A class of independently evolved transcriptional repressors in plant RNA viruses facilitates viral infection and vector feeding

Diverse roles of phytohormonal signaling in modulating plant-virus interactions

Virus infection brings about changes in the transcriptome, proteome, and metabolome status of the infected plant, wherein substantial alterations in the abundance of phytohormones and associated components involved in their signaling pathways have been observed. In recent years, extensive research in the field of plant virology has showcased the indisputable significance of phytohormone signaling during plant-virus interactions. Apart from acting as growth regulators, phytohormones elicit a robust immune response, which restricts viral multiplication within the plant and propagation by vectors. Interestingly, these pathways have been shown to act not only as isolated mechanisms but also as complex intertwined regulatory cascades where the crosstalk among different phytohormones and with other antiviral pathways takes place during plant-virus interaction. Viruses disrupt phytohormone homeostasis via their multifunctional effectors, which seems to be a 'smart' approach adopted by viruses to circumvent phytohormone-mediated plant immune responses. In this review, we summarize current understanding of role of phytohormone signaling pathways during plant-virus interactions in activating plant antiviral immune responses and how viruses exploit these signaling pathways to favor their pathogenesis.

A novel peptide encoded by a rice circular RNA confers broad-spectrum disease resistance in rice plants

Circular RNAs (circRNAs) are a significant class of endogenous RNAs that exert crucial biological functions in human and animal systems, but little is currently understood regarding their roles in plants. Here, we identified a circRNA originating from the back-splicing of exon 4 and exon 5 of a rice gene, OsWRKY9, and named it circ-WRKY9. It is upregulated in rice stripe mosaic virus (RSMV)-infected rice plants. Notably, circ-WRKY9 contains two open reading frames with an internal ribosome entry site. We found that circ-WRKY9 encoded a peptide of 88 amino acids (aa) and named it WRKY9-88aa. Overexpression of WRKY9-88aa suppresses RSMV infection in rice plants, with increased reactive oxygen species production. Furthermore, WRKY9-88aa enhances resistance to blast disease and bacterial leaf blight, suggesting its potential to provide broad-spectrum disease resistance. Our findings provide the first evidence of a peptide encoded by a circRNA in planta and highlight its potential application to control a wide spectrum of plant diseases.

A rice DELLA protein OsSLR1 positively regulates rice resistance to southern rice black-streaked dwarf virus infection

BACKGROUND

In the course of long-term confrontation with pathogens, plants have developed complex defense mechanisms to protect themselves from various pathogens. Previous studies have reported that the gibberellin (GA) signaling pathway negative regulator SLENDER RICE 1 (SLR1) in rice activates jasmonic acid (JA)-mediated broad-spectrum antiviral immunity, but the exploration regarding whether OsSLR1 exerts effects on alternative antiviral immune pathways remains limited.

RESULTS

Here, we identified that OsSLR1 was significantly induced after virus infection and overexpression of OsSLR1 in rice enhance the resistance of rice to southern rice black-streaked dwarf virus (SRBSDV) in rice. Transcriptome analysis revealed that a total of 2,336 differentially expressed genes (DEGs) were detected upon overexpression of OsSLR1 in rice, including 1,607 upregulated genes and 729 downregulated genes. Gene ontology (GO) enrichment analysis and RT-qPCR analysis revealed that genes related to JA and reactive oxygen species (ROS) were significantly upregulated, while genes associated with abscisic acid (ABA) were significantly downregulated.

CONCLUSIONS

These results suggest that OsSLR1 positively regulates the antiviral immunity of rice by modulating multiple pathways.

Evolutionary-Distinct Viral Proteins Subvert Rice Broad-Spectrum Antiviral Immunity Mediated by the RAV15-MYC2 Module

A variety of plant viruses employ different virulence strategies to achieve successful infection, resulting in abnormal plant development. However, the common pathogenicity of distinct viruses has rarely been studied. Here, it is shown that a plant-specific RAV-type transcription factor, OsRAV15, is specifically targeted by several distinct viral proteins for facilitating viral infection. OsRAV15 is found to activate jasmonic acid (JA)-mediated broad-spectrum antiviral immunity by physically associating with JA signaling essential components OsMYC2-OsJAZ complex. To facilitate viral infection, evolutionarily distinct viral proteins encoded by diverse rice viruses generally disrupt the OsRAV15-OsMYC2 complex to attenuate activation of JA signaling. Together, the results reveal a common counter-defense strategy used by different viruses to suppress the OsRAV15-OsMYC2 module that plays a vital role in fine-tuning JA-mediated antiviral defense.

