Geminiviruses employ host DNA glycosylases to subvert DNA methylation-mediated defense

Control of DNA demethylation by superoxide anion in plant stem cells

Superoxide anion is thought to be a natural by-product with strong oxidizing ability in all living organisms and was recently found to accumulate in plant meristems to maintain stem cells in the shoot and undifferentiated meristematic cells in the root. Here we show that the DNA demethylase repressor of silencing 1 (ROS1) is one of the direct targets of superoxide in stem cells. The Fe-S clusters in ROS1 are oxidized by superoxide to activate its DNA glycosylase/lyase activity. We demonstrate that superoxide extensively participates in the establishment of active DNA demethylation in the Arabidopsis genome and that ARABIDOPSIS RESPONSE REGULATOR 12 acts downstream of ROS1-mediated superoxide signaling to maintain stem cell fate. Our results provide a mechanistic framework for superoxide control of the stem cell niche and demonstrate how redox and DNA demethylation interact to define stem cell fate in plants.

Genome-Wide Analysis of DNA Demethylases in Land Plants and Their Expression Pattern in Rice

DNA demethylation is a very important biochemical pathway regulating a group of biological processes, such as embryo development, fruit ripening, and response to stress. Despite the essential role of DNA demethylases, their evolutionary relationship and detailed biological functions in different land plants remain unclear. In this study, 48 DNA demethylases in 12 land plants were identified and classified. A phylogenetic tree was constructed to demonstrate the evolutionary relationships among these DNA demethylases, indicating how they are related across different species. Conserved domain, protein motif, and gene structure analysis showed that these 48 DNA demethylases fell into the presently identified four classes of DNA demethylases. Amino acid alignment revealed conserved catalytic sites and a previously less-studied protein region (referred to as domain A) within the DNA demethylases. An analysis showed a conserved pattern of gene duplication for DNA demethylases throughout their evolutionary history, suggesting that these genes had been maintained due to their importance. The examination of promoter cis-elements displayed potential signaling and regulating pathways of DNA demethylases. Furthermore, the expression profile was analyzed to investigate the physiological role of rice DNA demethylase in different developmental stages, in tissues, and in response to stress and various phytohormone signals. The findings offer a deeper insight into the functional regions of DNA demethylases and their evolutionary relationships, which can guide future research directions. Understanding the role of DNA demethylases can lead to improved plant stress resistance and contribute to the development of better crop and fruit varieties.

Functional role of geminivirus encoded proteins in the host: Past and present

During plant-pathogen interaction, plant exhibits a strong defense system utilizing diverse groups of proteins to suppress the infection and subsequent establishment of the pathogen. However, in response, pathogens trigger an anti-silencing mechanism to overcome the host defense machinery. Among plant viruses, geminiviruses are the second largest virus family with a worldwide distribution and continue to be production constraints to food, feed, and fiber crops. These viruses are spread by a diverse group of insects, predominantly by whiteflies, and are characterized by a single-stranded DNA (ssDNA) genome coding for four to eight proteins that facilitate viral infection. The most effective means to managing these viruses is through an integrated disease management strategy that includes virus-resistant cultivars, vector management, and cultural practices. Dynamic changes in this virus family enable the species to manipulate their genome organization to respond to external changes in the environment. Therefore, the evolutionary nature of geminiviruses leads to new and novel approaches for developing virus-resistant cultivars and it is essential to study molecular ecology and evolution of geminiviruses. This review summarizes the multifunctionality of each geminivirus-encoded protein. These protein-based interactions trigger the abrupt changes in the host methyl cycle and signaling pathways that turn over protein normal production and impair the plant antiviral defense system. Studying these geminivirus interactions localized at cytoplasm-nucleus could reveal a more clear picture of host-pathogen relation. Data collected from this antagonistic relationship among geminivirus, vector, and its host, will provide extensive knowledge on their virulence mode and diversity with climate change.

