Interaction between a geminivirus replication protein and the plant sumoylation system

Post translational modifications at the verge of plant-geminivirus interaction

Plant-virus interaction is a complex phenomenon and involves the communication between plant and viral factors. Viruses have very limited coding ability yet, they are able to cause infection which results in huge agro-economic losses throughout the globe each year. Post-translational modifications (PTMs) are covalent modifications of proteins that have a drastic effect on their conformation, stability and function. Like the host proteins, geminiviral proteins are also subject to PTMs and these modifications greatly expand the diversity of their functions. Additionally, these viral proteins can also interact with the components of PTM pathways and modulate them. Several studies have highlighted the importance of PTMs such as phosphorylation, ubiquitination, SUMOylation, myristoylation, S-acylation, acetylation and methylation in plant-geminivirus interaction. PTMs also regulate epigenetic modifications during geminivirus infection which determines viral gene expression. In this review, we have summarized the role of PTMs in regulating geminiviral protein function, influence of PTMs on viral gene expression and how geminiviral proteins interact with the components of PTM pathways to modulate their function.

Replication mechanisms of circular ssDNA plant viruses and their potential implication in viral gene expression regulation

The replication of members of the two circular single-stranded DNA (ssDNA) virus families Geminiviridae and Nanoviridae, the only ssDNA viruses infecting plants, is believed to be processed by rolling-circle replication (RCR) and recombination-dependent replication (RDR) mechanisms. RCR is a ubiquitous replication mode for circular ssDNA viruses and involves a virus-encoded Replication-associated protein (Rep) which fulfills multiple functions in the replication mechanism. Two key genomic elements have been identified for RCR in Geminiviridae and Nanoviridae: (i) short iterative sequences called iterons which determine the specific recognition of the viral DNA by the Rep and (ii) a sequence enabling the formation of a stem-loop structure which contains a conserved motif and constitutes the origin of replication. In addition, studies in Geminiviridae provided evidence for a second replication mode, RDR, which has also been documented in some double-stranded DNA viruses. Here, we provide a synthesis of the current understanding of the two presumed replication modes of Geminiviridae and Nanoviridae, and we identify knowledge gaps and discuss the possibility that these replication mechanisms could regulate viral gene expression through modulation of gene copy number.

REPercussions: how geminiviruses recruit host factors for replication

Circular single-stranded DNA viruses of the family Geminiviridae encode replication-associated protein (Rep), which is a multifunctional protein involved in virus DNA replication, transcription of virus genes, and suppression of host defense responses. Geminivirus genomes are replicated through the interaction between virus Rep and several host proteins. The Rep also interacts with itself and the virus replication enhancer protein (REn), which is another essential component of the geminivirus replicase complex that interacts with host DNA polymerases α and δ. Recent studies revealed the structural and functional complexities of geminivirus Rep, which is believed to have evolved from plasmids containing a signature domain (HUH) for single-stranded DNA binding with nuclease activity. The Rep coding sequence encompasses the entire coding sequence for AC4, which is intricately embedded within it, and performs several overlapping functions like Rep, supporting virus infection. This review investigated the structural and functional diversity of the geminivirus Rep.

SUMOylation in Phytopathogen Interactions: Balancing Invasion and Resistance

Plants are constantly confronted by a multitude of biotic stresses involving a myriad of pathogens. In crops, pathogen infections result in significant agronomical losses worldwide posing a threat to food security. In order to enter plant tissues and establish a successful infection, phytopathogens have to surpass several physical, and chemical defense barriers. In recent years, post-translational modification (PTM) mechanisms have emerged as key players in plant defense against pathogens. PTMs allow a highly dynamic and rapid response in front of external challenges, increasing the complexity and precision of cellular responses. In this review, we focus on the role of SUMO conjugation (SUMOylation) in plant immunity against fungi, bacteria, and viruses. In plants, SUMO regulates multiple biological processes, ranging from development to responses arising from environmental challenges. During pathogen attack, SUMO not only modulates the activity of plant defense components, but also serves as a target of pathogen effectors, highlighting its broad role in plant immunity. Here, we summarize known pathogenic strategies targeting plant SUMOylation and, the plant SUMO conjugates involved in host-pathogen interactions. We also provide a catalog of candidate SUMO conjugates according to their role in defense responses. Finally, we discuss the complex role of SUMO in plant defense, focusing on key biological and experimental aspects that contribute to some controversial conclusions, and the opportunities for improving agricultural productivity by engineering SUMOylation in crop species.

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.

Epigenetic regulation of geminivirus pathogenesis: a case of relentless recalibration of defence responses in plants

Geminiviruses constitute one of the largest families of plant viruses and they infect many economically important crops. The proteins encoded by the single-stranded DNA genome of these viruses interact with a wide range of host proteins to cause global dysregulation of cellular processes and help establish infection in the host. Geminiviruses have evolved numerous mechanisms to exploit host epigenetic processes to ensure the replication and survival of the viral genome. Here, we review our current knowledge of diverse epigenetic processes that have been implicated in the regulation of geminivirus pathogenesis, including DNA methylation, histone post-transcriptional modification, chromatin remodelling, and nucleosome repositioning. In addition, we discuss the currently limited evidence of host epigenetic defence responses that are aimed at counteracting geminivirus infection, and the potential for exploiting these responses for the generation of resistance against geminiviruses in crop species.

Ubiquitin-Conjugating Enzyme E2 E Inhibits the Accumulation of Rice Stripe Virus in Laodelphax striatellus (Fallén)

The ubiquitin-proteasome system (UPS) is an essential protagonist in host-pathogen interactions. Among the three classes of enzymes in the UPS, ubiquitin-conjugating enzyme E2 plays a dual role in viral pathogenesis; however, the role of insect E2s in interactions with plant viruses is unclear. Twenty E2-encoding genes in Laodelphax striatellus, the small brown planthopper, were identified and classified into 17 groups by transcriptomic and phylogenetic analysis. Full-length cDNAs of four LstrE2s (LstrE2 A/E/G2/H) were obtained by rapid-amplification of cDNA ends (RACE-PCR) analysis. Expression of the four LstrE2s showed tissue- and development-specific patterns. RT-qPCR analyses revealed that Rice stripe viruse (RSV) infection increased the level of LstrE2 A/E/G2/H. Further study indicated that repression of LstrE2 E via RNAi caused significant increases in the expression of RSV coat protein mRNA and protein levels. These findings suggest that LstrE2 E inhibits RSV accumulation in the planthopper body. Understanding the function of LstrE2 E in RSV accumulation may ultimately result in the development of novel antiviral strategies.