Non-Specific Lipid Transfer Protein StLTP6 Promotes Virus Infection by Inhibiting Jasmonic Acid Signalling Pathway in Response to PVS TGB1

Plant viruses rely on host factors for successful infection. Non-specific lipid transfer proteins (nsLTPs) play critical roles in plant-pathogen interactions; however, their functions and underlying molecular mechanisms in viral infections remain largely unknown. Jasmonic acid (JA) is a crucial regulatory hormone in the process of plant resistance to viral infection. In this study, we screened and verified that StLTP6, a previously identified pro-viral factor, interacts with the silencing suppressor triple gene block1 (TGB1) of potato virus S (PVS). The PVS TGB1 induces the expression of StLTP6, and both co-localize in the cytoplasm. Furthermore, StLTP6 interacts with allene oxide cyclase and inhibits its accumulation, thereby suppressing JA synthesis and attenuating RNA silencing antiviral resistance. In summary, we elucidated the molecular mechanism by which PVS TGB1 interacts with StLTP6 to facilitate PVS infection. These findings broaden our understanding of the biological roles of nsLTPs and provide a new antiviral target for potato research.

A new scheme for boosting in situ Fenton-like reaction in plant pathogenic tissues for selective structural degradation of capsid proteins

Safe prevention and control of plant viruses is a global challenge. Inducing viral capsid protein (CP) degradation via hydroxyl radicals (∙OH) generated by an in situ Fenton-like reaction within plant pathogenic tissues is proposed for combating plant viruses in this study. We designed a new Fenton-like reaction inducer, tCuinter-bCDs, which utilizes an internal doping strategy that reduces copper content by 89.89% compared to conventional doping methods, while still achieving a high coexistence of multivalent copper ions. Our research demonstrated that tCuinter-bCDs possessed functional activity to specifically recognize and proximally degrade CP. tCuinter-bCDs form complexes with CP monomers through supramolecular bonds characterized by significant electrostatic components. Within 10 min, the complex induced complete degradation of tertiary structure pockets composed of α-helices and β-sheets located at residues MET1-GLU23, TYR73-ARG93, and SER147-PRO157. Based on a high-resolution 2.91 Å CP model that was constructed for the first time, this degradation process is likely driven by hydrophobic interactions between tCuinter-bCDs and CP residues MET1, VAL5, THR55, and THR58, along with hydrogen bonds formed with THR4, VAL5, GLY15, PRO57, and ALA59, thereby promoting degradation of adjacent peptide segments. This represents the first study demonstrating in situ Fenton-like reactions to combat pathogens in plant systems. Our findings provide a new, efficient, and environmentally friendly approach for plant virus control.

Rice Stripe Mosaic Virus Encoded P6 Interacts with Heading Protein OsHAPL1 to Promote Viral Infection

Rice stripe mosaic virus (RSMV) is the sole cytoplasmic rhabdovirus documented in naturally infected rice plants. It encodes P6, which induces delayed heading and reduces yield in infected rice plants. P6 of RSMV interacts with OsHAPL1, facilitating the interaction between OsHAPL1 and DTH8, resulting in delayed rice heading under long day conditions. Additionally, OsHAPL1 plays a dual role in RSMV infection by positively influencing viral infection while negatively regulating the expression of defense signal genes. These findings elucidate a novel molecular mechanism through which a virus manipulates the host defense system to enhance viral infection and transmission, shedding new light on the crosstalk between rice heading and disease resistance pathways.

Advances in understanding the interaction between Solanaceae NLR resistance proteins and the viral effector Avr

The rising prevalence of viral-induced diseases, particularly those caused by certain strains, poses a substantial risk to the genetic diversity of Solanaceae crops and the overall safety of horticultural produce. According to the "gene-for-gene" hypothesis, resistance proteins are capable of selectively identifying nontoxic effectors produced by pathogens, as they are under purview of the host's immune defenses. The sensitivity and responsiveness of Solanaceae plants to viral attacks play a crucial role in shaping the outcomes of their interactions with viruses. Pathogenic organisms, devise an array of infection tactics aimed at circumventing or neutralizing the host's immune defenses to facilitate effective invasion. The invasion often accomplishes by suppressing or disrupting the host's defensive mechanisms or immune signals, which are integral to the infection strategies of such invading pathogens. This comprehensive review delves into the myriad approaches that pathogenic viruses employ to infiltrate and overcome the sophisticated immune system of tomatoes. Furthermore, the review explores the possibility of utilizing these viral strategies to bolster the resilience of horticultural crops, presenting a hopeful direction for forthcoming progress in plant health and agricultural stability.