Molecular characterization and pathogenicity of a novel monopartite geminivirus infecting tobacco in China

The occurrence of geminiviruses causes significant economic losses in many economically important crops. In this study, a novel geminivirus isolated from tobacco in Sichuan province of China, named tomato leaf curl Chuxiong virus (TLCCxV), was characterized by small RNA-based deep sequencing. The full-length of TLCCxV genome was determined to be 2744 nucleotides (nt) encoding six open reading frames. Phylogenetic and genome-wide pairwise identity analysis revealed that TLCCxV shared less than 91% identities with reported geminiviruses. A TLCCxV infectious clone was constructed and successfully infected Nicotiana benthamiana, N. tabacum, N. glutinosa, Solanum lycopersicum and Petunia hybrida plants. Furthermore, expression of the V2, C1 and C4 proteins through a potato virus X vector caused severe chlorosis or necrosis symptom in N. benthamiana. Taken together, we identified a new geminivirus in tobacco plants, and found that V2, C1 and C4 contribute to symptom development.

A viral movement protein targets host catalases for 26S proteasome-mediated degradation to facilitate viral infection and aphid transmission in wheat

The infection of host plants by many different viruses causes reactive oxygen species (ROS) accumulation and yellowing symptoms, but the mechanisms through which plant viruses counteract ROS-mediated immunity to facilitate infection and symptom development have not been fully elucidated. Most plant viruses are transmitted by insect vectors in the field, but the molecular mechanisms underlying virus‒host-insect interactions are unclear. In this study, we investigated the interactions among wheat, barley yellow dwarf virus (BYDV), and its aphid vector and found that the BYDV movement protein (MP) interacts with both wheat catalases (CATs) and the 26S proteasome ubiquitin receptor non-ATPase regulatory subunit 2 homolog (PSMD2) to facilitate the 26S proteasome-mediated degradation of CATs, promoting viral infection, disease symptom development, and aphid transmission. Overexpression of the BYDV MP gene in wheat enhanced the degradation of CATs, which leading to increased accumulation of ROS and thereby enhanced viral infection. Interestingly, transgenic wheat lines overexpressing BYDV MP showed significantly reduced proliferation of wingless aphids and an increased number of winged aphids. Consistent with this observation, silencing of CAT genes also enhanced viral accumulation and reduced the proliferation of wingless aphids but increased the occurrence of winged aphids. In contrast, transgenic wheat plants overexpressing TaCAT1 exhibited the opposite changes and showed increases in grain size and weight upon infection with BYDV. Biochemical assays demonstrated that BYDV MP interacts with PSMD2 and promotes 26S proteasome-mediated degradation of TaCAT1 likely in a ubiquitination-independent manner. Collectively, our study reveals a molecular mechanism by which a plant virus manipulates the ROS production system of host plants to facilitate viral infection and transmission, shedding new light on the sophisticated interactions among viruses, host plants, and insect vectors.

Plant virology in the 21st century in China: Recent advances and future directions

Plant viruses are a group of intracellular pathogens that persistently threaten global food security. Significant advances in plant virology have been achieved by Chinese scientists over the last 20 years, including basic research and technologies for preventing and controlling plant viral diseases. Here, we review these milestones and advances, including the identification of new crop-infecting viruses, dissection of pathogenic mechanisms of multiple viruses, examination of multilayered interactions among viruses, their host plants, and virus-transmitting arthropod vectors, and in-depth interrogation of plant-encoded resistance and susceptibility determinants. Notably, various plant virus-based vectors have also been successfully developed for gene function studies and target gene expression in plants. We also recommend future plant virology studies in China.

Phytophthora sojae Effector PsCRN108 Targets CAMTA2 to Suppress HSP40 Expression and ROS Burst

Oomycete pathogens secrete numerous crinkling and necrosis proteins (CRNs) to manipulate plant immunity and promote infection. However, the functional mechanism of CRN effectors is still poorly understood. Previous research has shown that the Phytophthora sojae effector PsCRN108 binds to the promoter of HSP90s and inhibits their expression, resulting in impaired plant immunity. In this study, we found that in addition to HSP90, PsCRN108 also suppressed other Heat Shock Protein (HSP) family genes, including HSP40. Interestingly, PsCRN108 inhibited the expression of NbHSP40 through its promoter, but did not directly bind to its promoter. Instead, PsCRN108 interacted with NbCAMTA2, a negative regulator of plant immunity. NbCAMTA2 was a negative regulator of NbHSP40 expression, and PsCRN108 could promote such inhibition activity of NbCAMTA2. Our results elucidated the multiple roles of PsCRN108 in the suppression of plant immunity and revealed a new mechanism by which the CRN effector hijacked transcription factors to affect immunity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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

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

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

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

Geminiviral betasatellites: critical viral ammunition to conquer plant immunity

Geminiviruses have mastered plant cell modulation and immune invasion to ensue prolific infection. Encoding a relatively small number of multifunctional proteins, geminiviruses rely on satellites to efficiently re-wire plant immunity, thereby fostering virulence. Among the known satellites, betasatellites have been the most extensively investigated. They contribute significantly to virulence, enhance virus accumulation, and induce disease symptoms. To date, only two betasatellite proteins, βC1, and βV1, have been shown to play a crucial role in virus infection. In this review, we offer an overview of plant responses to betasatellites and counter-defense strategies deployed by betasatellites to overcome those responses.

