Virus altered rice attractiveness to planthoppers is mediated by volatiles and related to virus titre and expression of defence and volatile-biosynthesis genes

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.

The Antigenic Membrane Protein (Amp) of Rice Orange Leaf Phytoplasma Suppresses Host Defenses and Is Involved in Pathogenicity

Phytoplasmas are uncultivable, phloem-limited, phytopathogenic bacteria that represent a major threat to agriculture worldwide. Phytoplasma membrane proteins are in direct contact with hosts and presumably play a crucial role in phytoplasma spread within the plant as well as by the insect vector. Three highly abundant types of immunodominant membrane proteins (IDP) have been identified within the phytoplasmas: immunodominant membrane protein (Imp), immunodominant membrane protein A (IdpA), and antigenic membrane protein (Amp). Although recent results indicate that Amp is involved in host specificity by interacting with host proteins such as actin, little is known about the pathogenicity of IDP in plants. In this study, we identified an antigenic membrane protein (Amp) of rice orange leaf phytoplasma (ROLP), which interacts with the actin of its vector. In addition, we generated Amp-transgenic lines of rice and expressed Amp in tobacco leaves by the potato virus X (PVX) expression system. Our results showed that the Amp of ROLP can induce the accumulation of ROLP and PVX in rice and tobacco plants, respectively. Although several studies have reported interactions between major phytoplasma antigenic membrane protein (Amp) and insect vector proteins, this example demonstrates that Amp protein can not only interact with the actin protein of its insect vector but can also directly inhibit host defense responses to promote the infection. The function of ROLP Amp provides new insights into the phytoplasma-host interaction.

A viral protein orchestrates rice ethylene signaling to coordinate viral infection and insect vector-mediated transmission

Arthropod-borne viruses cause serious threats to human health and global agriculture by rapidly spreading via insect vectors. Southern rice black-streaked dwarf virus (SRBSDV) is the most damaging rice-infecting virus that is frequently transmitted by planthoppers. However, the molecular mechanisms underlying its propagation in the host plants and epidemics in the field are largely unknown. Here, we showed that the SRBSDV-encoded P6 protein is a key effector that regulates rice ethylene signaling to coordinate viral infection and transmission. In early SRBSDV infection, P6 interacts with OsRTH2 in the cytoplasm to activate ethylene signaling and enhance SRBSDV proliferation; this also repels the insect vector to reduce infestation. In late infection, P6 enters the nucleus, where it interacts with OsEIL2, a key transcription factor of ethylene signaling. The P6-OsEIL2 interaction suppresses ethylene signaling by preventing the dimerization of OsEIL2, thereby facilitating viral transmission by attracting the insect vector. Collectively, these findings reveal a novel molecular mechanism by which an arbovirus modulates the host defense system to promote viral infection and transmission.

Virus-induced plant volatiles mediate the olfactory behaviour of its insect vectors

Plant viruses can manipulate their hosts to release odours that are attractive or repellent to their insect vectors. However, the volatile organic compounds (VOCs), either individually or as mixtures, which play a key role in the olfactory behaviour of insect vectors remains largely unknown. Our study focused on green rice leafhoppers (GRLHs) vectoring rice dwarf virus (RDV) revealed that RDV infection significantly induced the emission of (E)-β-caryophyllene and 2-heptanol by rice plants, which influenced the olfactory behaviour of both non-viruliferous and viruliferous GRLHs. (E)-β-caryophyllene attracted non-viruliferous GRLHs to settle on RDV-infected plants, but neither attracted nor repelled viruliferous GRLHs. In contrast, 2-heptanol repelled viruliferous GRLHs to settle on RDV-infected plants, but neither repelled nor attracted non-viruliferous GRLHs. Suppression of (E)-β-caryophyllene synthase OsCAS via CRISPR-Cas9 to generate oscas-1 plants enabled us to confirm the important role played by (E)-β-caryophyllene in modulating the virus-vector-host plant interaction. These novel results reveal the role of these virus-induced VOCs in modulating the behaviour of its GRLH insect vector and may facilitate the design of new strategies for disease control through manipulation of plant volatile emissions.