Crop antiviral defense: Past and future perspective

Viral pathogens not only threaten the health and life of humans and animals but also cause enormous crop yield losses and contribute to global food insecurity. To defend against viral pathogens, plants have evolved an intricate immune system to perceive and cope with such attacks. Although most of the fundamental studies were carried out in model plants, more recent research in crops has provided new insights into the antiviral strategies employed by crop plants. We summarize recent advances in understanding the biological roles of cellular receptors, RNA silencing, RNA decay, hormone signaling, autophagy, and ubiquitination in manipulating crop host-mediated antiviral responses. The potential functions of circular RNAs, the rhizosphere microbiome, and the foliar microbiome of crops in plant-virus interactions will be fascinating research directions in the future. These findings will be beneficial for the development of modern crop improvement strategies.

A viral p3a protein targets and inhibits TaDOF transcription factors to promote the expression of susceptibility genes and facilitate viral infection

The interactions among viruses and host plants are complex and fascinating because these organisms interact with and adapt to each other continuously. Many plant transcription factors play important roles in plant growth and development and in the resistance to viral infection. To facilitate the infection of plants, some viral proteins typically target and inhibit the function of plant transcription factors. In this study, we found an interesting phenomenon wherein the p3a protein of barley yellow dwarf virus (BYDV) can interact with the zinc finger domain of the TaDOF transcription factor in wheat; the zinc finger domain of TaDOF can interact with the promoter of TaHSP70 and inhibit the transcription of the TaHSP70 gene; and p3a interacts with the TaDOF zinc finger domain through competitive binding, alleviating TaDOF zinc finger domain-mediated inhibition of the TaHSP70 promoter, thereby promoting TaHSP70 expression and promoting infection by BYDV. This study demonstrates that BYDV p3a is an immunosuppressive factor and enriches our understanding of the pathogenesis of BYDV.

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.

Two different viral proteins suppress NUCLEAR FACTOR-YC-mediated antiviral immunity during infection in rice

Plant viruses have multiple strategies to counter and evade the host's antiviral immune response. However, limited research has been conducted on the antiviral defense mechanisms commonly targeted by distinct types of plant viruses. In this study, we discovered that NUCLEAR FACTOR-YC (NF-YC) and NUCLEAR FACTOR-YA (NF-YA), 2 essential components of the NF-Y complex, were commonly targeted by viral proteins encoded by 2 different rice (Oryza sativa L.) viruses, rice stripe virus (RSV, Tenuivirus) and southern rice black streaked dwarf virus (SRBSDV, Fijivirus). In vitro and in vivo experiments showed that OsNF-YCs associate with OsNF-YAs and inhibit their transcriptional activation activity, resulting in the suppression of OsNF-YA-mediated plant susceptibility to rice viruses. Different viral proteins RSV P2 and SRBSDV SP8 directly disrupted the association of OsNF-YCs with OsNF-YAs, thereby suppressing the antiviral defense mediated by OsNF-YCs. These findings suggest an approach for conferring broad-spectrum disease resistance in rice and reveal a common mechanism employed by viral proteins to evade the host's antiviral defense by hindering the antiviral capabilities of OsNF-YCs.

Managing Super Pests: Interplay between Pathogens and Symbionts Informs Biocontrol of Whiteflies

Bemisia tabaci is distributed globally and incurs considerable economic and ecological costs as an agricultural pest and viral vector. The entomopathogenic fungus Metarhizium anisopliae has been known for its insecticidal activity, but its impacts on whiteflies are understudied. We investigated how infection with the semi-persistently transmitted Cucurbit chlorotic yellows virus (CCYV) affects whitefly susceptibility to M. anisopliae exposure. We discovered that viruliferous whiteflies exhibited increased mortality when fungus infection was present compared to non-viruliferous insects. High throughput 16S rRNA sequencing also revealed significant alterations of the whitefly bacterial microbiome diversity and structure due to both CCYV and fungal presence. Specifically, the obligate symbiont Portiera decreased in relative abundance in viruliferous whiteflies exposed to M. anisopliae. Facultative Hamiltonella and Rickettsia symbionts exhibited variability across groups but dominated in fungus-treated non-viruliferous whiteflies. Our results illuminate triangular interplay between pest insects, their pathogens, and symbionts-dynamics which can inform integrated management strategies leveraging biopesticides This work underscores the promise of M. anisopliae for sustainable whitefly control while laying the groundwork for elucidating mechanisms behind microbe-mediated shifts in vector competence.

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.

