Geminivirus C4 proteins inhibit GA signaling via prevention of NbGAI degradation, to promote viral infection and symptom development in N. benthamiana

Interplay between tobacco curly shoot virus vsiRNA24 and triosephosphate isomerase: implications for Nicotiana benthamiana viral defense

Virus-derived small interfering RNAs (vsiRNAs) play an important role in viral infection by regulating the expression of host genes. At present, research on the regulation of plant primary metabolic pathways by vsiRNAs is very limited. TvsiRNA24 derived from tobacco curly shoot virus (TbCSV) was amplified by reverse transcription polymerase chain reaction, and its target gene NbTPI (triosephosphate isomerase) was verified using reverse transcription quantitative polymerase chain reaction and GFP fluorescence observation. The effect of the interaction between TvsiRNA24 and NbTPI on TbCSV infection was analyzed by virus mediated, genetic transformation, western blotting, and quantitative detection. The expression of TvsiRNA24 retards the growth of Nicotiana benthamiana and enhances TbCSV accumulation within N. benthamiana. The overexpression of NbTPI attenuates the accumulation of TbCSV, and the silencing of NbTPI leads to the growth retardation of N. benthamiana and intensifies symptoms post-TbCSV infection. Moreover, the expression of some genes related to photosynthesis, primary metabolism and immune response is regulated by NbTPI. Our results unveil the specific role of TvsiRNA24-NbTPI in the pathogenicity of TbCSV, resulting in hindrance to plant growth and facilitation of viral infection. The identification of this regulatory pathway provides valuable insights that can be utilized to devise novel antiviral approaches targeting the reduction of viral pathogenicity.

SlUPA-like, a bHLH Transcription Factor in Tomato (Solanum lycopersicum), Serves as the Crosstalk of GA, JA and BR

The bHLH (basic Helix-Loop-Helix) transcription factor serves as pivotal controller in plant growth and development. In a previous study, the overexpression of SlUPA-like in Solanum lycopersicum L. Ailsa Craig (AC++) altered the JA (Jasmonic acid) response and endogenous GA (Gibberellic acid) content. However, the detailed regulation mechanism was not fully explored. In the present research, we found that the overexpression of SlUPA-like influenced the accumulation of GA, JA and BR (Brassinolide). RNA-Seq data illustrated that the expression levels of genes related to these plant hormones were significantly affected. Additionally, the interaction of SlUPA-like with SlMYB21, SlMYC2 and SlDELLA was characterized by employing Y2H (Yeast Two-Hybrid) and BiFC (Bimolecular Fluorescence Complementation) assay. Furthermore, Dual-LUC (Dual-Luciferase) assay and EMSA (Electrophoretic Mobility Shift Assay) identified that SlUPA-like directly targeted the E-box motif in the promoter of SlGID2 and activated the transcription of SlGID2. These results shed light on the potential role of SlUPA-like in mediating crosstalk among multiple plant hormones and established a robust theoretical framework for further unraveling the functions of SlUPA-like transcription factors in the context of plant growth and hormone signal transduction.

Functional Characterization of the Gibberellin (GA) Receptor ScGID1 in Sugarcane

Sugarcane smut caused by Sporisorium scitamineum represents the most destructive disease in the sugarcane industry, causing host hormone disruption and producing a black whip-like sorus in the apex of the stalk. In this study, the gibberellin metabolic pathway was found to respond to S. scitamineum infection, and the contents of bioactive gibberellins were significantly reduced in the leaves of diseased plants. The gibberellin receptor gene ScGID1 was identified and significantly downregulated. ScGID1 localized in both the nucleus and cytoplasm and had the highest expression level in the leaves. Eight proteins that interact with ScGID1 were screened out using a yeast two-hybrid assay. Novel DELLA proteins named ScGAI1a and ScGA20ox2, key enzymes in GA biosynthesis, were both found to interact with ScGID1 in a gibberellin-independent manner. Transcription factor trapping with a yeast one-hybrid system identified 50 proteins that interacted with the promoter of ScGID1, among which ScS1FA and ScPLATZ inhibited ScGID1 transcription, while ScGDSL promoted transcription. Overexpression of ScGID1 in transgenic Nicotiana benthamiana plants could increase plant height and promote flowering. These results not only contribute to improving our understanding of the metabolic regulatory network of sugarcane gibberellin but also expand our knowledge of the interaction between sugarcane and pathogens.

