Interplay between drought and plant viruses co-infecting melon plants

Drought affects crops directly, and indirectly by affecting the activity of insect pests and the transmitted pathogens. Here, we established an experiment with well-watered or water-stressed melon plants, later single infected with either cucumber mosaic virus (CMV: non-persistent), or cucurbit aphid-borne yellow virus (CABYV: persistent), or both CMV and CABYV, and mock-inoculated control. We tested whether i) the relation between CMV and CABYV is additive, and ii) the relationship between water stress and virus infection is antagonistic, i.e., water stress primes plants for enhanced tolerance to virus infection. Water stress increased leaf greenness and temperature, and reduced leaf water potential, shoot biomass, stem dimensions, rate of flowering, CABYV symptom severity, and marketable fruit yield. Virus infection reduced leaf water potential transiently in single infected plants and persistently until harvest in double-infected plants. Double-virus infection caused the largest and synergistic reduction of marketable fruit yield. The relationship between water regime and virus treatment was additive in 12 out of 15 traits at harvest, with interactions for leaf water content, leaf:stem ratio, and fruit set. We conclude that both virus-virus relations in double infection and virus-drought relations cannot be generalized because they vary with virus, trait, and plant ontogeny.

Inter-virus relationships in mixed infections and virus-drought relationships in plants: a quantitative review

Inter-virus relationships in mixed infections and virus-drought relationships are important in agriculture and natural vegetation. In this quantitative review, we sampled published factorial experiments to probe for relationships against the null hypothesis of additivity. Our sample captured antagonistic, additive and synergistic inter-virus relationships in double infections. Virus-drought relationships in our sample were additive or antagonistic, reinforcing the notion that viruses have neutral or positive effects on droughted plants, or that drought enhances plant tolerance to viruses. Both inter-virus and virus-drought relationships vary with virus species, host plant to the level of cultivar or accession, timing of infection, plant age and trait and growing conditions. The trait-dependence of these relationships has implications for resource allocation in plants. Owing to lagging theories, more experimental research in these fields is bound to return phenomenological outcomes. Theoretical work can advance in two complementary directions. First, the effective theory models the behaviour of the system without specifying all the underlying causes that lead to system state change. Second, mechanistic theory based on a nuanced view of the plant phenotype that explicitly considers downward causation; the influence of the plant phenotype on inter-virus relations and vice versa; the impact of timing, intensity and duration of drought interacting with viruses to modulate the plant phenotype; both the soil (moisture) and atmospheric (vapour pressure deficit) aspects of drought. Theories should scale in time, from short term to full growing season, and in levels of organisation up to the relevant traits: crop yield in agriculture and fitness in nature.

Unveiling the dynamic relationship of viruses and/or symbiotic bacteria with plant resilience in abiotic stress

In the ecosphere, plants interact with environmental biotic and abiotic partners, where unbalanced interactions can induce unfavourable stress conditions. Abiotic factors (temperature, water, and salt) are primarily required for plants healthy survival, and any change in their availability is reflected as a stress signal. In certain cases, the presence of infectious pathogens such as viruses, bacteria, fungi, protozoa, nematodes, and insects can also create stress conditions in plants, leading to the emergence of disease or deficiency symptoms. While these symptoms are often typical of abiotic or biotic stress, however, there are instances where they can intensify under specific conditions. Here, we primarily summarize the viral interactions with plants during abiotic stress to understand how these associations are linked together during viral pathogenesis. Secondly, focus is given to the beneficial effects of root-associated symbiotic bacteria in fulfilling the basic needs of plants during normal as well as abiotic stress conditions. The modulations of plant functional proteins, and their occurrence/cross-talk, with pathogen (virus) and symbiont (bacteria) molecules are also discussed. Furthermore, we have highlighted the biochemical and systematic adaptations that develop in plants due to bacterial symbiosis to encounter stress hallmarks. Lastly, directions are provided towards exploring potential rhizospheric bacteria to maintain plant-microbes ecosystem and manage abiotic stress in plants to achieve better trait health in the horticulture crops.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

The Potyviral Protein 6K2 from Turnip Mosaic Virus Increases Plant Resilience to Drought

Virus infection can increase drought tolerance of infected plants compared with noninfected plants; however, the mechanisms mediating virus-induced drought tolerance remain unclear. In this study, we demonstrate turnip mosaic virus (TuMV) infection increases Arabidopsis thaliana survival under drought compared with uninfected plants. To determine if specific TuMV proteins mediate drought tolerance, we cloned the coding sequence for each of the major viral proteins and generated transgenic A. thaliana that constitutively express each protein. Three TuMV proteins, 6K1, 6K2, and NIa-Pro, enhanced drought tolerance of A. thaliana when expressed constitutively in plants compared with controls. While in the control plant, transcripts related to abscisic acid (ABA) biosynthesis and ABA levels were induced under drought, there were no changes in ABA or related transcripts in plants expressing 6K2 under drought compared with well-watered conditions. Expression of 6K2 also conveyed drought tolerance in another host plant, Nicotiana benthamiana, when expressed using a virus overexpression construct. In contrast to ABA, 6K2 expression enhanced salicylic acid (SA) accumulation in both Arabidopsis and N. benthamiana. These results suggest 6K2-induced drought tolerance is mediated through increased SA levels and SA-dependent induction of plant secondary metabolites, osmolytes, and antioxidants that convey drought tolerance. [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.

