for each genome component. Changes in the 5' intergenic region of DNA-A resembled the sequence of the cognate DNA-B sequence. The majority of nucleotide changes in coding regions were non-synonymous, most with predicted deleterious effects on protein structure, indicative of relaxed selection pressure over six vegetative passages. Overall, these results underscore the importance of knowing how cropping practices affect viral evolution and disease progression.

Construction of Infectious Clones of Begomoviruses: Strategies, Techniques and Applications

Simple Summary

Begomovirus has a wide host range and threatens a significant amount of economic damage to many important crops such as tomatoes, beans, cassava, squash and cotton. There are many efforts directed at controlling this disease including the use of insecticides to control the insect vector as well as screening the resistant varieties. The use of synthetic virus or infectious clones approaches has allowed plant virologists to characterize and exploit the genome virus at the molecular and biological levels. By exploiting the DNA of the virus using the infectious clones strategy, the viral genome can be manipulated at specific regions to study functional genes for host–virus interactions. Thus, this review will provide an overview of the strategy to construct infectious clones of Begomovirus. The significance of established infectious clones in Begomovirus study will also be discussed.

Abstract

Begomovirus has become a potential threat to the agriculture sector. It causes significant losses to several economically important crops. Given this considerable loss, the development of tools to study viral genomes and function is needed. Infectious clones approaches and applications have allowed the direct exploitation of virus genomes. Infectious clones of DNA viruses are the critical instrument for functional characterization of the notable and newly discovered virus. Understanding of structure and composition of viruses has contributed to the evolution of molecular plant pathology. Therefore, this review provides extensive guidelines on the strategy to construct infectious clones of Begomovirus. Also, this technique’s impacts and benefits in controlling and understanding the Begomovirus infection will be discussed.

Evolutionary dynamics of bipartite begomoviruses revealed by complete genome analysis

Several key evolutionary events marked the evolution of geminiviruses, culminating with the emergence of divided (bipartite) genomes represented by viruses classified in the genus Begomovirus. This genus represents the most abundant group of multipartite viruses, contributing significantly to the observed abundance of multipartite species in the virosphere. Although aspects related to virus-host interactions and evolutionary dynamics have been extensively studied, the bipartite nature of these viruses has been little explored in evolutionary studies. Here, we performed a parallel evolutionary analysis of the DNA-A and DNA-B segments of New World begomoviruses. A total of 239 full-length DNA-B sequences obtained in this study, combined with 292 DNA-A and 76 DNA-B sequences retrieved from GenBank, were analysed. The results indicate that the DNA-A and DNA-B respond differentially to evolutionary processes, with the DNA-B being more permissive to variation and more prone to recombination than the DNA-A. Although a clear geographic segregation was observed for both segments, differences in the genetic structure between DNA-A and DNA-B were also observed, with cognate segments belonging to distinct genetic clusters. DNA-B coding regions evolve under the same selection pressures than DNA-A coding regions. Together, our results indicate an interplay between reassortment and recombination acting at different levels across distinct subpopulations and segments.

Molecular phylogenetics and evolutionary analysis of a highly recombinant begomovirus, Cotton leaf curl Multan virus, and associated satellites

Cotton leaf curl Multan virus (CLCuMuV) and its associated satellites are a major part of the cotton leaf curl disease (CLCuD) caused by the begomovirus species complex. Despite the implementation of potential disease management strategies, the incessant resurgence of resistance-breaking variants of CLCuMuV imposes a continuous threat to cotton production. Here, we present a focused effort to map the geographical prevalence, genomic diversity, and molecular evolutionary endpoints that enhance disease complexity by facilitating the successful adaptation of CLCuMuV populations to the diversified ecosystems. Our results demonstrate that CLCuMuV populations are predominantly distributed in China, while the majority of alphasatellites and betasatellites exist in Pakistan. We demonstrate that together with frequent recombination, an uneven genetic variation mainly drives CLCuMuV and its satellite's virulence and evolvability. However, the pattern and distribution of recombination breakpoints greatly vary among viral and satellite sequences. The CLCuMuV, Cotton leaf curl Multan alphasatellite, and Cotton leaf curl Multan betasatellite populations arising from distinct regions exhibit high mutation rates. Although evolutionarily linked, these populations are independently evolving under strong purifying selection. These findings will facilitate to comprehensively understand the standing genetic variability and evolutionary patterns existing among CLCuMuV populations across major cotton-producing regions of the world.