Insights Into Insect Vector Transmission and Epidemiology of Plant-Infecting Fijiviruses

Viruses in genus Fijivirus (family Reoviridae) have caused serious damage to rice, maize and sugarcane in American, Asian, European and Oceanian countries, where seven plant-infecting and two insect-specific viruses have been reported. Because the planthopper vectors are the only means of virus spread in nature, their migration and efficient transmission of these viruses among different crops or gramineous weeds in a persistent propagative manner are obligatory for virus epidemics. Understanding the mechanisms of virus transmission by these insect vectors is thus key for managing the spread of virus. This review describes current understandings of main fijiviruses and their insect vectors, transmission characteristics, effects of viruses on the behavior and physiology of vector insects, molecular transmission mechanisms. The relationships among transmission, virus epidemics and management are also discussed. To better understand fijivirus-plant disease system, research needs to focus on the complex interactions among the virus, insect vector, insect microbes, and plants.

Magnaporthe oryzae systemic defense trigger 1 (MoSDT1)-mediated metabolites regulate defense response in Rice

BACKGROUND

Some of the pathogenic effector proteins play an active role in stimulating the plant defense system to strengthen plant resistance.

RESULTS

In this study, ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC/Q-TOF-MS) was implemented to identify altered metabolites in transgenic rice containing over-expressed M. oryzae Systemic Defense Trigger 1 (MoSDT1) that was infected at three-time points. The characterized dominating metabolites were organic acids and their derivatives, organic oxygen compounds, lipids, and lipid-like molecules. Among the identified metabolites, shikimate, galactinol, trehalose, D-mannose, linolenic acid, dopamine, tyramine, and L-glutamine are precursors for the synthesis of many secondary defense metabolites Carbohydrate, as well as amino acid metabolic, pathways were revealed to be involved in plant defense responses and resistance strengthening.

CONCLUSION

The increasing salicylic acid (SA) and jasmonic acid (JA) content enhanced interactions between JA synthesis/signaling gene, SA synthesis/receptor gene, raffinose/fructose/sucrose synthase gene, and cell wall-related genes all contribute to defense response in rice. The symptoms of rice after M. oryzae infection were significantly alleviated when treated with six identified metabolites, i.e., galactol, tyramine, L-glutamine, L-tryptophan, α-terpinene, and dopamine for 72 h exogenously. Therefore, these metabolites could be utilized as an optimal metabolic marker for M. oryzae defense.

Post-acquisition effects of viruses on vector behavior are important components of manipulation strategies

A growing number of studies suggest that plant viruses manipulate host plant phenotypes to increase transmission-conducive behaviors by vectors. Studies on this phenomenon frequently omit examination of interactions that occur after vectors acquire virions, which provides an incomplete understanding of the ecology of plant virus manipulation. Here, by taking a full factorial approach that considered both the infection status of the host (Montia perfoliata) and viruliferous status of the aphid (Myzus persicae), we explored the effects of a circulative, non-propagative virus (Turnip yellows virus [TuYV]) on a suite of behavior and performance metrics that are relevant for virus transmission. Our results demonstrate that viruliferous aphids exhibited an increased velocity of movement and increased activity levels in locomotor and dispersal-retention assays. They also had increased fecundity and showed a capacity to more efficiently exploit resources by taking less time to reach the phloem and ingesting more sap, regardless of plant infection status. In contrast, non-viruliferous aphids only exhibited enhanced fecundity and biomass on TuYV-infected hosts, and had overall reduced dispersal and locomotor activity relative to viruliferous aphids. In this pathosystem, post-acquisition effects were stronger and more conducive to virus transmission than the purely pre-acquisition effects mediated by virus effects on the host plant. Our study provides additional support for the hypothesis that virus manipulation of vector behavior includes both pre- and post-acquisition effects and demonstrates the importance of considering both components when studying putative virus manipulation strategies.