A transcription factor ZmGLK36 confers broad resistance to maize rough dwarf disease in cereal crops

Maize rough dwarf disease (MRDD), caused by maize rough dwarf virus (MRDV) or rice black-streaked dwarf virus (RBSDV), seriously threatens worldwide production of all major cereal crops, including maize, rice, wheat and barley. Here we report fine mapping and cloning of a previously reported major quantitative trait locus (QTL) (qMrdd2) for RBSDV resistance in maize. Subsequently, we show that qMrdd2 encodes a G2-like transcription factor named ZmGLK36 that promotes resistance to RBSDV by enhancing jasmonic acid (JA) biosynthesis and JA-mediated defence response. We identify a 26-bp indel located in the 5' UTR of ZmGLK36 that contributes to differential expression and resistance to RBSDV in maize inbred lines. Moreover, we show that ZmDBF2, an AP2/EREBP family transcription factor, directly binds to the 26-bp indel and represses ZmGLK36 expression. We further demonstrate that ZmGLK36 plays a conserved role in conferring resistance to RBSDV in rice and wheat using transgenic or marker-assisted breeding approaches. Our results provide insights into the molecular mechanisms of RBSDV resistance and effective strategies to breed RBSDV-resistant cereal crops.

A systematic review of southern rice black-streaked dwarf virus in the age of omics

Southern rice black-streaked dwarf virus (SRBSDV) is one of the most damaging rice viruses. The virus decreases rice quality and yield, and poses a serious threat to food security. From this perspective, this review performed a survey of published studies in recent years to understand the current status of SRBSDV and white-backed planthopper (WBPH, Sogatella furcifera) transmission processes in rice. Recent studies have shown that the interactions between viral virulence proteins and rice susceptibility factors shape the transmission of SRBSDV. Moreover, the transmission of SRBSDV is influenced by the interactions between viral virulence proteins and S. furcifera susceptibility factors. This review focused on the molecular mechanisms of key genes or proteins associated with SRBSDV infection in rice via the S. furcifera vector, and the host defense response mechanisms against viral infection. A sustainable control strategy using RNAi was summarized to address this pest. Finally, we also present a model for screening anti-SRBSDV inhibitors using viral proteins as targets. © 2023 Society of Chemical Industry.

NLRs derepress MED10b- and MED7-mediated repression of jasmonate-dependent transcription to activate immunity

Plant intracellular nucleotide-binding domain, leucine-rich repeat-containing receptors (NLRs) activate a robust immune response upon detection of pathogen effectors. How NLRs induce downstream immune defense genes remains poorly understood. The Mediator complex plays a central role in transducing signals from gene-specific transcription factors to the transcription machinery for gene transcription/activation. In this study, we demonstrate that MED10b and MED7 of the Mediator complex mediate jasmonate-dependent transcription repression, and coiled-coil NLRs (CNLs) in Solanaceae modulate MED10b/MED7 to activate immunity. Using the tomato CNL Sw-5b, which confers resistance to tospovirus, as a model, we found that the CC domain of Sw-5b directly interacts with MED10b. Knockout/down of MED10b and other subunits including MED7 of the middle module of Mediator activates plant defense against tospovirus. MED10b was found to directly interact with MED7, and MED7 directly interacts with JAZ proteins, which function as transcriptional repressors of jasmonic acid (JA) signaling. MED10b-MED7-JAZ together can strongly repress the expression of JA-responsive genes. The activated Sw-5b CC interferes with the interaction between MED10b and MED7, leading to the activation of JA-dependent defense signaling against tospovirus. Furthermore, we found that CC domains of various other CNLs including helper NLR NRCs from Solanaceae modulate MED10b/MED7 to activate defense against different pathogens. Together, our findings reveal that MED10b/MED7 serve as a previously unknown repressor of jasmonate-dependent transcription repression and are modulated by diverse CNLs in Solanaceae to activate the JA-specific defense pathways.

A Jasmonic Acid-Related Mechanism Affects ARGONAUTE5 Expression and Antiviral Defense Against Potato Virus X in Arabidopsis thaliana

During virus infection, Argonaute (AGO) proteins bind to Dicer-produced virus small interfering RNAs and target viral RNA based on sequence complementarity, thereby limiting virus proliferation. The Arabidopsis AGO2 protein is important for resistance to multiple viruses, including potato virus X (PVX). In addition, AGO5 is important in systemic defense against PVX. Normally AGO5 is expressed only in reproductive tissues, and its induction by virus infection is thought to be important for its participation in antiviral defense. However, it is unclear what mechanisms induce AGO5 expression in response to virus infection. Here, we show that dde2-2, a mutant compromised in jasmonic acid (JA) biosynthesis, displays constitutive upregulation of AGO5. This mutant also showed increased resistance to PVX and this resistance was dependent on a functional AGO5 gene. Furthermore, methyl jasmonate treatment ablated AGO5 expression in leaves during virus infection and resulted in increased susceptibility to virus. Our results further support a role for AGO5 in antiviral RNA silencing and a negative regulation by JA, a plant hormone associated with defense against plant-feeding arthropods, which are often the vectors of plant viruses. [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.