Identification of Tomato Proteins That Interact With Replication Initiator Protein (Rep) of the Geminivirus TYLCV

Geminiviruses are plant-infecting DNA viruses that reshape the intracellular environment of their host in order to create favorable conditions for viral replication and propagation. Viral manipulation is largely mediated via interactions between viral and host proteins. Identification of this protein network helps us to understand how these viruses manipulate their host and therefore provides us potentially with novel leads for resistance against this class of pathogens, as genetic variation in the corresponding plant genes could subvert viral manipulation. Different studies have already yielded a list of host proteins that interact with one of the geminiviral proteins. Here, we use affinity purification followed by mass spectrometry (AP-MS) to further expand this list of interacting proteins, focusing on an important host (tomato) and the Replication initiator protein (Rep, AL1, C1) from Tomato yellow leaf curl virus (TYLCV). Rep is the only geminiviral protein proven to be essential for geminiviral replication and it forms an integral part of viral replisomes, a protein complex that consists of plant and viral proteins that allows for viral DNA replication. Using AP-MS, fifty-four 'high confidence' tomato proteins were identified that specifically co-purified with Rep. For two of them, an unknown EWS-like RNA-binding protein (called Geminivirus Rep interacting EWS-like protein 1 or GRIEP1) and an isoform of the THO complex subunit 4A (ALY1), we were able to confirm this interaction with Rep in planta using a second method, bimolecular fluorescence complementation (BiFC). The THO subunit 4 is part of the THO/TREX (TRanscription-EXport) complex, which controls RNA splicing and nuclear export of mRNA to the cytoplasm and is also connected to plant disease resistance. This work represents the first step towards characterization of novel host factors with a putative role in the life cycle of TYLCV and possibly other geminiviruses.

Hijacking of host cellular components as proviral factors by plant-infecting viruses

Plant viruses are important pathogens that cause serious crop losses worldwide. They are obligate intracellular parasites that commandeer a wide array of proteins, as well as metabolic resources, from infected host cells. In the past two decades, our knowledge of plant-virus interactions at the molecular level has exploded, which provides insights into how plant-infecting viruses co-opt host cellular machineries to accomplish their infection. Here, we review recent advances in our understanding of how plant viruses divert cellular components from their original roles to proviral functions. One emerging theme is that plant viruses have versatile strategies that integrate a host factor that is normally engaged in plant defense against invading pathogens into a viral protein complex that facilitates viral infection. We also highlight viral manipulation of cellular key regulatory systems for successful virus infection: posttranslational protein modifications for fine control of viral and cellular protein dynamics; glycolysis and fermentation pathways to usurp host resources, and ion homeostasis to create a cellular environment that is beneficial for viral genome replication. A deeper understanding of viral-infection strategies will pave the way for the development of novel antiviral strategies.

RNA silencing of South African cassava mosaic virus in transgenic cassava expressing AC1/AC4 hp- RNA induces tolerance

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A hairpin-RNA construct targeting South African cassava mosaic virus AC1/AC4 overlapping genome region confers tolerance in cassava.

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Viral load and symptom severity were significantly lower in transgenic lines transformed with AC1/AC4 construct compared to wildtype cv.60444.

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SACMV transgenic lines infected with SACMV displayed a recovery phenotype similar to cassava mosaic disease-tolerant cassava landrace TME3.

Cassava mosaic disease (CMD), caused by geminiviruses, is a major hurdle to cassava production. Due to the heterozygous nature of cassava, breeding for virus resistance is difficult, but cassava has been shown to be a good candidate for genetic engineering using RNA interference (RNAi). T This study reports on the ability of a transgene-derived RNA hairpin, homologous to an overlapping region of the SACMV replication associated protein and putative virus suppressor of silencing protein (AC1/AC4), to confer tolerance in the CMD-susceptible model cassava cultivar 60444. Three of the fourteen transgenic lines expressing SACMV AC1/AC4 hairpin-derived siRNAs showed decreased symptoms and viral loads compared to untransformed control plants. Expression of SACMV AC1/AC4 homologous siRNAs showed that this tolerance is most likely associated with post-transcriptional gene silencing of the virus. This is the first report of targeting the overlapping AC1 and AC4 genes of SACMV conferring CMD tolerance in cassava.

A Lysine Residue Essential for Geminivirus Replication Also Controls Nuclear Localization of the Tomato Yellow Leaf Curl Virus Rep Protein

Geminiviruses are single-stranded DNA (ssDNA) viruses that infect a wide range of plants. To promote viral replication, geminiviruses manipulate the host cell cycle. The viral protein Rep is essential to reprogram the cell cycle and then initiate viral DNA replication by interacting with a plethora of nuclear host factors. Even though many protein domains of Rep have been characterized, little is known about its nuclear targeting. Here, we show that one conserved lysine in the N-terminal part of Rep is pivotal for nuclear localization of the Rep protein from Tomato yellow leaf curl virus (TYLCV), with two other lysines also contributing to its nuclear import. Previous work had identified that these residues are essential for Rep from Tomato golden mosaic virus (TGMV) to interact with the E2 SUMO-conjugating enzyme (SCE1). We here show that mutating these lysines leads to nuclear exclusion of TYLCV Rep without compromising its interaction with SCE1. Moreover, the ability of TYLCV Rep to promote viral DNA replication also depends on this highly conserved lysine independently of its role in nuclear import of Rep. Our data thus reveal that this lysine potentially has a broad role in geminivirus replication, but its role in nuclear import and SCE1 binding differs depending on the Rep protein examined.IMPORTANCE Nuclear activity of the replication initiator protein (Rep) of geminiviruses is essential for viral replication. We now define that one highly conserved lysine is important for nuclear import of Rep from three different begomoviruses. To our knowledge, this is the first time that nuclear localization has been mapped for any geminiviral Rep protein. Our data add another key function to this lysine residue, besides its roles in viral DNA replication and interaction with host factors, such as the SUMO E2-conjugating enzyme.

Genome wide molecular evolution analysis of begomoviruses reveals unique diversification pattern in coat protein gene of Old World and New World viruses

Begomoviruses (Family-Geminiviridae) are plant infecting single stranded DNA viruses known to evolve very fast. Here, we have analysed the DNA-A sequences of 302 begomoviruses reported as 'type isolates' from different countries following the list of International Committee on Taxonomy of Viruses till 2017. Phylogenetic analysis was performed which revealed two major evolutionarily distinct groups namely Old World (OW) and New World (NW) viruses. Our work present evidence that cp gene has varied degree of diversification among the viruses reported from NW and OW. The NW viruses are more conserved in their cp gene sequences than that of OW viruses irrespective of host plant families. Further analysis reveals that cp gene differs in its recombination pattern among OW and NW viruses whereas rep gene is highly recombination prone in both OW and NW viruses. The sequence conservation in cp gene in NW viruses is a result of meagre recombination and subsequent low substitution rate in comparison to OW viruses. Our results demonstrated that the cp gene in NW viruses is less likely to possess nuclear localisation sequences than OW cp gene. Further we present evidence that the NW-cp is under the influence of strong purifying selection. We propose that the precoat protein (pcp) gene present exclusively in the 5' of cp gene in OW viruses is highly diversified and strong positive selection working on pcp gene might be attributing largely to the diversity of OW-cp gene.