Functional identification of a novel C7 protein of tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) is a monopartite geminivirus, and one of the most devastating plant viruses in the world. TYLCV is traditionally known to encode six viral proteins in bidirectional and partially overlapping open reading frames (ORFs). However, recent studies have shown that TYLCV encodes additional small proteins with specific subcellular localizations and potential virulence functions. Here, a novel protein named C7, encoded by a newly-described ORF in the complementary strand, was identified as part of the TYLCV proteome using mass spectrometry. The C7 protein localized to the nucleus and cytoplasm, both in the absence and presence of the virus. C7 was found to interact with two other TYLCV-encoded proteins: with C2 in the nucleus, and with V2 in the cytoplasm, forming conspicuous granules. Mutation of C7 start codon ATG to ACG to block the translation of C7 delayed the onset of viral infection, and the mutant virus caused milder virus symptoms and less accumulations of viral DNAs and proteins. Using the potato virus X (PVX)-based recombinant vector, we found that ectopic overexpression of C7 resulted in more severe mosaic symptoms and promoted a higher accumulation of PVX-encoded coat protein in the late virus infection stage. In addition, C7 was also found to inhibit GFP-induced RNA silencing moderately. This study demonstrates that the novel C7 protein encoded by TYLCV is a pathogenicity factor and a weak RNA silencing suppressor, and that it plays a critical role during TYLCV infection.

Begomoviruses: what is the secret(s) of their success?

Begomoviruses constitute an extremely successful group of emerging plant viruses transmitted by whiteflies of the Bemisia tabaci complex. Hosts include important vegetable, root, and fiber crops grown in the tropics and subtropics. Factors contributing to the ever-increasing diversity and success of begomoviruses include their predisposition to recombine their genomes, interaction with DNA satellites recruited throughout their evolution, presence of wild plants as a virus reservoir and a source of speciation, and extreme polyphagia and continuous movement of the insect vectors to temperate regions. These features as well as some controversial issues (replication in the insect vector, putative seed transmission, transmission by insects other than B. tabaci, and expansion of the host range to monocotyledonous plants) will be analyzed in this review.

Geminiviral C2 proteins inhibit active autophagy to facilitate virus infection by impairing the interaction of ATG7 and ATG8

Autophagy is a conserved intracellular degradation process that plays an active role in plant response to virus infections. Here we report that geminiviruses counteract activated autophagy-mediated antiviral defense in plant cells through the C2 proteins they encode. We found that, in Nicotiana benthamiana plants, tomato leaf curl Yunnan virus (TLCYnV) infection upregulated the transcription levels of autophagy-related genes (ATGs). Overexpression of NbATG5, NbATG7, or NbATG8a in N. benthamiana plants decreased TLCYnV accumulation and attenuated viral symptoms. Interestingly, transgenic overexpression of NbATG7 promoted the growth of N. benthamiana plants and enhanced plant resistance to TLCYnV. We further revealed that the C2 protein encoded by TLCYnV directly interacted with the ubiquitin-activating domain of ATG7. This interaction competitively disrupted the ATG7-ATG8 binding in N. benthamiana and Solanum lycopersicum plants, thereby inhibiting autophagy activity. Furthermore, we uncovered that the C2-mediated autophagy inhibition mechanism was conserved in three other geminiviruses. In summary, we discovered a novel counter-defensive strategy employed by geminiviruses that enlists their C2 proteins as disrupters of ATG7-ATG8 interactions to defeat antiviral autophagy.