Rice stripe mosaic virus hijacks rice heading-related gene to promote the overwintering of its insect vector

Rice stripe mosaic virus (RSMV) is an emerging pathogen which significantly reduces rice yields in the southern region of China. It is transmitted by the leafhopper Recilia dorsalis, which overwinters in rice fields. Our field investigations revealed that RSMV infection causes delayed rice heading, resulting in a large number of green diseased plants remaining in winter rice fields. This creates a favorable environment for leafhoppers and viruses to overwinter, potentially contributing to the rapid spread and epidemic of the disease. Next, we explored the mechanism by which RSMV manipulates the developmental processes of the rice plant. A rice heading-related E3 ubiquitin ligase, Heading date Associated Factor 1 (HAF1), was found to be hijacked by the RSMV-encoded P6. The impairment of HAF1 function affects the ubiquitination and degradation of downstream proteins, HEADING DATE 1 and EARLY FLOWERING3, leading to a delay in rice heading. Our results provide new insights into the development regulation-based molecular interactions between virus and plant, and highlights the importance of understanding virus-vector-plant tripartite interactions for effective disease management strategies.

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.

Gibberellin Positively Regulates Tomato Resistance to Tomato Yellow Leaf Curl Virus (TYLCV)

Tomato yellow leaf curl virus (TYLCV) is a prominent viral pathogen that adversely affects tomato plants. Effective strategies for mitigating the impact of TYLCV include isolating tomato plants from the whitefly, which is the vector of the virus, and utilizing transgenic lines that are resistant to the virus. In our preliminary investigations, we observed that the use of growth retardants increased the rate of TYLCV infection and intensified the damage to the tomato plants, suggesting a potential involvement of gibberellic acid (GA) in the conferring of resistance to TYLCV. In this study, we employed an infectious clone of TYLCV to inoculate tomato plants, which resulted in leaf curling and growth inhibition. Remarkably, this inoculation also led to the accumulation of GA3 and several other phytohormones. Subsequent treatment with GA3 effectively alleviated the TYLCV-induced leaf curling and growth inhibition, reduced TYLCV abundance in the leaves, enhanced the activity of antioxidant enzymes, and lowered the reactive oxygen species (ROS) levels in the leaves. Conversely, the treatment with PP333 exacerbated TYLCV-induced leaf curling and growth suppression, increased TYLCV abundance, decreased antioxidant enzyme activity, and elevated ROS levels in the leaves. The analysis of the gene expression profiles revealed that GA3 up-regulated the genes associated with disease resistance, such as WRKYs, NACs, MYBs, Cyt P450s, and ERFs, while it down-regulated the DELLA protein, a key agent in GA signaling. In contrast, PP333 induced gene expression changes that were the opposite of those caused by the GA3 treatment. These findings suggest that GA plays an essential role in the tomato's defense response against TYLCV and acts as a positive regulator of ROS scavenging and the expression of resistance-related genes.

Comprehensive phytohormone metabolomic and transcriptomic analysis of tobacco (Nicotiana tabacum) infected by tomato spotted wilt virus (TSWV)

Tomato spotted wilt virus (TSWV) is ranked among the top 10 most destructive viruses globally. It results in abnormal leaf growth, stunting, and even death, significantly affecting crop yield and quality. Phytohormones play a crucial role in regulating plant-virus interactions. However, there is still limited research on the effect of TSWV on phytohormone levels, particularly growth hormones and genes involved in the phytohormone pathway. In our study, we combined phytohormone metabolomics and transcriptomics to examine the impact of TSWV infection on phytohormone content and gene expression profile. Metabolomic results showed that 41 metabolites, including major phytohormones and their precursors and derivatives were significantly altered after 14 days of TSWV inoculation tobacco plants cvK326, with 31 being significantly increased and 10 significantly reduced. Specifically, the levels of abscisic acid (ABA) and jasmonoyl-isoleucine (JA-Ile) were significantly reduced. The levels of indole-3-acetic acid (IAA) have remained unchanged. However, the levels of cytokinin isopentenyladenine (iP) and salicylic acid (SA) significantly increased. The transcriptome analysis revealed 2,746 genes with significant changes in expression. Out of these, 1,072 genes were significantly downregulated, while 1,674 genes were significantly upregulated. Among them, genes involved in ABA synthesis and signaling pathways, such as 9-cis-epoxycarotenoid dioxygenase (NCED), protein phosphatase 2C (PP2C), serine/threonine-protein kinase (SnRK2), and abscisic acid responsive element binding factor (ABF), exhibited significant downregulation. Additionally, expression of the lipoxygenase gene LOX, Jasmonate ZIM domain-containing protein gene JAZ, and transcription factor gene MYC were significantly down-regulated. In the cytokinin pathway, while there were no significant changes in the expression of the cytokinin synthesis genes, a significant downregulation of transcriptionally active factor type-B response regulators (type-B RRs) was observed. In terms of SA synthesis and signaling pathways, the isochorismate synthase gene ICS1 and the pathogenesis-related gene PR1 were significantly upregulated. These results can strengthen the theoretical foundation for understanding the interaction between TSWV and tobacco and provide new insights for the future prevention and control of TSWV.