Proteomic and Phosphoproteomic Analysis in Tobacco Mosaic Virus-Infected Tobacco (Nicotiana tabacum)

Tobacco mosaic virus (TMV) is a common source of biological stress that significantly affects plant growth and development. It is also useful as a model in studies designed to clarify the mechanisms involved in plant viral disease. Plant responses to abiotic stress were recently reported to be regulated by complex mechanisms at the post-translational modification (PTM) level. Protein phosphorylation is one of the most widespread and major PTMs in organisms. Using immobilized metal ion affinity chromatography (IMAC) enrichment, high-pH C18 chromatography fraction, and high-accuracy mass spectrometry (MS), a set of proteins and phosphopeptides in both TMV-infected tobacco and control tobacco were identified. A total of 4905 proteins and 3998 phosphopeptides with 3063 phosphorylation sites were identified. These 3998 phosphopeptides were assigned to 1311 phosphoproteins, as some proteins carried multiple phosphorylation sites. Among them, 530 proteins and 337 phosphopeptides corresponding to 277 phosphoproteins differed between the two groups. There were 43 upregulated phosphoproteins, including phosphoglycerate kinase, pyruvate phosphate dikinase, protein phosphatase 2C, and serine/threonine protein kinase. To the best of our knowledge, this is the first phosphoproteomic analysis of leaves from a tobacco cultivar, K326. The results of this study advance our understanding of tobacco development and TMV action at the protein phosphorylation level.

Geminivirus Replication Protein Impairs SUMO Conjugation of Proliferating Cellular Nuclear Antigen at Two Acceptor Sites

Geminiviruses are DNA viruses that replicate in nuclei of infected plant cells using the plant DNA replication machinery, including PCNA (proliferating cellular nuclear antigen), a cofactor that orchestrates genome duplication and maintenance by recruiting crucial players to replication forks. These viruses encode a multifunctional protein, Rep, which is essential for viral replication, induces the accumulation of the host replication machinery, and interacts with several host proteins, including PCNA and the SUMO E2 conjugation enzyme (SCE1). Posttranslational modification of PCNA by ubiquitin or SUMO plays an essential role in the switching of PCNA between interacting partners during DNA metabolism processes (e.g., replication, recombination, and repair, etc.). In yeast, PCNA sumoylation has been associated with DNA repair involving homologous recombination (HR). Previously, we reported that ectopic Rep expression results in very specific changes in the sumoylation pattern of plant cells. In this work, we show, using a reconstituted sumoylation system in Escherichia coli, that tomato PCNA is sumoylated at two residues, K254 and K164, and that coexpression of the geminivirus protein Rep suppresses sumoylation at these lysines. Finally, we confirm that PCNA is sumoylated in planta and that Rep also interferes with PCNA sumoylation in plant cells.IMPORTANCE SUMO adducts have a key role in regulating the activity of animal and yeast PCNA on DNA repair and replication. Our work demonstrates for the first time that sumoylation of plant PCNA occurs in plant cells and that a plant virus interferes with this modification. This work marks the importance of sumoylation in allowing viral infection and replication in plants. Moreover, it constitutes a prime example of how viral proteins interfere with posttranslational modifications of selected host factors to create a proper environment for infection.

Geminivirus Replication Protein Impairs SUMO Conjugation of Proliferating Cellular Nuclear Antigen at Two Acceptor Sites

Geminiviruses are DNA viruses that replicate in nuclei of infected plant cells using the plant DNA replication machinery, including PCNA (proliferating cellular nuclear antigen), a cofactor that orchestrates genome duplication and maintenance by recruiting crucial players to replication forks. These viruses encode a multifunctional protein, Rep, which is essential for viral replication, induces the accumulation of the host replication machinery, and interacts with several host proteins, including PCNA and the SUMO E2 conjugation enzyme (SCE1). Posttranslational modification of PCNA by ubiquitin or SUMO plays an essential role in the switching of PCNA between interacting partners during DNA metabolism processes (e.g., replication, recombination, and repair, etc.). In yeast, PCNA sumoylation has been associated with DNA repair involving homologous recombination (HR). Previously, we reported that ectopic Rep expression results in very specific changes in the sumoylation pattern of plant cells. In this work, we show, using a reconstituted sumoylation system in Escherichia coli, that tomato PCNA is sumoylated at two residues, K254 and K164, and that coexpression of the geminivirus protein Rep suppresses sumoylation at these lysines. Finally, we confirm that PCNA is sumoylated in planta and that Rep also interferes with PCNA sumoylation in plant cells.IMPORTANCE SUMO adducts have a key role in regulating the activity of animal and yeast PCNA on DNA repair and replication. Our work demonstrates for the first time that sumoylation of plant PCNA occurs in plant cells and that a plant virus interferes with this modification. This work marks the importance of sumoylation in allowing viral infection and replication in plants. Moreover, it constitutes a prime example of how viral proteins interfere with posttranslational modifications of selected host factors to create a proper environment for infection.

Multifunctional roles of geminivirus encoded replication initiator protein

Geminivirus infection has been a threat to cultivation worldwide by causing huge losses to the crop. The single-stranded DNA genome of a geminivirus possesses a limited coding potential and many of the open reading frames (ORFs) are overlapping. Out of 5-7 ORFs that a geminivirus genome codes for, the AC1 ORF encodes for the replication initiator protein (Rep) which is involved in the replication of virus within the infected plant cell. Rep is the only viral protein absolutely required for the in planta viral replication. Across different genera of the Geminiviridae family, the AC1 ORF exhibits a high degree of sequence conservation thus it has been used as an effective target for developing broad spectrum resistance against the invading geminiviruses. This multifunctional protein is required for initiation, elongation as well as termination of the viral replication process. Rep is also involved in stimulation of viral transcription. In addition, it also functions as suppressor of gene silencing and is involved in the process of transcription by regulating the expression of certain viral genes. Rep protein also interacts with few viral proteins such as coat protein, replication enhancer protein and with several host factors involved in different pathways and processes for its replication and efficient infection. This review will summarise our current understanding about the role of this early viral protein in viral propagation as well as in establishment of pathogenesis in a permissive host.

Infectivity of cloned begomoviral DNAs: an appraisal

Infectivity of cloned begomoviral DNAs is an important criterion to establish the etiology of the disease it causes, to study viral gene functions and host-virus interactions. Three main methods have been employed to study infectivity; mechanical inoculation with cloned viral DNA using abrasives, Agrobacterium-mediated inoculation (agroinoculation) of cloned viral DNA and bombardment using microprojectiles coated with cloned viral DNA (biolistics). Each method has its own advantages and disadvantages and the adoption of one over the other for demonstrating infectivity depends on various factors. This review compares the various features associated with the above three methods.

Two SUMO Proteases SUMO PROTEASE RELATED TO FERTILITY1 and 2 Are Required for Fertility in Arabidopsis

In plants, the posttranslational modification small ubiquitin-like modifier (SUMO) is involved in regulating several important developmental and cellular processes, including flowering time control and responses to biotic and abiotic stresses. Here, we report two proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2, that regulate male and female gamete and embryo development and remove SUMO from proteins in vitro and in vivo. spf1 mutants exhibit abnormal floral structures and embryo development, while spf2 mutants exhibit largely a wild-type phenotype. However, spf1 spf2 double mutants exhibit severe abnormalities in microgametogenesis, megagametogenesis, and embryo development, suggesting that the two genes are functionally redundant. Mutation of SPF1 and SPF2 genes also results in misexpression of generative- and embryo-specific genes. In vitro, SPF1 and SPF2 process SUMO1 precursors into a mature form, and as expected in vivo, spf1 and spf2 mutants accumulate SUMO conjugates. Using a yeast two-hybrid screen, we identified EMBRYO SAC DEVELOPMENT ARREST9 (EDA9) as an SPF1-interacting protein. In vivo, we demonstrate that EDA9 is sumolyated and that, in spf1 mutants, EDA9-SUMO conjugates increase in abundance, demonstrating that EDA9 is a substrate of SPF1. Together, our results demonstrate that SPF1 and SPF2 are two SUMO proteases important for plant development in Arabidopsis (Arabidopsis thaliana).