Heat-Killed Tobacco Mosaic Virus Mitigates Plant Abiotic Stress Symptoms

Since the discovery of the tobacco mosaic virus in the 1890s, awareness has grown in regard to how viruses affect the environment. Viral infections are now known to cause various effects besides pathogenicity, with some viruses in fact having a beneficial impact on plants. Although research has focused on disease-causing viruses that can infect plants, many wild plants are also infected with non-pathogenic viral agents. Traditionally, abiotic, and biotic stresses have been studied as isolated stimuli that trigger signaling pathways within the plant. However, both biotic and abiotic stress can trigger complex molecular interactions within plants, which in turn drive interconnected response pathways. Here, we demonstrate that heat-killed tobacco mosaic virus (TMV) can increase abiotic stress tolerance in plants, an effect that could potentially be implemented in challenging growth environments. To our knowledge, this is the first report of plant abiotic stress tolerance following treatment with heat-killed viral particles.

Drought stress affects interactions between potato plants, psyllid vectors, and a bacterial pathogen

Transmission of insect-borne pathogens is mediated by interactions between insects and plants across variable environments. Water stress, for example, affects the physiology, defense, chemistry, and nutritional balance of plants in ways that alter their tolerance to herbivores and pathogens. However, few studies have explored interactions between water stress and insect-borne pathogens as well as the molecular mechanisms mediating these interactions. Here, we address these knowledge gaps by assessing effects of plant water stress on the transmission of a bacterial pathogen, Candidatus Liberibacter solanacearum (CLs), by the vector Bactericera cockerelli Šulc (potato psyllid). We hypothesized that plant water stress would promote pathogen transmission by inducing plant gene transcripts and phytohormones involved in defense. Our results showed water stress was associated with decreased CLs titer with two psyllid haplotypes. Our analysis of plant gene transcripts suggested water stress affected phytohormone pathways in ways that altered plant tolerance to the CLs pathogen. Our study shows that abiotic stressors like drought may mediate the spread of plant pathogens by altering plant signaling pathways in ways that affect pathogen transmission.

Ecology of Yellow Dwarf Viruses in Crops and Grasslands: Interactions in the Context of Climate Change

Our understanding of the ecological interactions between plant viruses, their insect vectors, and their host plants has increased rapidly over the past decade. The suite of viruses known collectively as the yellow dwarf viruses infect an extensive range of cultivated and noncultivated grasses worldwide and is one of the best-studied plant virus systems. The yellow dwarf viruses are ubiquitous in cereal crops, where they can significantly limit yields, and there is growing recognition that they are also ubiquitous in grassland ecosystems, where they can influence community dynamics. Here, we discuss recent research that has explored (a) the extent and impact of yellow dwarf viruses in a diversity of plant communities, (b) the role of vector behavior in virus transmission, and (c) the prospects for impacts of climate change-including rising temperatures, drought, and elevated CO₂-on the epidemiology of yellow dwarf viruses.

Water Deficit Improves Reproductive Fitness in Nicotiana benthamiana Plants Infected by Cucumber mosaic virus

Plants are concurrently exposed to biotic and abiotic stresses, including infection by viruses and drought. Combined stresses result in plant responses that are different from those observed for each individual stress. We investigated compensatory effects induced by virus infection on the fitness of hosts grown under water deficit, and the hypothesis that water deficit improves tolerance, estimated as reproductive fitness, to virus infection. Our results show that infection by Turnip mosaic virus (TuMV) or Cucumber mosaic virus (CMV) promotes drought tolerance in Arabidopsis thaliana and Nicotiana benthamiana. However, neither CMV nor TuMV had a positive impact on host reproductive fitness following withdrawal of water, as determined by measuring the number of individuals producing seeds, seed grains, and seed germination rates. Importantly, infection by CMV but not by TuMV improved the reproductive fitness of N. benthamiana plants when exposed to drought compared to watered, virus-infected plants. However, no such conditional phenotype was found in Arabidopsis plants infected with CMV. Water deficit did not affect the capacity of infected plants to transmit CMV through seeds. These findings highlight a conditional improvement in biological efficacy of N. benthamiana plants infected with CMV under water deficit, and lead to the prediction that plants can exhibit increased tolerance to specific viruses under some of the projected climate change scenarios.