Identification and Molecular Characterization of a Novel Carlavirus Infecting Chrysanthemum morifolium in China

Chrysanthemum (Chrysanthemum morifolium) is an important ornamental and medicinal plant suffering from many viruses and viroids worldwide. In this study, a new carlavirus, tentatively named Chinese isolate of Carya illinoinensis carlavirus 1 (CiCV1-CN), was identified from chrysanthemum plants in Zhejiang Province, China. The genome sequence of CiCV1-CN was 8795 nucleotides (nt) in length, with a 68-nt 5'-untranslated region (UTR) and a 76-nt 3'-UTR, which contained six predicted open reading frames (ORFs) that encode six corresponding proteins of various sizes. Phylogenetic analyses based on full-length genome and coat protein sequences revealed that CiCV1-CN is in an evolutionary branch with chrysanthemum virus R (CVR) in the Carlavirus genus. Pairwise sequence identity analysis showed that, except for CiCV1, CiCV1-CN has the highest whole-genome sequence identity of 71.3% to CVR-X6. At the amino acid level, the highest identities of predicted proteins encoded by the ORF1, ORF2, ORF3, ORF4, ORF5, and ORF6 of CiCV1-CN were 77.1% in the CVR-X21 ORF1, 80.3% in the CVR-X13 ORF2, 74.8% in the CVR-X21 ORF3, 60.9% in the CVR-BJ ORF4, 90.2% in the CVR-X6 and CVR-TX ORF5s, and 79.4% in the CVR-X21 ORF6. Furthermore, we also found a transient expression of the cysteine-rich protein (CRP) encoded by the ORF6 of CiCV1-CN in Nicotiana benthamiana plants using a potato virus X-based vector, which can result in a downward leaf curl and hypersensitive cell death over the time course. These results demonstrated that CiCV1-CN is a pathogenic virus and C. morifolium is a natural host of CiCV1.

Discovery of Brassica Yellows Virus and Porcine Reproductive and Respiratory Syndrome Virus in Diaphorina citri and Changes in Virome Due to Infection with 'Ca. L. asiaticus'

Detection of new viruses or new virus hosts is essential for the protection of economically important agroecosystems and human health. Increasingly, metatranscriptomic data are being used to facilitate this process. Such data were obtained from adult Asian citrus psyllids (ACP) (Diaphorina citri Kuwayama) that fed solely on mandarin (Citrus ×aurantium L.) plants grafted with buds infected with 'Candidatus Liberibacter asiaticus' (CLas), a phloem-limited bacterium associated with the severe Asian variant of huanglongbing (HLB), the most destructive disease of citrus. Brassica yellows virus (BrYV), the causative agent of yellowing or leafroll symptoms in brassicaceous plants, and its associated RNA (named as BrYVaRNA) were detected in ACP. In addition, the porcine reproductive and respiratory syndrome virus (PRRSV), which affects pigs and is economically important to pig production, was also found in ACP. These viruses were not detected in insects feeding on plants grafted with CLas-free buds. Changes in the concentrations of insect-specific viruses within the psyllid were caused by coinfection with CLas. IMPORTANCE The cross transmission of pathogenic viruses between different farming systems or plant communities is a major threat to plants and animals and, potentially, human health. The use of metagenomics is an effective approach to discover viruses and vectors. Here, we collected buds from the CLas-infected and CLas-free mandarin (Citrus ×aurantium L. [Rutaceae: Aurantioideae: Aurantieae]) trees from a commercial orchard and grafted them onto CLas-free mandarin plants under laboratory conditions. Through metatranscriptome sequencing, we first identified the Asian citrus psyllids feeding on plants grafted with CLas-infected buds carried the plant pathogen, brassica yellows virus and its associated RNA, and the swine pathogen, porcine reproductive and respiratory syndrome virus. These discoveries indicate that both viruses can be transmitted by grafting and acquired by ACP from CLas+ mandarin seedlings.

Genome-wide identification and expression analysis of wall-associated kinase (WAK) and WAK-like kinase gene family in response to tomato yellow leaf curl virus infection in Nicotiana benthamiana

BACKGROUND

Tomato yellow leaf curl virus (TYLCV) is a major monopartite virus in the family Geminiviridae and has caused severe yield losses in tomato and tobacco planting areas worldwide. Wall-associated kinases (WAKs) and WAK-like kinases (WAKLs) are a subfamily of the receptor-like kinase family implicated in cell wall signaling and transmitting extracellular signals to the cytoplasm, thereby regulating plant growth and development and resistance to abiotic and biotic stresses. Recently, many studies on WAK/WAKL family genes have been performed in various plants under different stresses; however, identification and functional survey of the WAK/WAKL gene family of Nicotiana benthamiana have not yet been performed, even though its genome has been sequenced for several years. Therefore, in this study, we aimed to identify the WAK/WAKL gene family in N. benthamiana and explore their possible functions in response to TYLCV infection.