Geminiviruses and Plant Hosts: A Closer Examination of the Molecular Arms Race

Geminiviruses are plant-infecting viruses characterized by a single-stranded DNA (ssDNA) genome. Geminivirus-derived proteins are multifunctional and effective regulators in modulating the host cellular processes resulting in successful infection. Virus-host interactions result in changes in host gene expression patterns, reprogram plant signaling controls, disrupt central cellular metabolic pathways, impair plant's defense system, and effectively evade RNA silencing response leading to host susceptibility. This review summarizes what is known about the cellular processes in the continuing tug of war between geminiviruses and their plant hosts at the molecular level. In addition, implications for engineered resistance to geminivirus infection in the context of a greater understanding of the molecular processes are also discussed. Finally, the prospect of employing geminivirus-based vectors in plant genome engineering and the emergence of powerful genome editing tools to confer geminivirus resistance are highlighted to complete the perspective on geminivirus-plant molecular interactions.

The replication initiator protein of a geminivirus interacts with host monoubiquitination machinery and stimulates transcription of the viral genome

Geminiviruses constitute a group of plant viruses, with a ssDNA genome, whose replication in the nucleus of an infected cell requires the function of geminivirus-encoded replication initiator protein (Rep). Our results suggest that monoubiquitinated histone 2B (H2B-ub) promotes tri-methylation of histone 3 at lysine 4 (H3-K4me3) on the promoter of Chilli leaf curl virus (ChiLCV). We isolated homologues of two major components of the monoubiquitination machinery: UBIQUITIN-CONJUGATING ENZYME2 (NbUBC2) and HISTONE MONOUBIQUITINATION1 (NbHUB1) from N. benthamiana. ChiLCV failed to cause disease in NbUBC2-, and NbHUB1-silenced plants, at the same time, H2B-ub and H3-K4me3 modifications were decreased, and the occupancy of RNA polymerase II on the viral promoter was reduced as well. In further investigations, Rep protein of ChiLCV was found to re-localize NbUBC2 from the cytoplasm to the nucleoplasm, like NbHUB1, the cognate partner of NbUBC2. Rep was observed to interact and co-localize with NbHUB1 and NbUBC2 in the nuclei of the infected cells. In summary, the current study reveals that the ChiLCV Rep protein binds the viral genome and interacts with NbUBC2 and NbHUB1 for the monoubiquitination of histone 2B that subsequently promotes trimethylation of histone 3 at lysine 4 on ChiLCV mini-chromosomes and enhances transcription of the viral genes.

Viral manipulation of the cellular sumoylation machinery

Viruses exploit various cellular processes for their own benefit, including counteracting anti-viral responses and regulating viral replication and propagation. In the past 20 years, protein sumoylation has emerged as an important post-translational modification that is manipulated by viruses to modulate anti-viral responses, viral replication, and viral pathogenesis. The process of sumoylation is a multi-step cascade where a small ubiquitin-like modifier (SUMO) is covalently attached to a conserved ΨKxD/E motif within a target protein, altering the function of the modified protein. Here we review how viruses manipulate the cellular machinery at each step of the sumoylation process to favor viral survival and pathogenesis.

SUMOylation Inhibition Mediated by Disruption of SUMO E1-E2 Interactions Confers Plant Susceptibility to Necrotrophic Fungal Pathogens

Protein modification by SUMO modulates essential biological processes in eukaryotes. SUMOylation is facilitated by sequential action of the E1-activating, E2-conjugating, and E3-ligase enzymes. In plants, SUMO regulates plant development and stress responses, which are key determinants in agricultural productivity. To generate additional tools for advancing our knowledge about the SUMO biology, we have developed a strategy for inhibiting in vivo SUMO conjugation based on disruption of SUMO E1-E2 interactions through expression of E1 SAE2^UFDCt domain. Targeted mutagenesis and phylogenetic analyses revealed that this inhibition involves a short motif in SAE2^UFDCt highly divergent across kingdoms. Transgenic plants expressing the SAE2^UFDCt domain displayed dose-dependent inhibition of SUMO conjugation, and have revealed the existence of a post-transcriptional mechanism that regulates SUMO E2 conjugating enzyme levels. Interestingly, these transgenic plants displayed increased susceptibility to necrotrophic fungal infections by Botrytis cinerea and Plectosphaerella cucumerina. Early after fungal inoculation, host SUMO conjugation was post-transcriptionally downregulated, suggesting that targeting SUMOylation machinery could constitute a novel mechanism for fungal pathogenicity. These findings support the role of SUMOylation as a mechanism involved in plant protection from environmental stresses. In addition, the strategy for inhibiting SUMO conjugation in vivo described in this study might be applicable in important crop plants and other non-plant organisms regardless of their genetic complexity.

The diversification of begomovirus populations is predominantly driven by mutational dynamics

Begomoviruses (single-stranded DNA plant viruses) are responsible for serious agricultural threats. Begomovirus populations exhibit a high degree of within-host genetic variation and evolve as quickly as RNA viruses. Although the recombination-prone nature of begomoviruses has been extensively demonstrated, the relative contribution of recombination and mutation to the genetic variation of begomovirus populations has not been assessed. We estimated the genetic variability of begomovirus datasets from around the world. An uneven distribution of genetic variation across the length of the cp and rep genes due to recombination was evident from our analyses. To estimate the relative contributions of recombination and mutation to the genetic variability of begomoviruses, we mapped all substitutions over maximum likelihood trees and counted the number of substitutions on branches which were associated with recombination (η_r) and mutation (η_μ). In addition, we also estimated the per generation relative rates of both evolutionary mechanisms (r/μ) to express how frequently begomovirus genomes are affected by recombination relative to mutation. We observed that the composition of genetic variation in all begomovirus datasets was dominated by mutation. Additionally, the low correlation between the estimates indicated that the relative contributions of recombination and mutation are not necessarily a function of their relative rates. Our results show that, although a considerable fraction of the genetic variation levels could be assigned to recombination, it was always lower than that due to mutation, indicating that the diversification of begomovirus populations is predominantly driven by mutational dynamics.

Plant immunity against viruses: antiviral immune receptors in focus

BACKGROUND

Among the environmental limitations that affect plant growth, viruses cause major crop losses worldwide and represent serious threats to food security. Significant advances in the field of plant-virus interactions have led to an expansion of potential strategies for genetically engineered resistance in crops during recent years. Nevertheless, the evolution of viral virulence represents a constant challenge in agriculture that has led to a continuing interest in the molecular mechanisms of plant-virus interactions that affect disease or resistance.