Overexpression of OsF3H modulates WBPH stress by alteration of phenylpropanoid pathway at a transcriptomic and metabolomic level in Oryza sativa

The whitebacked planthopper (WBPH), has become a devastating pest for rice crops, causes serious yield losses each year, and urgently needs biological control. Here, we developed a WBPH-resistant rice cultivar by overexpressing the OsF3H gene. A genetic functional analysis of the OsF3H gene confirmed its role in facilitating flavonoid contents and have indicated that the expression of the OsF3H gene is involved in regulation of the downstream genes (OsDFR and OsFLS) of the flavonoid pathway and genes (OsSLR1 and OsWRKY13) involved in other physiological pathways. OxF3H (OsF3H transgenic) plants accumulated significant amounts of the flavonols kaempferol (Kr) and quercetin (Qu) and the anthocyanins delphinidin and cyanidin, compared to the wild type, in response to the stress induced by WBPH. Similarly, OsF3H-related proteins were significantly expressed in OxF3H lines after WBPH infestation. The present study, indicated that the regulation of JA in OxF3H plants was suppressed due the overexpression of the OsF3H gene, which induced the expression of downstream genes related to anthocyanin. Similarly, the OsWRKY13 transcriptional factor was significantly suppressed in OxF3H plants during WBPH infestation. Exogenous application of Kr and Qu increased the survival rates of susceptible TN1 lines in response to WBPH, while decreased the survival rate of first instar WBPHs, indicating that both flavonols exhibit pesticide activity. Phenotypic demonstration also affirms that OxF3H plants show strong resistance to WBPH compared with wild type. Collectively, our result suggested that OsF3H overexpression led to the up-regulation of defense related genes and enhanced rice resistance to WBPH infestation.

A synthesis of virus-vector associations reveals important deficiencies in studies on host and vector manipulation by plant viruses

Plant viruses face many challenges in agricultural environments. Although crop fields appear to be abundant resources for these pathogens, it may be difficult for viruses to "escape" from crop environments prior to host senescence or harvesting. One way for viruses to increase the odds of persisting outside of agricultural fields across seasons is by evolving traits that increase transmission opportunities between crops and wild plant communities. There is accumulating evidence that some viruses can achieve this by manipulating crop plant phenotypes in ways that enhance transmission by vectors. Putative manipulations occur through alteration of plant cues (color, size, texture, foliar volatiles, in-leaf metabolites, defenses, and leaf cuticles) that mediate vector orientation, feeding, and dispersal behaviors. Virus effects on host phenotypes are not uniform but appear to exhibit convergence depending on virus traits underlying transmission, particularly the duration of probing and feeding required to acquire and inoculate distinct types of plant viruses. This shared congruence in manipulation strategies and mechanisms across divergent virus lineages suggests that such effects may be adaptive. To discern if this is the case, researchers must consider molecular and environmental constraints on virus evolution, including those imposed by insect vectors from organismal to landscape scales. In this review, we synthesize applied research on vector-borne virus transmission in laboratory and field settings to identify the main factors determining transmission opportunities for plant viruses, and thus, selection pressure to evolve manipulative traits. We then examine these outputs in the context of studies reporting putative instances of plant virus manipulation. Our synthesis reveals important disconnects between virus manipulation studies and actual selection pressures imposed by vectors in real-world contexts.