Different viral effectors suppress hormone-mediated antiviral immunity of rice coordinated by OsNPR1

Salicylic acid (SA) and jasmonic acid (JA) are plant hormones that typically act antagonistically in dicotyledonous plants and SA and JA signaling is often manipulated by pathogens. However, in monocotyledonous plants, the detailed SA-JA interplay in response to pathogen invasion remains elusive. Here, we show that different types of viral pathogen can disrupt synergistic antiviral immunity mediated by SA and JA via OsNPR1 in the monocot rice. The P2 protein of rice stripe virus, a negative-stranded RNA virus in the genus Tenuivirus, promotes OsNPR1 degradation by enhancing the association of OsNPR1 and OsCUL3a. OsNPR1 activates JA signaling by disrupting the OsJAZ-OsMYC complex and boosting the transcriptional activation activity of OsMYC2 to cooperatively modulate rice antiviral immunity. Unrelated viral proteins from different rice viruses also interfere with the OsNPR1-mediated SA-JA interplay to facilitate viral pathogenicity, suggesting that this may be a more general strategy in monocot plants. Overall, our findings highlight that distinct viral proteins convergently obstruct JA-SA crosstalk to facilitate viral infection in monocot rice.

A Rice Receptor-like Protein Negatively Regulates Rice Resistance to Southern Rice Black-Streaked Dwarf Virus Infection

Plants rely on various receptor-like proteins and receptor-like kinases to recognize and defend against invading pathogens. However, research on the role of receptor-like proteins in plant antiviral defense, particularly in rice-virus interactions, is limited. In this study, we identified a receptor-like gene, OsBAP1, which was significantly induced upon infection with southern rice black-streaked dwarf virus (SRBSDV) infection. A viral inoculation assay showed that the OsBAP1 knockout mutant exhibited enhanced resistance to SRBSDV infection, indicating that OsBAP1 plays a negatively regulated role in rice resistance to viral infection. Transcriptome analysis revealed that the genes involved in plant-pathogen interactions, plant hormone signal transduction, oxidation-reduction reactions, and protein phosphorylation pathways were significantly enriched in OsBAP1 mutant plants (osbap1-cas). Quantitative real-time PCR (RT-qPCR) analysis further demonstrated that some defense-related genes were significantly induced during SRBSDV infection in osbap1-cas mutants. Our findings provide new insights into the role of receptor-like proteins in plant immune signaling pathways, and demonstrate that OsBAP1 negatively regulates rice resistance to SRBSDV infection.

ZmMYC2s play important roles in maize responses to simulated herbivory and jasmonate

Both herbivory and jasmonic acid (JA) activate the biosynthesis of defensive metabolites in maize, but the mechanism underlying this remains unclear. We generated maize mutants in which ZmMYC2a and ZmMYC2b, two transcription factor genes important in JA signaling, were individually or both knocked out. Genetic and biochemical analyses were used to elucidate the functions of ZmMYC2 proteins in the maize response to simulated herbivory and JA. Compared with the wild-type (WT) maize, the double mutant myc2ab was highly susceptible to insects, and the levels of benzoxazinoids and volatile terpenes, and the levels of their biosynthesis gene transcripts, were much lower in the mutants than in the WT maize after simulated insect feeding or JA treatment. Moreover, ZmMYC2a and ZmMYC2b played a redundant role in maize resistance to insects and JA signaling. Transcriptome and Cleavage Under Targets and Tagmentation-Sequencing (CUT&Tag-Seq) analysis indicated that ZmMYC2s physically targeted 60% of the JA-responsive genes, even though only 33% of these genes were transcriptionally ZmMYC2-dependent. Importantly, CUT&Tag-Seq and dual luciferase assays revealed that ZmMYC2s transactivate the benzoxazinoid and volatile terpene biosynthesis genes IGPS1/3, BX10/11/12/14, and TPS10/2/3/4/5/8 by directly binding to their promoters. Furthermore, several transcription factors physically targeted by ZmMYC2s were identified, and these are likely to function in the regulation of benzoxazinoid biosynthesis. This work reveals the transcriptional regulatory landscapes of both JA signaling and ZmMYC2s in maize and provides comprehensive mechanistic insight into how JA signaling modulates defenses in maize responses to herbivory through ZmMYC2s.