SCOPE AND CONCLUSION

This review summarizes the molecular mechanisms of the antiviral immune system in plants and the latest breakthroughs reported in plant defence against viruses. Particular attention is given to the immune receptors and transduction pathways in antiviral innate immunity. Plants counteract viral infection with a sophisticated innate immune system that resembles the non-viral pathogenic system, which is broadly divided into pathogen-associated molecular pattern (PAMP)-triggered immunity and effector-triggered immunity. An additional recently uncovered virus-specific defence mechanism relies on host translation suppression mediated by a transmembrane immune receptor. In all cases, the recognition of the virus by the plant during infection is central for the activation of these innate defences, and, conversely, the detection of host plants enables the virus to activate virulence strategies. Plants also circumvent viral infection through RNA interference mechanisms by utilizing small RNAs, which are often suppressed by co-evolving virus suppressors. Additionally, plants defend themselves against viruses through hormone-mediated defences and activation of the ubiquitin-26S proteasome system (UPS), which alternatively impairs and facilitates viral infection. Therefore, plant defence and virulence strategies co-evolve and co-exist; hence, disease development is largely dependent on the extent and rate at which these opposing signals emerge in host and non-host interactions. A deeper understanding of plant antiviral immunity may facilitate innovative biotechnological, genetic and breeding approaches for crop protection and improvement.

Sumoylation of Turnip mosaic virus RNA Polymerase Promotes Viral Infection by Counteracting the Host NPR1-Mediated Immune Response

Sumoylation is a transient, reversible dynamic posttranslational modification that regulates diverse cellular processes including plant-pathogen interactions. Sumoylation of NPR1, a master regulator of basal and systemic acquired resistance to a broad spectrum of plant pathogens, activates the defense response. Here, we report that NIb, the only RNA-dependent RNA polymerase of Turnip mosaic virus (TuMV) that targets the nucleus upon translation, interacts exclusively with and is sumoylated by SUMO3 (SMALL UBIQUITIN-LIKE MODIFIER3), but not the three other Arabidopsis thaliana SUMO paralogs. TuMV infection upregulates SUMO3 expression, and the sumoylation of NIb by SUMO3 regulates the nuclear-cytoplasmic partitioning of NIb. We identified the SUMO-interacting motif in NIb that is essential for its sumoylation and found that knockout or overexpression of SUMO3 suppresses TuMV replication and attenuates viral symptoms, suggesting that SUMO3 plays dual roles as a host factor of TuMV and as an antiviral defender. Sumoylation of NIb by SUMO3 is crucial for its role in suppressing the host immune response. Taken together, our findings reveal that sumoylation of NIb promotes TuMV infection by retargeting NIb from the nucleus to the cytoplasm where viral replication takes place and by suppressing host antiviral responses through counteracting the TuMV infection-induced, SUMO3-activated, NPR1-mediated resistance pathway.

Plant responses to tomato chlorotic mottle virus: Proteomic view of the resistance mechanisms to a bipartite begomovirus in tomato

Tomato chlorotic mottle virus (ToCMoV) is a widespread bipartite Begomovirus species found in tomato fields in Brazil. In this study, plant responses and putative mechanisms associated with the 'Tyking'-derived recessive resistance to ToCMoV were investigated. Changes in the protein profile in the inoculated plants of two near isogenic tomato lines resistant ('LAM 157') and susceptible ('Santa Clara') to ToCMoV were analyzed. Seedlings were biolistically inoculated with an infectious ToCMoV clone. Leaves from infected plants (confirmed by PCR) were sampled at 15days after inoculation. Proteins were extracted using phenol and analyzed by shotgun MS (2D-nanoUPLC/HDMS^E). Out of the 534 identified proteins, 82 presented statistically significant differences in abundance, including 35 unique proteins displayed in the resistant tomato inoculated with ToCMoV. Proteins associated to chromatin structure, cytoskeleton structure, cuticle biosynthesis, and ubiquitin pathway were identified and their putative roles during virus infection process were discussed. The protein profile analysis allowed for the development of a hypothetical model showing how the resistant host cell responds to ToCMoV infection. The data obtained provide a better understanding of resistant mechanisms used by the host plant to contain viral infection and could be the basis for further investigation in other plant-begomovirus pathosystems.

BIOLOGICAL SIGNIFICANCE

In this study we propose a model of resistance to begomovirus in tomato and highlight host proteins, which could be targets for future investigations in plant-begomovirus pathosystems.

Tobacco RING E3 Ligase NtRFP1 Mediates Ubiquitination and Proteasomal Degradation of a Geminivirus-Encoded βC1

The βC1 protein encoded by the Tomato yellow leaf curl China virus-associated betasatellite functions as a pathogenicity determinant. To better understand the molecular basis whereby βC1 functions in pathogenicity, a yeast two-hybrid screen of a tobacco cDNA library was carried out using βC1 as the bait. The screen revealed that βC1 interacts with a tobacco RING-finger protein designated NtRFP1, which was further confirmed by the bimolecular fluorescence complementation and co-immunoprecipitation assays in Nicotiana benthamiana cells. Expression of NtRFP1 was induced by βC1, and in vitro ubiquitination assays showed that NtRFP1 is a functional E3 ubiquitin ligase that mediates βC1 ubiquitination. In addition, βC1 was shown to be ubiquitinated in vivo and degraded by the plant 26S proteasome. After viral infection, plants overexpressing NtRFP1 developed attenuated symptoms, whereas plants with silenced expression of NtRFP1 showed severe symptoms. Other lines of evidence showed that NtRFP1 attenuates βC1-induced symptoms through promoting its degradation by the 26S proteasome. Taken together, our results suggest that tobacco RING E3 ligase NtRFP1 attenuates disease symptoms by interacting with βC1 to mediate its ubiquitination and degradation via the ubiquitin/26S proteasome system.

Tomato plant cell death induced by inhibition of HSP90 is alleviated by Tomato yellow leaf curl virus infection

To ensure a successful long-term infection cycle, begomoviruses must restrain their destructive effect on host cells and prevent drastic plant responses, at least in the early stages of infection. The monopartite begomovirus Tomato yellow leaf curl virus (TYLCV) does not induce a hypersensitive response and cell death on whitefly-mediated infection of virus-susceptible tomato plants until diseased tomatoes become senescent. The way in which begomoviruses evade plant defences and interfere with cell death pathways is still poorly understood. We show that the chaperone HSP90 (heat shock protein 90) and its co-chaperone SGT1 (suppressor of the G2 allele of Skp1) are involved in the establishment of TYLCV infection. Inactivation of HSP90, as well as silencing of the Hsp90 and Sgt1 genes, leads to the accumulation of damaged ubiquitinated proteins and to a cell death phenotype. These effects are relieved under TYLCV infection. HSP90-dependent inactivation of 26S proteasome degradation and the transcriptional activation of the heat shock transcription factors HsfA2 and HsfB1 and of the downstream genes Hsp17 and Apx1/2 are suppressed in TYLCV-infected tomatoes. Following suppression of the plant stress response, TYLCV can replicate and accumulate in a permissive environment.

Interplay between the virus and the ubiquitin-proteasome system: molecular mechanism of viral pathogenesis

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Many viruses have evolved to utilize the host UPS for their own benefit.

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Viruses subvert the UPS to maintain optimal level/function of viral proteins.

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Viruses exploit the UPS to degrade host proteins which impede viral growth.

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The UPS serves as an important host anti-viral defense mechanism.

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The UPS is inhibited by some viruses to prevent viral clearance.