Two strains of a novel begomovirus encoding Rep proteins with identical β1 strands but different β5 strands are not compatible in replication

Geminiviruses have genomes composed of single-stranded DNA molecules and encode a rolling-circle replication (RCR) initiation protein ("Rep"), which has multiple functions. Rep binds to specific repeated DNA motifs ("iterons"), which are major determinants of virus-specific replication. The particular amino acid (aa) residues that determine the preference of a geminivirus Rep for specific iterons (i.e., the trans-acting replication "specificity determinants", or SPDs) are largely unknown, but diverse lines of evidence indicate that most of them are closely associated with the so-called RCR motif I (FLTYP), located in the first 12-19 aa residues of the protein. In this work, we characterized two strains of a novel begomovirus, rhynchosia golden mosaic Sinaloa virus (RhGMSV), that were incompatible in replication in pseudorecombination experiments. Systematic comparisons of the Rep proteins of both RhGMSV strains in the DNA-binding domain allowed the aa residues at positions 71 and 74 to be identified as the residues most likely to be responsible for differences in replication specificity. Residue 71 is part of the β-5 strand structural element, which was predicted in previous studies to contain Rep SPDs. Since the Rep proteins encoded by both RhGMSV strains are identical in their first 24 aa residues, where other studies have mapped potential SPDs, this is the first study lending direct support to the notion that geminivirus Rep proteins contain separate SPDs in their N-terminal domain.

Protein modification: A critical modulator in the interaction between geminiviruses and host plants

Geminiviruses are a large group of single-stranded DNA viruses that infect plants and cause severe agricultural losses worldwide. Given geminiviruses only have small genomes that encode a few proteins, viral factors have to interact with host components to establish an environment suitable for virus infection, whilst the host immunity system recognizes and targets these viral components during infection. Post-translational protein modifications, such as phosphorylation, lipidation, ubiquitination, SUMOylation, acetylation and methylation, have been reported to be critical during the interplay between host plants and geminiviruses. Here we summarize the research progress, including phosphorylation and lipidation which usually control the activity and localization of viral factors; as well as ubiquitination and histone modification which are predominantly interfered with by viral components. We also discuss the dynamic competition on protein modifications between host defence and geminivirus efficient infection, as well as potential applications of protein modifications in geminivirus resistance. The summary and perspective of this topic will improve our understanding on the mechanism of geminivirus-plant interaction and contribute to further protection of plants from virus infection.

Positive selection and intrinsic disorder are associated with multifunctional C4(AC4) proteins and geminivirus diversification

Viruses within the Geminiviridae family cause extensive agricultural losses. Members of four genera of geminiviruses contain a C4 gene (AC4 in geminiviruses with bipartite genomes). C4(AC4) genes are entirely overprinted on the C1(AC1) genes, which encode the replication-associated proteins. The C4(AC4) proteins exhibit diverse functions that may be important for geminivirus diversification. In this study, the influence of natural selection on the evolutionary diversity of 211 C4(AC4) genes relative to the C1(AC1) sequences they overlap was determined from isolates of the Begomovirus and Curtovirus genera. The ratio of nonsynonymous (d_N) to synonymous (d_S) nucleotide substitutions indicated that C4(AC4) genes are under positive selection, while the overlapped C1(AC1) sequences are under purifying selection. Ninety-one of 200 Begomovirus C4(AC4) genes encode elongated proteins with the extended regions being under neutral selection. C4(AC4) genes from begomoviruses isolated from tomato from native versus exotic regions were under similar levels of positive selection. Analysis of protein structure suggests that C4(AC4) proteins are entirely intrinsically disordered. Our data suggest that non-synonymous mutations and mutations that increase the length of C4(AC4) drive protein diversity that is intrinsically disordered, which could explain C4/AC4 functional variation and contribute to both geminivirus diversification and host jumping.

Plant DNA polymerases α and δ mediate replication of geminiviruses

Geminiviruses are causal agents of devastating diseases in crops. Geminiviruses have circular single-stranded (ss) DNA genomes that are replicated in the nucleus of the infected plant cell through double-stranded (ds) DNA intermediates by the plant DNA replication machinery. Which host DNA polymerase mediates geminiviral multiplication, however, has so far remained elusive. Here, we show that subunits of the nuclear replicative DNA polymerases α and δ physically interact with the geminivirus-encoded replication enhancer protein, C3, and that these polymerases are required for viral replication. Our results suggest that, while DNA polymerase α is essential to generate the viral dsDNA intermediate, DNA polymerase δ mediates the synthesis of new copies of the geminiviral ssDNA genome, and that the virus-encoded C3 may act selectively, recruiting DNA polymerase δ over ε to favour productive replication.

Virus-induced phytohormone dynamics and their effects on plant-insect interactions

Attacks on plants by both viruses and their vectors is common in nature. Yet the dynamics of the plant-virus-vector tripartite system, in particular the effects of viral infection on plant-insect interactions, have only begun to emerge in the last decade. Viruses can modulate the interactions between insect vectors and plants via the jasmonate, salicylic acid and ethylene phytohormone pathways, resulting in changes in fitness and viral transmission capacity of their insect vectors. Virus infection of plants may also modulate other phytohormones, such as auxin, gibberellins, cytokinins, brassinosteroids and abscisic acid, with yet undefined consequences on plant-insect interactions. Moreover, virus infection in plants may incur changes to other plant traits, such as nutrition and secondary metabolites, that potentially contribute to virus-associated, phytohormone-mediated manipulation of plant-insect interactions. In this article, we review the research progress, discuss issues related to the complexity and variability of the viral modulation of plant interactions with insect vectors, and suggest future directions of research in this field.