The transcriptional regulator JAZ8 interacts with the C2 protein from geminiviruses and limits the geminiviral infection in Arabidopsis

Jasmonates (JAs) are phytohormones that finely regulate critical biological processes, including plant development and defense. JASMONATE ZIM-DOMAIN (JAZ) proteins are crucial transcriptional regulators that keep JA-responsive genes in a repressed state. In the presence of JA-Ile, JAZ repressors are ubiquitinated and targeted for degradation by the ubiquitin/proteasome system, allowing the activation of downstream transcription factors and, consequently, the induction of JA-responsive genes. A growing body of evidence has shown that JA signaling is crucial in defending against plant viruses and their insect vectors. Here, we describe the interaction of C2 proteins from two tomato-infecting geminiviruses from the genus Begomovirus, tomato yellow leaf curl virus (TYLCV) and tomato yellow curl Sardinia virus (TYLCSaV), with the transcriptional repressor JAZ8 from Arabidopsis thaliana and its closest orthologue in tomato, SlJAZ9. Both JAZ and C2 proteins colocalize in the nucleus, forming discrete nuclear speckles. Overexpression of JAZ8 did not lead to altered responses to TYLCV infection in Arabidopsis; however, knock-down of JAZ8 favors geminiviral infection. Low levels of JAZ8 likely affect the viral infection specifically, since JAZ8-silenced plants neither display obvious developmental phenotypes nor present differences in their interaction with the viral insect vector. In summary, our results show that the geminivirus-encoded C2 interacts with JAZ8 in the nucleus, and suggest that this plant protein exerts an anti-geminiviral effect.

Rice stripe virus p2 protein interacts with ATG5 and is targeted for degradation by autophagy

Autophagy can be induced by viral infection and plays antiviral roles in plants, but the underlying mechanism is not well understood. In our previous reports, we have demonstrated that the plant ATG5 plays an essential role in activating autophagy in rice stripe virus (RSV)-infected plants. We also showed that eIF4A, a negative factor of autophagy, interacts with and inhibits ATG5. We here found that RSV p2 protein interacts with ATG5 and can be targeted by autophagy for degradation. Expression of p2 protein induced autophagy and p2 protein was shown to interfere with the interaction between ATG5 and eIF4A, while eIF4A had no effect on the interaction between ATG5 and p2. These results indicate an additional information on the induction of autophagy in RSV-infected plants.

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.

The P3N-PIPO Protein Encoded by Wheat Yellow Mosaic Virus Is a Pathogenicity Determinant and Promotes Its Pathogenicity through Interaction with NbRLK6 in Nicotiana benthamiana

Similarly to other potyvirids, the bymovirus wheat yellow mosaic virus (WYMV) encodes a P3N-PIPO protein that is expressed by frameshifting occurring within the open reading frame of the P3 protein. P3N-PIPO is known to be essential for the cell-to-cell movement of several potyviruses, but this has not yet been confirmed for the WYMV. Here, we show that the WYMV P3N-PIPO protein influences disease symptom formation. Infection of Nicotiana benthamiana plants with a potato virus X (PVX)-based vector carrying the WYMV P3N-PIPO gene induced more severe disease symptoms and resulted in higher virus accumulation levels than did infection with PVX lacking the P3N-PIPO gene. N. benthamiana P3N-PIPO-interacting proteins were identified through co-immunoprecipitation (Co-IP) coupled with LC-MS/MS (mass spectrometry), and the interaction between P3N-PIPO and the N. benthamiana receptor-like kinase NbRLK6 was further verified by Co-IP and bimolecular fluorescence complementation (BiFC) of transiently-expressed proteins. Furthermore, our investigation showed that the disease symptom severity and accumulation level of PVX-P3N-PIPO were decreased in N. benthamiana plants when NbRLK6 expression was reduced by tobacco rattle virus-induced gene silencing.

Hrip1 mediates rice cell wall fortification and phytoalexins elicitation to confer immunity against Magnaporthe oryzae

Magnaporthe oryzae is a potent fungus that adversely affects rice yield. Combinatorial techniques of prevention, toxic chemicals, and fungicide are used to remedy rice blast infection. We reported the role of Hrip1 in cell death elicitation and expression of systematic acquired resistance that could potentially stifle M. oryzae infection. In this study, transcriptome and metabolomic techniques were used to investigate the mechanism by which Hrip1 reprogramed the transcriptome of rice seedlings to confer immunity against M. oryzae. Our results showed that Hrip1 induces cell wall thickening and phytoalexin elicitation to confer immunity against M. oryzae infection. Hrip1 activates key lignin biosynthetic genes and myeloblastosis transcription factors that act as molecular switches for lignin production. Lignin content was increased by 68.46% and more after 48 h onwards in Hrip1-treated seedlings compared to the control treatment. Further analysis of cell wall morphology using the transmission electron microscopy technique revealed over 100% cell wall robustness. Hrip1 also induced the expression of 24 diterpene synthases. These include class I and II terpene synthases, cytochrome P450 subfamilies (OsCYP76M and OsCYP71Z), and momilactones synthases. The relationship between the expression of these genes and metabolic elicitation was analyzed using ultra-performance liquid chromatography-tandem mass spectrometry. Enhanced amounts of momilactones A and B, oryzalactone, and phytocassane A and G were detected in the Hrip1-treated leaves. We also identified seven benzoxazinoid genes (BX1-BX7) that could improve rice immunity. Our findings show that Hrip1 confers dual immunity by leveraging lignin and phytoalexins for physical and chemical resistance. This study provides novel insights into the mechanisms underlying Hrip1-treated plant immunity.