Glucose Utilization in the Regulation of Chitin Synthesis in Brown Planthopper

Glucose-6-phosphatase (G6Pase) and hexokinase (HK) are two key enzymes in the glycolysis and gluconeogenesis pathways, which catalyze the synthesis and degradation of glucose in insects, respectively. G6Pase and HK play an important role in insect growth by regulating the metabolism of glucose, leading to the efficient metabolism of other macromolecules. However, it is unclear whether these genes could be investigated for pest control through their actions on chitin metabolism. We studied the potential functions of G6Pase and HK genes in the regulation of chitin metabolism pathways by RNAi technology. Interference with G6Pase expression did not affect trehalose and chitin metabolism pathways in brown planthopper, Nilaparvata lugens (Stål). However, knockdown of the HK gene resulted in a significant decrease of expression of genes associated with the trehalose metabolic pathway but had no significant effect on trehalase activity, trehalose content, or glucogen content. Additionally, HK knockdown resulting in downregulation of the genes involved in chitin metabolism in the brown planthopper. These insects also showed wing deformities and difficulty in molting to varying degrees. We suggest that the silencing of HK expression directly inhibited the decomposition of glucose, leading to impaired chitin synthesis.

Progress and challenges in identifying molecular mechanisms underlying host and vector manipulation by plant viruses

Plant virus infection fundamentally alters chemical and behavioral phenotypes of hosts and vectors. These alterations often enhance virus transmission, leading researchers to surmise that such effects are manipulations caused by virus adaptations and not just by-products of pathology. But identification of the virus components behind manipulation is missing from most studies performed to date. Here, we evaluate causative empirical evidence that virus components are the drivers of manipulated host and vector phenotypes. To do so, we link findings and methodologies on virus pathology with observational and functional genomics studies on virus manipulation. Our synthesis provides an overview of progress, areas of synergy, and new approaches that will lead to an improved mechanistic understanding of host and vector manipulation by plant viruses.

Mal de Río Cuarto virus infection causes hormone imbalance and sugar accumulation in wheat leaves

BACKGROUND

Mal de Río Cuarto virus (MRCV) infects several monocotyledonous species including maize and wheat. Infected plants show shortened internodes, partial sterility, increased tillering and reduced root length. To better understand the molecular basis of the plant-virus interactions leading to these symptoms, we combined RNA sequencing with metabolite and hormone measurements.

RESULTS

More than 3000 differentially accumulated transcripts (DATs) were detected in MRCV-infected wheat plants at 21 days post inoculation compared to mock-inoculated plants. Infected plants exhibited decreased levels of TaSWEET13 transcripts, which are involved in sucrose phloem loading. Soluble sugars, starch, trehalose 6-phosphate (Tre6P), and organic and amino acids were all higher in MRCV-infected plants. In addition, several transcripts related to plant hormone metabolism, transport and signalling were increased upon MRCV infection. Transcripts coding for GA20ox, D14, MAX2 and SMAX1-like proteins involved in gibberellin biosynthesis and strigolactone signalling, were reduced. Transcripts involved in jasmonic acid, ethylene and brassinosteroid biosynthesis, perception and signalling and in auxin transport were also altered. Hormone measurements showed that jasmonic acid, brassinosteroids, abscisic acid and indole-3-acetic acid were significantly higher in infected leaves.

CONCLUSIONS

Our results indicate that MRCV causes a profound hormonal imbalance that, together with alterations in sugar partitioning, could account for the symptoms observed in MRCV-infected plants.

Metabolomic Changes in Sogatella furcifera under Southern rice black-streaked dwarf virus Infection and Temperature Stress

Southern rice black-streaked dwarf virus (SRBSDV) is a devastating newly emerged rice reovirus in Eastern and Southeastern Asia transmitted by a long-distance migratory pest, the white-backed planthopper (WBPH). We previously showed that SRBSDV infection decreased the cold tolerance but improved the heat tolerance of its vector, WBPH. Comparative metabolomic analysis was used to explore the potential mechanisms underlying these changes in temperature stress response. Fourth-generation WBPH nymphs were treated with SRBSDV and/or extreme temperature stress and were analyzed using gas chromatography-time of flight-mass spectrometry. A total of 605 distinguishable peaks were identified and 165, 207, and 202 differentially accumulated metabolites were identified in WBPH after virus infection, cold, or heat stress, respectively. The nucleic acids and fatty acids were the major categories of metabolites regulated by SRBSDV infection, whereas temperature stress regulated tricarboxylic acid cycle compounds, sugars, and polyols. For the WBPH samples infected with SRBSDV and subjected to temperature stress, amino acids, sugars, and polyols were the most significant regulated metabolites. The metabolomics study suggests that SRBSDV may influence the extreme temperature tolerance of WBPH by regulating the accumulation of amino acids, sugars, and polyols in the insect body.