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.

Size Restriction Is Required for Proper Functioning of a Bipartite Begomovirus AC4 Protein

Many geminiviruses, including members of the genus Begomovirus, produce a protein known as C4 or AC4. Whereas C4/AC4 typically consists of more than 80 amino acid residues, a few are much shorter. The significance of these shorter C4/AC4 proteins in viral infection and why the virus maintains their abbreviated length is not yet understood. The AC4 of the begomovirus Tomato leaf curl Hsinchu virus contains only 65 amino acids, but it extends to 96 amino acids when the natural termination codon is replaced with a normal codon. We discovered that both interrupting and extending AC4 were harmful to tomato leaf curl Hsinchu virus (ToLCHsV). The extended AC4 (EAC4) also showed a reduced ability to promote the infection of the heterologous virus Potato virus X than the wild-type AC4. When the wild-type AC4 was fused with yellow fluorescent protein (AC4-YFP), it was predominantly found in chloroplasts, whereas EAC4-YFP was mainly localized to the cell periphery. These results suggest that ToLCHsV's AC4 protein is important for viral infection, and the virus may benefit from the abbreviated length, because it may lead to chloroplast localization. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A shift of vector specificity acquired by a begomovirus through natural homologous recombination

Recombination is common in plant viruses such as geminiviruses, but the ecological and pathogenic consequences have been explored only in a few cases. Here, we found that a new begomovirus, tomato yellow leaf curl Shuangbai virus (TYLCSbV), probably originated from the recombination of Ageratum yellow vein China virus (AYVCNV) and tobacco curl shoot virus (TbCSV). Agrobacterium-mediated inoculation showed that TYLCSbV and AYVCNV have similar levels of infectivity on tomato and tobacco plants. However, the two viruses exhibit contrasting specificities for vector transmission, that is, TYLCSbV was efficiently transmitted by the whitefly Bemisia tabaci Mediterranean (MED) rather than by the whitefly B. tabaci Middle East-Asia Minor 1 (MEAM1), whereas AYVCNV was more efficiently transmitted by MEAM1. We also showed that the transmission efficiencies of TYLCSbV and AYVCNV are positively correlated with the accumulation of the viruses in whitefly whole bodies and organs/tissues. The key coat protein amino acids that determine their accumulation are between positions 147 and 256. Moreover, field surveys suggest that MED has displaced MEAM1 in some regions where TYLCSbV was collected. Viral competition assays indicated that TYLCSbV outcompeted AYVCNV when transmitted by MED, while the outcome was the opposite when transmitted by MEAM1. Our findings suggest that recombination has resulted in a shift of vector specificity that could provide TYLCSbV with a potential selective transmission advantage, and the population shift of whitefly cryptic species could have influenced virus evolution towards an extended trajectory of transmission.

Unraveling the Mechanisms of Virus-Induced Symptom Development in Plants

Plant viruses, as obligate intracellular parasites, induce significant changes in the cellular physiology of host cells to facilitate their multiplication. These alterations often lead to the development of symptoms that interfere with normal growth and development, causing USD 60 billion worth of losses per year, worldwide, in both agricultural and horticultural crops. However, existing literature often lacks a clear and concise presentation of the key information regarding the mechanisms underlying plant virus-induced symptoms. To address this, we conducted a comprehensive review to highlight the crucial interactions between plant viruses and host factors, discussing key genes that increase viral virulence and their roles in influencing cellular processes such as dysfunction of chloroplast proteins, hormone manipulation, reactive oxidative species accumulation, and cell cycle control, which are critical for symptom development. Moreover, we explore the alterations in host metabolism and gene expression that are associated with virus-induced symptoms. In addition, the influence of environmental factors on virus-induced symptom development is discussed. By integrating these various aspects, this review provides valuable insights into the complex mechanisms underlying virus-induced symptoms in plants, and emphasizes the urgency of addressing viral diseases to ensure sustainable agriculture and food production.