Insights Into the Effect of Rice Stripe Virus P2 on Rice Defense by Comparative Proteomic Analysis

Rice stripe virus (RSV) has a serious effect on rice production. Our previous research had shown that RSV P2 plays important roles in RSV infection, so in order to further understand the effect of P2 on rice, we used Tandem Mass Tag (TMT) quantitative proteomics experimental system to analyze the changes of protein in transgenic rice expressing P2 for the first time. The results of proteomics showed that a total of 4,767 proteins were identified, including 198 up-regulated proteins and 120 down-regulated proteins. Functional classification results showed that differentially expressed proteins (DEPs) were mainly localized in chloroplasts and mainly involved in the metabolic pathways. Functional enrichment results showed that DEPs are mainly involved in RNA processing and splicing. We also verified the expression of several DEPs at the mRNA level and the interaction of a transcription factor (B7EPB8) with RSV P2. This research is the first time to use proteomics technology to explore the mechanism of RSV infection in rice with the RSV P2 as breakthrough point. Our findings provide valuable information for the study of RSV P2 and RSV infection mechanism.

NF-YA transcription factors suppress jasmonic acid-mediated antiviral defense and facilitate viral infection in rice

NF-Y transcription factors are known to play many diverse roles in the development and physiological responses of plants but little is known about their role in plant defense. Here, we demonstrate the negative roles of rice NF-YA family genes in antiviral defense against two different plant viruses, Rice stripe virus (RSV, Tenuivirus) and Southern rice black-streaked dwarf virus (SRBSDV, Fijivirus). RSV and SRBSDV both induced the expression of OsNF-YA family genes. Overexpression of OsNF-YAs enhanced rice susceptibility to virus infection, while OsNF-YAs RNAi mutants were more resistant. Transcriptome sequencing showed that the expression of jasmonic acid (JA)-related genes was significantly decreased in plants overexpressing OsNF-YA when they were infected by viruses. qRT-PCR and JA sensitivity assays confirmed that OsNF-YAs play negative roles in regulating the JA pathway. Further experiments showed that OsNF-YAs physically interact with JA signaling transcription factors OsMYC2/3 and interfere with JA signaling by dissociating the OsMYC2/3-OsMED25 complex, which inhibits the transcriptional activation activity of OsMYC2/3. Together, our results reveal that OsNF-YAs broadly inhibit plant antiviral defense by repressing JA signaling pathways, and provide new insight into how OsNF-YAs are directly associated with the JA pathway.

Jasmonate Signaling Pathway Modulates Plant Defense, Growth, and Their Trade-Offs

Lipid-derived jasmonates (JAs) play a crucial role in a variety of plant development and defense mechanisms. In recent years, significant progress has been made toward understanding the JA signaling pathway. In this review, we discuss JA biosynthesis, as well as its core signaling pathway, termination mechanisms, and the evolutionary origin of JA signaling. JA regulates not only plant regeneration, reproductive growth, and vegetative growth but also the responses of plants to stresses, including pathogen as well as virus infection, herbivore attack, and abiotic stresses. We also focus on the JA signaling pathway, considering its crosstalk with the gibberellin (GA), auxin, and phytochrome signaling pathways for mediation of the trade-offs between growth and defense. In summary, JA signals regulate multiple outputs of plant defense and growth and act to balance growth and defense in order to adapt to complex environments.

Genome-Wide Identification and Gene Expression Analysis of the OTU DUB Family in Oryza sativa

Ovarian tumor domain (OTU)-containing deubiquitinating enzymes (DUBs) are an essential DUB to maintain protein stability in plants and play important roles in plant growth development and stress response. However, there is little genome-wide identification and analysis of the OTU gene family in rice. In this study, we identified 20 genes of the OTU family in rice genome, which were classified into four groups based on the phylogenetic analysis. Their gene structures, conserved motifs and domains, chromosomal distribution, and cis elements in promoters were further studied. In addition, OTU gene expression patterns in response to plant hormone treatments, including SA, MeJA, NAA, BL, and ABA, were investigated by RT-qPCR analysis. The results showed that the expression profile of OsOTU genes exhibited plant hormone-specific expression. Expression levels of most of the rice OTU genes were significantly changed in response to rice stripe virus (RSV), rice black-streaked dwarf virus (RBSDV), Southern rice black-streaked dwarf virus (SRBSDV), and Rice stripe mosaic virus (RSMV). These results suggest that the rice OTU genes are involved in diverse hormone signaling pathways and in varied responses to virus infection, providing new insights for further functional study of OsOTU genes.