A virus plays a role in partially suppressing plant defenses induced by the viruliferous vectors

Herbivorous attack induces plant defenses. There is evidence that some pests suppress these defenses by interfering with signaling pathways. We here report that infestation by the white-backed planthopper, Sogatella furcifera, induces defense responses in rice and infection of the southern rice black-streaked dwarf virus in the planthoppers partially suppresses the planthopper-induced plant defenses. Salicylic acid (SA) levels generally showed a temporal increase pattern while jasmonic acid (JA) levels generally exhibited a decrease pattern in the planthopper-infested plants, irrespective of virus infection status in the insects. The increase in SA was less while the decrease in JA was more in the viruliferous insect-infested plants than in the nonviruliferous insect-infested plants at both 48 and 72 h post infestation. The phytohormone levels corresponded to the patterns of relative expression levels of SA-marker genes (ICS1 and NPR1) and JA-marker gene (AOS2) in the plant treatments. Planthoppers performed better on the uninfested plants than on the previously infested plants and were of not significant increase in performance on the plants previously attacked by viruliferous planthoppers in comparison with the plants previously attacked by nonviruliferous insects. Our results indicate that the virus plays a role in partially suppressing the plant defenses induced by the planthopper. These findings provide a new perspective on plant-virus-vector interactions.

Expression of hybrid fusion protein (Cry1Ac::ASAL) in transgenic rice plants imparts resistance against multiple insect pests

To evolve rice varieties resistant to different groups of insect pests a fusion gene, comprising DI and DII domains of Bt Cry1Ac and carbohydrate binding domain of garlic lectin (ASAL), was constructed. Transgenic rice lines were generated and evaluated to assess the efficacy of Cry1Ac::ASAL fusion protein against three major pests, viz., yellow stem borer (YSB), leaf folder (LF) and brown planthopper (BPH). Molecular analyses of transgenic plants revealed stable integration and expression of the fusion gene. In planta insect bioassays on transgenics disclosed enhanced levels of resistance compared to the control plants. High insect mortality of YSB, LF and BPH was observed on transgenics compared to that of control plants. Furthermore, honeydew assays revealed significant decreases in the feeding ability of BPH on transgenic plants as compared to the controls. Ligand blot analysis, using BPH insects fed on cry1Ac::asal transgenic rice plants, revealed a modified receptor protein-binding pattern owing to its ability to bind to additional receptors in insects. The overall results authenticate that Cry1Ac::ASAL protein is endowed with remarkable entomotoxic effects against major lepidopteran and hemipteran insects. As such, the fusion gene appears promising and can be introduced into various other crops to control multiple insect pests.