The ubiquitin–proteasome system (UPS) plays a central role in a wide range of fundamental cellular functions by ensuring protein quality control and through maintaining a critical level of important regulatory proteins. Viruses subvert or manipulate this cellular machinery to favor viral propagation and to evade host immune response. The UPS serves as a double-edged sword in viral pathogenesis: on the one hand, the UPS is utilized by many viruses to maintain proper function and level of viral proteins; while on the other hand, the UPS constitutes a host defense mechanism to eliminate viral components. To combat this host anti-viral machinery, viruses have evolved to employ the UPS to degrade or inactivate cellular proteins that limit viral growth. This review will highlight our current knowledge pertaining to the different roles for the UPS in viral pathogenesis.

Insights into the functional characteristics of geminivirus rolling-circle replication initiator protein and its interaction with host factors affecting viral DNA replication

Geminiviruses are DNA viruses that infect several economically important crops, resulting in a reduction in their overall yield. These plant viruses have circular, single-stranded DNA genomes that replicate mainly by a rolling-circle mechanism. Geminivirus infection results in crosstalk between viral and cellular factors to complete the viral life cycle or counteract the infection as part of defense mechanisms of host plants. The geminiviral replication initiator protein Rep is the only essential viral factor required for replication. It is multifunctional and is known to interact with a number of host factors to modulate the cellular environment or to function as a part of the replication machinery. This review provides a holistic view of the research related to the viral Rep protein and various host factors involved in geminiviral DNA replication. Studies on the promiscuous nature of geminiviral satellite DNAs are also reviewed.

Arabidopsis thaliana NAC083 protein interacts with Mungbean yellow mosaic India virus (MYMIV) Rep protein

Geminiviral replication initiator protein (Rep) is a key player in geminiviral rolling circle mode of replication. However, the virus exploits various host cellular machineries for its replication. Study of these host factors is important to understand the geminiviral DNA replication in greater details. With this view, we screened for the peptides interacting with the Rep protein of a representative of geminivirus, namely, Mungbean yellow mosaic India virus (MYMIV), employing phage display technique. Through this screen, we have identified a host transcription factor, NAC083, as a potential MYMIV-Rep-binding partner. In silico docking studies also suggested possible binding of NAC083 peptide to MYMIV-Rep. We validated the interaction between MYMIV-Rep and Arabidopsis thaliana full-length NAC083 protein using in vitro pull-down assay and yeast two-hybrid analysis. NAC proteins are well-known transcription factors belonging to the largest gene families in plants. This study demonstrates for the first time the interaction of NAC083, a member of NAC transcription factor family, with MYMIV-Rep protein thereby indicating its possible role in MYMIV DNA replication.

Tomato yellow leaf curl Sardinia virus-resistant tomato plants expressing the multifunctional N-terminal domain of the replication-associated protein show transcriptional changes resembling stress-related responses

The N-terminal domain (amino acids 1-130) of the replication-associated protein (Rep130 ) of Tomato yellow leaf curl Sardinia virus (TYLCSV) retains the ability of full-length Rep to localize to the nucleus and to down-regulate C1 transcription when ectopically expressed in plants, both functions being required to inhibit homologous viral replication. In this study, we analysed the effect of Rep130 expression on virus resistance and the plant transcriptome in the natural and agronomically important host species of TYLCSV, Solanum lycopersicum. Tomato plants accumulating high levels of Rep130 were generated and proved to be resistant to TYLCSV. Using an in vitro assay, we showed that plant-expressed Rep130 also retains the catalytic activity of Rep, thus supporting the notion that this protein domain is fully functional. Interestingly, Rep130 -expressing tomatoes were characterized by an altered transcriptional profile resembling stress-related responses. Notably, the serine-type protease inhibitor (Ser-PI) category was over-represented among the 20 up-regulated genes. The involvement of Rep130 in the alteration of host mRNA steady-state levels was confirmed using a distinct set of virus-resistant transgenic tomato plants expressing the same TYLCSV Rep130 , but from a different, synthetic, gene. Eight genes were found to be up-regulated in both types of transgenic tomato and two encoded Ser-PIs. Four of these eight genes were also up-regulated in TYLCSV-infected wild-type tomato plants. Implications with regard to the ability of this Rep domain to interfere with viral infections and to alter the host transcriptome are discussed.

Replication of Plant Viruses

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses co-infecting cells.

Viruses replicate using both their own genetic information and host cell components and machinery. The different genome types have different replication pathways which contain controls on linking the process with translation and movement around the cell as well as not compromising the infected cell. This chapter discusses the replication mechanisms, faults in replication and replication of viruses coinfecting cells.

Effects of the crinivirus coat protein-interacting plant protein SAHH on post-transcriptional RNA silencing and its suppression

In plants, post-transcriptional gene silencing (PTGS) is a sequence-specific mechanism of RNA degradation induced by double-stranded RNA (dsRNA), which is processed into small interfering RNAs (siRNAs). siRNAs are methylated and, thereby, stabilized by the activity of the S-adenosylmethionine-dependent RNA methyltransferase HEN1. PTGS is amplified by host-encoded RNA-dependent RNA polymerases (RDR), which generate dsRNA that is processed into secondary siRNAs. To counteract this RNA silencing-mediated response of the host, plant viruses express proteins with silencing suppression activity. Here, we report that the coat protein (CP) of crinivirus (family Closteroviridae, genus Crinivirus) Tomato chlorosis virus, a known suppressor of silencing, interacts with S-adenosylhomocysteine hydrolase (SAHH), a plant protein essential for sustaining the methyl cycle and S-adenosylmethionine-dependent methyltransferase activity. Our results show that, by contributing to an increased accumulation of secondary siRNAs generated by the action of RDR6, SAHH enhances local RNA silencing. Although downregulation of SAHH prevents local silencing, it enhances the spread of systemic silencing. Our results also show that SAHH is important in the suppression of local RNA silencing not only by the crinivirus Tomato chlorosis virus CP but also by the multifunctional helper component-proteinase of the potyvirus Potato virus Y.

Geminivirus protein structure and function

Geminiviruses are a family of plant viruses that cause economically important plant diseases worldwide. These viruses have circular single-stranded DNA genomes and four to eight genes that are expressed from both strands of the double-stranded DNA replicative intermediate. The transcription of these genes occurs under the control of two bidirectional promoters and one monodirectional promoter. The viral proteins function to facilitate virus replication, virus movement, the assembly of virus-specific nucleoprotein particles, vector transmission and to counteract plant host defence responses. Recent research findings have provided new insights into the structure and function of these proteins and have identified numerous host interacting partners. Most of the viral proteins have been shown to be multifunctional, participating in multiple events during the infection cycle and have, indeed, evolved coordinated interactions with host proteins to ensure a successful infection. Here, an up-to-date review of viral protein structure and function is presented, and some areas requiring further research are identified.

Viral manipulation of cellular protein conjugation pathways: The SUMO lesson

Small ubiquitin-like modifier (SUMO)ylation is a key post-translational modification mechanism that controls the function of a plethora of proteins and biological processes. Given its central regulatory role, it is not surprising that it is widely exploited by viruses. A number of viral proteins are known to modify and/or be modified by the SUMOylation system to exert their function, to create a cellular environment more favorable for virus survival and propagation, and to prevent host antiviral responses. Since the SUMO pathway is a multi-step cascade, viral proteins engage with it at many levels, to advance and favor each stage of a typical infection cycle: replication, viral assembly and immune evasion. Here we review the current knowledge on the interplay between the host SUMO system and viral lifecycle.