Turnip mosaic virus P1 suppresses JA biosynthesis by degrading cpSRP54 that delivers AOCs onto the thylakoid membrane to facilitate viral infection

Jasmonic acid (JA) is a crucial hormone in plant antiviral immunity. Increasing evidence shows that viruses counter this host immune response by interfering with JA biosynthesis and signaling. However, the mechanism by which viruses affect JA biosynthesis is still largely unexplored. Here, we show that a highly conserved chloroplast protein cpSRP54 was downregulated in Nicotiana benthamiana infected by turnip mosaic virus (TuMV). Its silencing facilitated TuMV infection. Furthermore, cpSRP54 interacted with allene oxide cyclases (AOCs), key JA biosynthesis enzymes, and was responsible for delivering AOCs onto the thylakoid membrane (TM). Interestingly, TuMV P1 protein interacted with cpSRP54 and mediated its degradation via the 26S proteosome and autophagy pathways. The results suggest that TuMV has evolved a strategy, through the inhibition of cpSRP54 and its delivery of AOCs to the TM, to suppress JA biosynthesis and enhance viral infection. Interaction between cpSRP54 and AOCs was shown to be conserved in Arabidopsis and rice, while cpSRP54 also interacted with, and was degraded by, pepper mild mottle virus (PMMoV) 126 kDa protein and potato virus X (PVX) p25 protein, indicating that suppression of cpSRP54 may be a common mechanism used by viruses to counter the antiviral JA pathway.

Jasmonic acid pathway has emerged as one of the predominant battlefields between plants and viruses. Several studies have indicated that, in addition to interfering with JA signaling, plant viruses can also affect JA biosynthesis, but the direct molecular links between them remain elusive. Here, we identify a highly conserved chloroplast protein cpSRP54 as a key positive regulator in JA biosynthesis and a common target for viruses belong to different genera. Through associating with cpSRP54 and inducing its degradation using the protein they encoded, the viruses can inhibit the cpSRP54-facilitated delivery of AOCs to the thylakoid membrane and manipulation of JA-mediated defense. This capability of viruses might define a novel and effective strategy against the antiviral JA pathway.

Rice Stripe Mosaic Disease: Characteristics and Control Strategies

Rice stripe mosaic disease (RSMD) is caused by the rice stripe mosaic virus (RSMV; genus Cytorhabdovirus, family Rhabdoviridae). In recent years, significant progress has been made in understanding several aspects of the disease, especially its geographical distribution, symptoms, vectors, gene functions, and control measures. Since RSMD was first detected in southern China in 2015, it has been found in more and more rice growing areas and has become one of the most important rice diseases in southern China. RSMV is transmitted by the leafhopper Recilia dorsalis in a persistent-propagative manner, inducing yellow stripes, a slight distortion of leaves, increased tillers, and empty grains in rice plants. The virus has a negative-sense single-strand RNA genome of about 12.7 kb that encodes seven proteins: N, P, P3, M, G, P6, and L. Several molecular and serological tests have been developed to detect RSMV in plants and insects. The disease cycle can be described as follows: RSMV and its vector overwinter in infected plants; viruliferous R. dorsalis adults transmit the virus to spring rice and lay eggs on the infected seedlings; the next generation of R. dorsalis propagate on infected seedlings, become viruliferous, disperse, and cause new disease outbreaks. Control measures include monitoring and accurate forecasting, selecting disease-resistant varieties, improving cultivation systems, covering rice seedling nurseries with insect-proof nets, and using pesticides rationally. Inappropriate cultivation systems, pesticide overuse, and climatic conditions contribute to epidemics by affecting the development of vector insects and their population dynamics.

Genome-Wide Analysis of the RAV Transcription Factor Genes in Rice Reveals Their Response Patterns to Hormones and Virus Infection

The RAV family is part of the B3 superfamily and is one of the most abundant transcription factor families in plants. Members have highly conserved B3 or AP2 DNA binding domains. Although the RAV family genes of several species have been systematically identified from genome-wide studies, there has been no comprehensive study to identify rice RAV family genes. Here, we identified 15 genes of the RAV family in the rice genome and analyzed their phylogenetic relationships, gene structure, conserved domains, and chromosomal distribution. Based on domain similarity and phylogenetic topology, rice RAV transcription factors were phylogenetically clustered into four groups. qRT-PCR analyses showed that expression of these RAV genes was significantly up-regulated or down-regulated by plant hormone treatments, including BL, NAA, IAA, MeJA, and SA. Most of the rice RAV genes were dramatically down-regulated in response to rice stripe virus (RSV) and mostly up-regulated in response to Southern rice black-streaked dwarf virus (SRBSDV). These results suggest that the rice RAV genes are involved in diverse signaling pathways and in varied responses to virus infection.