Evolutionary Determinants of Host and Vector Manipulation by Plant Viruses

Plant viruses possess adaptations for facilitating acquisition, retention, and inoculation by vectors. Until recently, it was hypothesized that these adaptations are limited to virus proteins that enable virions to bind to vector mouthparts or invade their internal tissues. However, increasing evidence suggests that viruses can also manipulate host plant phenotypes and vector behaviors in ways that enhance their own transmission. Manipulation of vector-host interactions occurs through virus effects on host cues that mediate vector orientation, feeding, and dispersal behaviors, and thereby, the probability of virus transmission. Effects on host phenotypes vary by pathosystem but show a remarkable degree of convergence among unrelated viruses whose transmission is favored by the same vector behaviors. Convergence based on transmission mechanism, rather than phylogeny, supports the hypothesis that virus effects are adaptive and not just by-products of infection. Based on this, it has been proposed that viruses manipulate hosts through multifunctional proteins that facilitate exploitation of host resources and elicitation of specific changes in host phenotypes. But this proposition is rarely discussed in the context of the numerous constraints on virus evolution imposed by molecular and environmental factors, which figure prominently in research on virus-host interactions not dealing with host manipulation. To explore the implications of this oversight, we synthesized available literature to identify patterns in virus effects among pathogens with shared transmission mechanisms and discussed the results of this synthesis in the context of molecular and environmental constraints on virus evolution, limitations of existing studies, and prospects for future research.

Insect-Borne Plant Pathogens and Their Vectors: Ecology, Evolution, and Complex Interactions

The transmission of insect-borne plant pathogens, including viruses, bacteria, phytoplasmas, and fungi depends upon the abundance and behavior of their vectors. These pathogens should therefore be selected to influence their vectors to enhance their transmission, either indirectly, through the infected host plant, or directly, after acquisition of the pathogen by the vector. Accumulating evidence provides partial support for the occurrence of vector manipulation by plant pathogens, especially for plant viruses, for which a theoretical framework can explain patterns in the specific effects on vector behavior and performance depending on their modes of transmission. The variability in effects of pathogens on their vectors, however, suggests inconsistency in the occurrence of vector manipulation but also may reflect incomplete information about these systems. For example, manipulation can occur through combinations of specific effects, including direct and indirect effects on performance and behavior, and dynamics in those effects with disease progression or pathogen acquisition that together constitute syndromes that promote pathogen spread. Deciphering the prevalence and forms of vector manipulation by plant pathogens remains a compelling field of inquiry, but gaps and opportunities to advance it remain. A proposed research agenda includes examining vector manipulation syndromes comprehensively within pathosystems, expanding the taxonomic and genetic breadth of the systems studied, evaluating dynamic effects that occur during disease progression, incorporating the influence of biotic and abiotic environmental factors, evaluating the effectiveness of putative manipulation syndromes under field conditions, deciphering chemical and molecular mechanisms whereby pathogens can influence vectors, expanding the use of evolutionary and epidemiological models, and seeking opportunities to exploit these effects to improve management of insect-borne, economically important plant pathogens. We expect this field to remain vibrant and productive in its own right and as part of a wider inquiry concerning host and vector manipulation by plant and animal pathogens and parasites.

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation

By serving as vectors of transmission, insects play a key role in the infection cycle of many plant viruses. Viruses use sophisticated transmission strategies to overcome the spatial barrier separating plants and the impediment imposed by the plant cell wall. Interactions among insect vectors, viruses, and host plants mediate transmission by integrating all organizational levels, from molecules to populations. Best-examined on the molecular scale are two basic transmission modes wherein virus-vector interactions have been well characterized. Whereas association of virus particles with specific sites in the vector's mouthparts or in alimentary tract regions immediately posterior to them is required for noncirculative transmission, the cycle of particles through the vector body is necessary for circulative transmission. Virus transmission is also determined by interactions that are associated with changes in vector feeding behaviors and with alterations in plant host's morphology and/or metabolism that favor the attraction or deterrence of vectors. A recent concept in virus-host-vector interactions proposes that when vectors land on infected plants, vector elicitors and effectors "inform" the plants of the confluence of interacting entities and trigger signaling pathways and plant defenses. Simultaneously, the plant responses may also influence virus acquisition and inoculation by vectors. Overall, a picture is emerging where transmission depends on multilayered virus-vector-host interactions that define the route of a virus through the vector, and on the manipulation of the host and the vector. These interactions guarantee virus propagation until one or more of the interactants undergo changes through evolution or are halted by environmental interventions.