RESULTS

Thirty-eight putative WAK/WAKL genes were identified and named according to their locations in N. benthamiana. Phylogenetic analysis showed that NbWAK/WAKLs are clustered into five groups. The protein motifs and gene structure compositions of NbWAK/WAKLs appear to be highly conserved among the phylogenetic groups. Numerous cis-acting elements involved in phytohormone and/or stress responses were detected in the promoter regions of NbWAK/WAKLs. Moreover, gene expression analysis revealed that most of the NbWAK/WAKLs are expressed in at least one of the examined tissues, suggesting their possible roles in regulating the growth and development of plants. Virus-induced gene silencing and quantitative PCR analyses demonstrated that NbWAK/WAKLs are implicated in regulating the response of N. benthamiana to TYLCV, ten of which were dramatically upregulated in locally or systemically infected leaves of N. benthamiana following TYLCV infection.

CONCLUSIONS

Our study lays an essential base for the further exploration of the potential functions of NbWAK/WAKLs in plant growth and development and response to viral infections in N. benthamiana.

Plant Defense and Viral Counter-Defense during Plant-Geminivirus Interactions

Geminiviruses are the largest family of plant viruses that cause severe diseases and devastating yield losses of economically important crops worldwide. In response to geminivirus infection, plants have evolved ingenious defense mechanisms to diminish or eliminate invading viral pathogens. However, increasing evidence shows that geminiviruses can interfere with plant defense response and create a suitable cell environment by hijacking host plant machinery to achieve successful infections. In this review, we discuss recent findings about plant defense and viral counter-defense during plant-geminivirus interactions.

Geminiviral C4/AC4 proteins: An emerging component of the viral arsenal against plant defence

Virus infection triggers a plethora of defence reactions in plants to incapacitate the intruder. Viruses, in turn, have added additional functions to their genes so that they acquire capabilities to neutralize the above defence reactions. In plant-infecting viruses, the family Geminiviridae comprises members, majority of whom encode 6-8 genes in their small single-stranded DNA genomes. Of the above genes, one which shows the most variability in its amino acid sequence is the C4/AC4. Recent studies have uncovered evidence, which point towards a wide repertoire of functions performed by C4/AC4 revealing its role as a major player in suppressing plant defence. This review summarizes the various plant defence mechanisms against viruses and highlights how C4/AC4 has evolved to counter most of them.

Genome-wide analysis of epigenetic and transcriptional changes in the pathogenesis of RGSV in rice

Rice grassy stunt virus (RGSV), a typical negative single-stranded RNA virus, invades rice and generates several disease signs, including dwarfing, tillering, and sterility. Previous research has revealed that RGSV-encoded proteins can force the host's ubiquitin-proteasome system to utilize them for viral pathogenesis. However, most of the studies were limited to a single omics level and lacked multidimensional data collection and correlation analysis on the mechanisms of RGSV-rice interactions. Here, we performed a comprehensive association analysis of genome-wide methylation sequencing, transcriptome sequencing, and histone H3K9me3 modification in RGSV-infested as well as non-infested rice leaves, and the levels of all three cytosine contexts (CG, CHG and CHH) were found to be slightly lower in RGSV-infected rice leaves than in normal rice. Large proportions of DMRs were distributed in the promoter and intergenic regions, and most DMRs were enriched in the CHH context, where the number of CHH hypo-DMRs was almost twice as high as that of hyper-DMRs. Among the genes with down-regulated expression and hypermethylation, we analyzed and identified 11 transcripts involved in fertility, plant height and tillering, and among the transcribed up-regulated and hypermethylated genes, we excavated 7 transcripts related to fertility, plant height and tillering. By analyzing the changes of histone H3K9me3 modification before and after virus infestation, we found that the distribution of H3K9me3 modification in the whole rice genome was prevalent, mainly concentrated in the gene promoter and gene body regions, which was distinctly different from the characteristics of animals. Combined with transcriptomic data, H3K9me3 mark was found to favor targeting highly expressed genes. After RGSV infection, H3K9me3 modifications in several regions of CTK and BR hormone signaling-related genes were altered, providing important targets for subsequent studies.