Screening and Identification of Host Factors Interacting with the Virulence Factor P0 Encoded by Sugarcane Yellow Leaf Virus by Yeast Two-Hybrid Assay

Sugarcane yellow leaf virus (SCYLV), a member of the genus Polerovirus in the family Luteoviridae, causes severe damage and represents a great threat to sugarcane cultivation and sugar industry development. In this study, inoculation of Nicotiana benthamiana plants with a potato virus X (PVX)-based vector carrying the SCYLV P0 gene induced typical mosaic, leaf rolling symptoms and was associated with a hypersensitive-like response (HLR) necrosis symptom, which is accompanied with a systemic burst of H₂O₂ and also leads to higher PVX viral genome accumulation levels. Our results demonstrate that SCYLV P0 is a pathogenicity determinant and plays important roles in disease development. To further explore its function in pathogenic processes, a yeast two-hybrid assay was performed to screen the putative P0-interacting host factors. The recombinant plasmid pGBKT7-P0 was constructed as a bait and transformed into the yeast strain Y2HGold. The ROC22 cultivar (an important parental resource of the main cultivar in China) cDNA prey library was constructed and screened by co-transformation with the P0 bait. We identified 28 potential interacting partners including those involved in the optical signal path, plant growth and development, transcriptional regulation, host defense response, and viral replication. To our knowledge, this is the first time we have reported the host proteins interacting with the P0 virulence factor encoded by sugarcane yellow leaf virus. This study not only provides valuable insights into elucidating the molecular mechanism of the pathogenicity of SCYLV, but also sheds light on revealing the probable new pathogenesis of Polerovirus in the future.

A prophage-encoded effector from "Candidatus Liberibacter asiaticus" targets ASCORBATE PEROXIDASE6 in citrus to facilitate bacterial infection

Citrus huanglongbing (HLB), associated with the unculturable phloem-limited bacterium "Candidatus Liberibacter asiaticus" (CLas), is the most devastating disease in the citrus industry worldwide. However, the pathogenicity of CLas remains poorly understood. In this study, we show that AGH17488, a secreted protein encoded by the prophage region of the CLas genome, suppresses plant immunity via targeting the host ASCORBATE PEROXIDASE6 (APX6) protein in Nicotiana benthamiana and Citrus sinensis. The transient expression of AGH17488 reduced the chloroplast localization of APX6 and its enzyme activity, inhibited the accumulation of reactive oxygen species (H₂ O₂ and O₂ ^- ) and the lipid oxidation endproduct malondialdehyde in plants, and promoted the proliferation of Pseudomonas syringae pv. tomato DC3000 and Xanthomonas citri subsp. citri. This study reveals a novel mechanism underlying how CLas uses a prophage-encoded effector, AGH17488, to target a reactive oxygen species accumulation-related gene, APX6, in the host to facilitate its infection.

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.

Alpha-momorcharin preserves catalase activity to inhibit viral infection by disrupting the 2b-CAT interaction in Solanum lycopersicum

Many host factors of plants are used by viruses to facilitate viral infection. However, little is known about how alpha-momorcharin (αMMC) counters virus-mediated attack strategies in tomato. Our present research revealed that the 2b protein of cucumber mosaic virus (CMV) directly interacted with catalases (CATs) and inhibited their activities. Further analysis revealed that transcription levels of catalase were induced by CMV infection and that virus accumulation increased in CAT-silenced or 2b-overexpressing tomato plants compared with that in control plants, suggesting that the interaction between 2b and catalase facilitated the accumulation of CMV in hosts. However, both CMV accumulation and viral symptoms were reduced in αMMC transgenic tomato plants, indicating that αMMC engaged in an antiviral role in the plant response to CMV infection. Molecular experimental analysis demonstrated that αMMC interfered with the interactions between catalases and 2b in a competitive manner, with the expression of αMMC inhibited by CMV infection. We further demonstrated that the inhibition of catalase activity by 2b was weakened by αMMC. Accordingly, αMMC transgenic plants exhibited a greater ability to maintain redox homeostasis than wild-type plants when infected with CMV. Altogether, these results reveal that αMMC retains catalase activity to inhibit CMV infection by subverting the interaction between 2b and catalase in tomato.