SCE1, the SUMO-conjugating enzyme in plants that interacts with NIb, the RNA-dependent RNA polymerase of Turnip mosaic virus, is required for viral infection

SUMOylation, which is catalyzed by small ubiquitin-like modifier (SUMO) enzymes, is a transient, reversible posttranslational protein modification that regulates diverse cellular processes. Potyviruses, the largest group of known plant viruses, comprise many agriculturally important viruses, such as Turnip mosaic virus (TuMV). The potyviral genome encodes 11 mature proteins. To investigate if SUMOylation plays a role in potyvirus infection, a yeast two-hybrid screen was performed to examine possible interactions of each of the 11 viral proteins of TuMV with AtSCE1, the only SUMO-conjugating enzyme in Arabidopsis thaliana homologous to the key SUMO-conjugating enzyme E2 in mammalian cells or Ubc9 in yeast. A positive reaction was found between AtSCE1 and NIb, the potyviral RNA-dependent RNA polymerase. Further bimolecular fluorescence complementation (BiFC) and fluorescence resonance energy transfer (FRET) assays revealed that the NIb and AtSCE1 interaction occurred in both the cytoplasm and nuclei of epidermal cells of Nicotiana benthamiana. The interaction motif was mapped to a region encompassing NIb amino acids 171 to 300 which contains a potential negatively charged amino acid-dependent SUMOylation motif (NDSM). An Escherichia coli SUMOylation assay showed that NIb can be SUMOylated and that the lysine residue (K172) in the motif is a potent SUMOylation site. A TuMV infectious clone with an arginine (R) substitution mutation at K172 compromised TuMV infectivity in plants. In comparison with wild-type Arabidopsis plants, sce1 knockdown plants exhibited increased resistance to TuMV as well as a nonrelated RNA virus. To the best of our knowledge, this is the first report showing that the host SUMO modification system plays an essential role in infection by plant RNA viruses.

New insights into the role of the small ubiquitin-like modifier (SUMO) in plants

Small ubiquitin-like modifier (SUMO) is a small (∼12kDa) protein that occurs in all eukaryotes and participates in the reversible posttranslational modification of target cellular proteins. The three-dimensional structure of SUMO and ubiquitin (Ub) are superimposable although there is very little similarity in their primary amino acid sequences. In all organisms, conjugation and deconjugation of Ub and SUMO proceed by the same reactions while using pathway-specific enzymes. SUMO conjugation in plants is a part of the controls governing important biological processes such as growth, development, flowering, environmental (abiotic) stress responses, and response to pathogen infection. Most of the evidence for this comes from genetic analyses. Recent efforts to dissect the function of sumoylation have focused on uncovering targets of SUMO conjugation by using either a yeast two-hybrid screen employing components of the SUMO cycle as bait or by using affinity purification of SUMO-conjugated proteins followed by identification of these proteins by mass spectrometry. This chapter reviews the current knowledge regarding sumoylation in plants, with special focus on the model plant Arabidopsis thaliana.

Arabidopsis thaliana MCM2 plays role(s) in mungbean yellow mosaic India virus (MYMIV) DNA replication

Geminiviruses are plant pathogens with single-stranded (ss) DNA genomes of about 2.7 kb in size. They replicate primarily via rolling-circle replication (RCR) with the help of a few virally encoded factors and various host-cell machineries. The virally encoded replication initiator protein (Rep) is essential for geminivirus replication. In this study, by interaction screening of an Arabidopsis thaliana cDNA library, we have identified a host factor, MCM2, that interacts with the Rep protein of the geminivirus mungbean yellow mosaic India virus (MYMIV). Using yeast two-hybrid, β-galactosidase and co-immunoprecipitation assays, we demonstrated an interaction between MYMIV-Rep and the host factor AtMCM2. We investigated the possible role of AtMCM2 in geminiviral replication using a yeast-based geminivirus DNA replication restoration assay and observed that the AtMCM2 protein complemented the mcm2∆ mutation of S. cerevisiae. Our data suggest the involvement of AtMCM2 in the replication of MYMIV ex vivo. The role of MCM2 in replication was confirmed in planta by a transient replication assay in both wild-type and mutant Arabidopsis plants through agroinoculation. Our data provide evidence for the involvement of AtMCM2 in geminiviral DNA replication, presumably in conjunction with other host factors, and suggest its importance in MYMIV DNA replication.

Regulating DNA replication in plants

Chromosomal DNA replication in plants has requirements and constraints similar to those in other eukaryotes. However, some aspects are plant-specific. Studies of DNA replication control in plants, which have unique developmental strategies, can offer unparalleled opportunities of comparing regulatory processes with yeast and, particularly, metazoa to identify common trends and basic rules. In addition to the comparative molecular and biochemical studies, genomic studies in plants that started with Arabidopsis thaliana in the year 2000 have now expanded to several dozens of species. This, together with the applicability of genomic approaches and the availability of a large collection of mutants, underscores the enormous potential to study DNA replication control in a whole developing organism. Recent advances in this field with particular focus on the DNA replication proteins, the nature of replication origins and their epigenetic landscape, and the control of endoreplication will be reviewed.

A stress inducible SUMO conjugating enzyme gene (SaSce9) from a grass halophyte Spartina alterniflora enhances salinity and drought stress tolerance in Arabidopsis

BACKGROUND

SUMO (Small Ubiquitin related Modifier) conjugation is a post translational regulatory process found in all eukaryotes, mediated by SUMO activating enzyme, SUMO conjugating enzyme, and SUMO ligase for the attachment of SUMO to its target protein. Although the mechanism for regulation of SUMO conjugation pathway genes under abiotic stress has been studied to certain extent, the role of SUMO conjugating enzyme in improving abiotic stress tolerance to plant is largely unexplored. Here, we have characterized a SUMO conjugating enzyme gene 'SaSce9' from a halophytic grass Spartina alterniflora and investigated its role in imparting abiotic stress tolerance.

RESULTS

SaSce9 gene encodes for a polypeptide of 162 amino acids with a molecular weight of ~18 kD and isoelectric point 8.43. Amino acid sequence comparisons of SaSce9 with its orthologs from other plant species showed high degree (~85-93%) of structural conservation among each other. Complementation analysis using yeast SCE mutant, Ubc9, revealed functional conservation of SaSce9 between yeast and S. alterniflora. SaSce9 transcript was inducible by salinity, drought, cold, and exogenously supplied ABA both in leaves and roots of S. alterniflora. Constitutive overexpression of SaSce9 in Arabidopsis through Agrobacterium mediated transformation improved salinity and drought tolerance of Arabidopsis. SaSce9 overexpressing Arabidopsis plants retained more chlorophyll and proline both under salinity and drought stress. SaSce9 transgenic plants accumulated lower levels of reactive oxygen under salinity stress. Expression analysis of stress responsive genes in SaSce9 Arabidopsis plants revealed the increased expression of antioxidant genes, AtSOD and AtCAT, ion antiporter genes, AtNHX1 and AtSOS1, a gene involved in proline biosynthesis, AtP5CS, and a gene involved in ABA dependent signaling pathway, AtRD22.