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

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

Active DNA demethylation in plants: 20 years of discovery and beyond

Maintaining proper DNA methylation levels in the genome requires active demethylation of DNA. However, removing the methyl group from a modified cytosine is chemically difficult and therefore, the underlying mechanism of demethylation had remained unclear for many years. The discovery of the first eukaryotic DNA demethylase, Arabidopsis thaliana REPRESSOR OF SILENCING 1 (ROS1), led to elucidation of the 5-methylcytosine base excision repair mechanism of active DNA demethylation. In the 20 years since ROS1 was discovered, our understanding of this active DNA demethylation pathway, as well as its regulation and biological functions in plants, has greatly expanded. These exciting developments have laid the groundwork for further dissecting the regulatory mechanisms of active DNA demethylation, with potential applications in epigenome editing to facilitate crop breeding and gene therapy.

Geminiviruses boost active DNA demethylation for counter-defense

DNA methylation regulates gene expression under abiotic and biotic stresses. Recently, Gui et al. discovered that geminiviruses subverted DNA methylation-mediated defense through boosting the active DNA demethylation mediated by host DNA glycosylases to promote viral virulence. Their findings reveal a distinctive counter-defense strategy exploited by invading pathogens to achieve successful infection.

P25 and P37 proteins encoded by firespike leafroll-associated virus are viral suppressors of RNA silencing

Firespike leafroll-associated virus (FLRaV) is a major pathogen associated with firespike (Odontonema tubaeforme) leafroll disease. Phylogenetic analysis showed that FLRaV possesses typical traits of subgroup II members of ampeloviruses, but encodes two additional proteins, P25 and P37. Here, we determined the microfilament localization of P25 protein. Posttranscriptional gene silencing (PTGS) assay showed that both FLRaV P25 and P37 were able to suppress the local and systemic PTGS and FLRaV P25 was capable of suppressing the green fluorescent protein (GFP) gene silencing triggered by both sense RNA-induced PTGS (S-PTGS) and inverted repeat RNA-induced PTGS (IR-PTGS). In contrast, FLRaV P37 was only able to inhibit the GFP silencing triggered by the S-PTGS but not the IR-PTGS. In the transcriptional gene silencing (TGS) assay, only FLRaV P25 was found to be able to reverse established TGS-mediated silencing of GFP in 16-TGS plants. We also found that FLRaV P25 could aggravate the disease symptom and viral titer of potato virus X in N. benthamiana. These results suggest that FLRaV P25 and P37 may have crucial roles in overcoming host RNA silencing, which provides key insights into our understanding of the molecular mechanisms underlying FLRaV infection.

Turnip mosaic virus manipulates DRM2 expression to regulate host CHH and CHG methylation for robust infection

DNA methylation is an important epigenetic marker for the suppression of transposable elements (TEs) and the regulation of plant immunity. However, little is known how RNA viruses counter defense such antiviral machinery. In this study, the change of DNA methylation in turnip mosaic virus (TuMV)-infected cells was analyzed by whole genome bisulfite sequencing. Results showed that the total number of methylated sites of CHH and CHG increased in TuMV-infected cells, the majority of differentially methylated regions (DMRs) in the CHH and CHG contexts were associated with hypermethylation. Gene expression analysis showed that the expression of two methylases (DRM2 and CMT3) and three demethylases (ROS3, DML2, DML3) was significantly increased and decreased in TuMV-infected cells, respectively. Pathogenicity tests showed that the enhanced resistance to TuMV of the loss-of-function mutant of DRM2 is associated with unregulated expression of several defense-related genes. Finally, we found TuMV-encoded NIb, the viral RNA-dependent RNA polymerase, was able to induce the expression of DRM2. In conclusion, this study discovered that TuMV can modulate host DNA methylation by regulating the expression of DRM2 to promote virus infection.

Autophagy in plant viral infection

Autophagy is a conserved degradation pathway that delivers dysfunctional cellular organelles or other cytosol components to degradative vesicular structures (vacuoles in plants and yeasts, lysosomes in mammals) for degradation and recycling. Viruses are intracellular parasites that hijack their host to live. Research on regulation of the trade-off between plant cells and viruses has indicated that autophagy is an integral part of the host responses to virus infection. Meanwhile, plants have evolved a diverse array of defense responses to counter pathogenic viruses. In this review, we focus on the roles of autophagy in plant virus infection and offer a glimpse of recent advances about how plant viruses evade autophagy or manipulate host autophagy pathways to complete their replication cycle.

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

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