Sugarcane mosaic virus orchestrates the lactate fermentation pathway to support its successful infection

Viruses often establish their own infection by altering host metabolism. How viruses co-opt plant metabolism to support their successful infection remains an open question. Here, we used untargeted metabolomics to reveal that lactate accumulates immediately before and after robust sugarcane mosaic virus (SCMV) infection. Induction of lactate-involved anaerobic glycolysis is beneficial to SCMV infection. The enzyme activity and transcriptional levels of lactate dehydrogenase (LDH) were up-regulated by SCMV infection, and LDH is essential for robust SCMV infection. Moreover, LDH relocates in viral replicase complexes (VRCs) by interacting with SCMV-encoded 6K2 protein, a key protein responsible for inducing VRCs. Additionally, lactate could promote SCMV infection by suppressing plant defense responses. Taken together, we have revealed a viral strategy to manipulate host metabolism to support replication compartment but also depress the defense response during the process of infection.

A R2R3 MYB Transcription Factor, TaMYB391, Is Positively Involved in Wheat Resistance to Puccinia striiformis f. sp. tritici

A biotrophic fungus, Puccinia striiformis f.sp. tritici (Pst), which causes stripe rust disease in wheat is the most yield-limiting factor in wheat production. Plants have complex defense mechanisms against invading pathogens. Hypersensitive response (HR), a kind of programmed cell death (PCD) at the infection site, is among these defense mechanisms. Transcription factors (TFs) play a crucial role in plant defense response against invading pathogens. Myeloblastosis (MYB) TFs are among the largest TFs families that are involved in response to both biotic and abiotic stresses. However, little is known about the mechanisms of MYB TFs during the interaction between wheat and the stripe rust fungus. Here, we identified an R2R3 MYB TF from wheat, designated as TaMYB391, and characterized its functional role during wheat-Pst interaction. Our data indicated that TaMYB391 is induced by Pst infection and exogenous application of salicylic acid (SA) and abscisic acid (ABA). TaMYB391 is localized in the nucleus of both wheat and Nicotiana benthamiana. Transient overexpression of TaMYB391 in N. benthamiana triggered HR-related PCD accompanied by increased electrolyte leakage, high accumulation of reactive oxygen species (ROS), and transcriptional accumulation of SA defense-related genes and HR-specific marker genes. Overexpression of TaMYB391 in wheat significantly enhanced wheat resistance to stripe rust fungus through the induction of pathogenesis-related (PR) genes, ROS accumulation and hypersensitive cell death. On the other hand, RNAi-mediated silencing of TaMYB391 decreased the resistance of wheat to Pst accompanied by enhanced growth of the pathogen. Together our findings demonstrate that TaMYB391 acts as a positive regulator of HR-associated cell death and positively contributes to the resistance of wheat to the stripe rust fungus by regulating certain PR genes, possibly through SA signaling pathways.

Independently evolved viral effectors convergently suppress DELLA protein SLR1-mediated broad-spectrum antiviral immunity in rice

Plant viruses adopt diverse virulence strategies to inhibit host antiviral defense. However, general antiviral defense directly targeted by different types of plant viruses have rarely been studied. Here, we show that the single rice DELLA protein, SLENDER RICE 1 (SLR1), a master negative regulator in Gibberellin (GA) signaling pathway, is targeted by several different viral effectors for facilitating viral infection. Viral proteins encoded by different types of rice viruses all directly trigger the rapid degradation of SLR1 by promoting association with the GA receptor OsGID1. SLR1-mediated broad-spectrum resistance was subverted by these independently evolved viral proteins, which all interrupted the functional crosstalk between SLR1 and jasmonic acid (JA) signaling. This decline of JA antiviral further created the advantage of viral infection. Our study reveals a common viral counter-defense strategy in which different types of viruses convergently target SLR1-mediated broad-spectrum resistance to benefit viral infection in the monocotyledonous crop rice.