CONCLUSIONS

These results highlight the prospect of improving abiotic stress tolerance in plants through genetic engineering of the sumoylation pathway. The study provides evidence that the overexpression of SaSce9 in plant can improve salinity and drought stress tolerance by protecting the plant through scavenging of ROS, accumulation of an osmolyte, proline, and expression of stress responsive genes. In addition, this study demonstrates the potential of the halophyte grass S. alterniflora as a reservoir of abiotic stress related genes for crop improvement.

A novel role for RAD54: this host protein modulates geminiviral DNA replication

Geminiviruses primarily encode only few factors, such as replication initiator protein (Rep), and need various host cellular machineries for rolling-circle replication (RCR) and/or recombination-dependent replication (RDR). We have identified a host factor, RAD54, in a screen for Rep-interacting partners and observed its role in DNA replication of the geminivirus mungbean yellow mosaic India virus (MYMIV). We identified the interacting domains ScRAD54 and MYMIV-Rep and observed that ScRAD54 enhanced MYMIV-Rep nicking, ATPase, and helicase activities. An in vitro replication assay demonstrated that the geminiviral DNA replication reaction depends on the viral Rep protein, viral origin of replication sequences, and host cell-cycle proteins. Rad54-deficient yeast nuclear extract did not support in vitro viral DNA replication, while exogenous addition of the purified ScRAD54 protein enhanced replication. The role of RAD54 in in planta replication was confirmed by the transient replication assay; i.e., agroinoculation studies. RAD54 is a well-known recombination/repair protein that uses its DNA-dependent ATPase activity in conjunction with several other host factors. However, this study demonstrates for the first time that the eukaryotic rolling-circle replicon depends on the RAD54 protein.

Tomato SlSnRK1 protein interacts with and phosphorylates βC1, a pathogenesis protein encoded by a geminivirus β-satellite

The βC1 protein of tomato yellow leaf curl China β-satellite functions as a pathogenicity determinant. To better understand the molecular basis of βC1 in pathogenicity, a yeast two-hybrid screen of a tomato (Solanum lycopersicum) cDNA library was carried out using βC1 as bait. βC1 interacted with a tomato SUCROSE-NONFERMENTING1-related kinase designated as SlSnRK1. Their interaction was confirmed using a bimolecular fluorescence complementation assay in Nicotiana benthamiana cells. Plants overexpressing SnRK1 were delayed for symptom appearance and contained lower levels of viral and satellite DNA, while plants silenced for SnRK1 expression developed symptoms earlier and accumulated higher levels of viral DNA. In vitro kinase assays showed that βC1 is phosphorylated by SlSnRK1 mainly on serine at position 33 and threonine at position 78. Plants infected with βC1 mutants containing phosphorylation-mimic aspartate residues in place of serine-33 and/or threonine-78 displayed delayed and attenuated symptoms and accumulated lower levels of viral DNA, while plants infected with phosphorylation-negative alanine mutants contained higher levels of viral DNA. These results suggested that the SlSnRK1 protein attenuates geminivirus infection by interacting with and phosphorylating the βC1 protein.

Self-interaction of Abutilon mosaic virus replication initiator protein (Rep) in plant cell nuclei

Geminiviruses replicate their circular single-stranded DNA genome in nuclei of infected plant cells. Their replication initiator proteins (Reps) possess interaction domains for homo- and hetero-oligomerization as shown previously by in vitro studies and yeast two hybrid assays. Here, homo-oligomerization and cellular localization of the Abutilon mosaic virus (AbMV) Rep was analysed with bimolecular fluorescence complementation (BiFC) in epidermal tissues of Nicotiana benthamiana. BiFC revealed that Rep oligomers accumulated within the nucleoplasm, but were excluded from nucleoli as indicated by a nucleoli/cajal body marker. A similar subcellular distribution was observed for Rep fused to full-length cyan fluorescent protein. To examine whether tagged Reps were functionally active, N. benthamiana plants transgenic for a dimeric AbMV DNA B were inoculated with the BiFC expression constructs and nucleic acids were analysed by rolling circle amplification/restriction fragment length polymorphism as well as Southern blot hybridization. The results confirmed that the modified AbMV Rep was able to transreplicate DNA B.

Interaction between geminivirus replication protein and the SUMO-conjugating enzyme is required for viral infection

Geminiviruses are small DNA viruses that replicate in nuclei of infected plant cells by using plant DNA polymerases. These viruses encode a protein designated AL1, Rep, or AC1 that is essential for viral replication. AL1 is an oligomeric protein that binds to double-stranded DNA, catalyzes the cleavage and ligation of single-stranded DNA, and induces the accumulation of host replication machinery. It also interacts with several host proteins, including the cell cycle regulator retinoblastoma-related protein (RBR), the DNA replication protein PCNA (proliferating cellular nuclear antigen), and the sumoylation enzyme that conjugates SUMO to target proteins (SUMO-conjugating enzyme [SCE1]). The SCE1-binding motif was mapped by deletion to a region encompassing AL1 amino acids 85 to 114. Alanine mutagenesis of lysine residues in the binding region either reduced or eliminated the interaction with SCE1, but no defects were observed for other AL1 functions, such as oligomerization, DNA binding, DNA cleavage, and interaction with AL3 or RBR. The lysine mutations reduced or abolished virus infectivity in plants and viral DNA accumulation in transient-replication assays, suggesting that the AL1-SCE1 interaction is required for viral DNA replication. Ectopic AL1 expression did not result in broad changes in the sumoylation pattern of plant cells, but specific changes were detected, indicating that AL1 modifies the sumoylation state of selected host proteins. These results established the importance of AL1-SCE1 interactions during geminivirus infection of plants and suggested that AL1 alters the sumoylation of selected host factors to create an environment suitable for viral infection.

SUMO and SUMOylation in plants

The traditional focus on the central dogma of molecular biology, from gene through RNA to protein, has now been replaced by the recognition of an additional mechanism. The new regulatory mechanism, post-translational modifications to proteins, can actively alter protein function or activity introducing additional levels of functional complexity by altering cellular and sub-cellular location, protein interactions and the outcome of biochemical reaction chains. Modifications by ubiquitin (Ub) and ubiquitin-like modifiers systems are conserved in all eukaryotic organisms. One of them, small ubiquitin-like modifier (SUMO) is present in plants. The SUMO mechanism includes several isoforms of proteins that are involved in reactions of sumoylation and de-sumoylation. Sumoylation affects several important processes in plants. Outstanding among those are responses to environmental stresses. These may be abiotic stresses, such as phosphate deficiency, heat, low temperature, and drought, or biotic stressses, as well including defense reactions to pathogen infection. Also, the regulations of flowering time, cell growth and development, and nitrogen assimilation have recently been added to this list. Identification of SUMO targets is material to characterize the function of sumoylation or desumoylation. Affinity purification and mass spectrometric identification have been done lately in plants. Further SUMO noncovalent binding appears to have function in other model organisms and SUMO interacting proteins in plants will be of interest to plant biologists who dissect the dynamic function of SUMO. This review will discuss results of recent insights into the role of sumoylation in plants.

How do plant viruses induce disease? Interactions and interference with host components

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection-defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.