Influence of virus-host interactions on plant response to abiotic stress

Environmental factors play a significant role in controlling growth, development and defense responses of plants. Changes in the abiotic environment not only significantly alter the physiological and molecular pathways in plants, but also result in attracting the insect pests that carry a payload of viruses. Invasion of plants by viruses triggers the RNA silencing based defense mechanism in plants. In counter defense the viruses have gained the ability to suppress the host RNA silencing activities. A new paradigm has emerged, with the recognition that plant viruses also have the intrinsic capacity to modulate host plant response to environmental cues, in an attempt to favour their own survival. Thus, plant-virus interactions provide an excellent system to understand the signals in crosstalk between biotic (virus) and abiotic stresses. In this review, we have summarized the basal plant defense responses to pathogen invasion while emphasizing on the role of RNA silencing as a front line of defense response to virus infection. The emerging knowledge indicates overlap between RNA silencing with the innate immune responses during antiviral defense. The suppressors of RNA silencing serve as Avr proteins, which can be recognized by the host R proteins. The defense signals also function in concert with the phytohormones to influence plant responses to abiotic stresses. The current evidence on the role of virus induced host tolerance to abiotic stresses is also discussed.

Plant virus evolution under strong drought conditions results in a transition from parasitism to mutualism

Environmental conditions are an important factor driving pathogens' evolution. Here, we explore the effects of drought stress in plant virus evolution. We evolved turnip mosaic potyvirus in well-watered and drought conditions in Arabidopsis thaliana accessions that differ in their response to virus infection. Virus adaptation occurred in all accessions independently of watering status. Drought-evolved viruses conferred a significantly higher drought tolerance to infected plants. By contrast, nonsignificant increases in tolerance were observed in plants infected with viruses evolved under standard watering. The magnitude of this effect was dependent on the plant accessions. Differences in tolerance were correlated to alterations in the expression of host genes, some involved in regulation of the circadian clock, as well as in deep changes in the balance of phytohormones regulating defense and growth signaling pathways. Our results show that viruses can promote host survival in situations of abiotic stress, with the magnitude of such benefit being a selectable trait.

Yield Losses Caused by Barley Yellow Dwarf Virus-PAV Infection in Wheat and Barley: A Three-Year Field Study in South-Eastern Australia

Barley yellow dwarf virus (BYDV) is transmitted by aphids and significantly reduces the yield and quality of cereals worldwide. Four experiments investigating the effects of barley yellow dwarf virus-PAV (BYDV-PAV) infection on either wheat or barley were conducted over three years (2015, 2017, and 2018) under typical field conditions in South-Eastern Australia. Plants inoculated with BYDV-PAV using viruliferous aphids (Rhopalosiphum padi) were harvested at maturity then grain yield and yield components were measured. Compared to the non-inoculated control, virus infection severely reduced grain yield by up to 84% (1358 kg/ha) in wheat and 64% (1456 kg/ha) in barley. The yield component most affected by virus infection was grain number, which accounted for a large proportion of the yield loss. There were no significant differences between early (seedling stage) and later (early-tillering stage) infection for any of the parameters measured (plant height, biomass, yield, grain number, 1000-grain weight or grain size) for either wheat or barley. Additionally, this study provides an estimated yield loss value, or impact factor, of 0.91% (72 kg/ha) for each one percent increase in natural BYDV-PAV background infection. Yield losses varied considerably between experiments, demonstrating the important role of cultivar and environmental factors in BYDV epidemiology and highlighting the importance of conducting these experiments under varying conditions for specific cultivar–vector–virus combinations.

The Epidemiology of Plant Virus Disease: Towards a New Synthesis

Epidemiology is the science of how disease develops in populations, with applications in human, animal and plant diseases. For plant diseases, epidemiology has developed as a quantitative science with the aims of describing, understanding and predicting epidemics, and intervening to mitigate their consequences in plant populations. Although the central focus of epidemiology is at the population level, it is often necessary to recognise the system hierarchies present by scaling down to the individual plant/cellular level and scaling up to the community/landscape level. This is particularly important for diseases caused by plant viruses, which in most cases are transmitted by arthropod vectors. This leads to range of virus-plant, virus-vector and vector-plant interactions giving a distinctive character to plant virus epidemiology (whilst recognising that some fungal, oomycete and bacterial pathogens are also vector-borne). These interactions have epidemiological, ecological and evolutionary consequences with implications for agronomic practices, pest and disease management, host resistance deployment, and the health of wild plant communities. Over the last two decades, there have been attempts to bring together these differing standpoints into a new synthesis, although this is more apparent for evolutionary and ecological approaches, perhaps reflecting the greater emphasis on shorter often annual time scales in epidemiological studies. It is argued here that incorporating an epidemiological perspective, specifically quantitative, into this developing synthesis will lead to new directions in plant virus research and disease management. This synthesis can serve to further consolidate and transform epidemiology as a key element in plant virus research.

Identification of an Emaravirus in a Common Oak (Quercus robur L.) Conservation Seed Orchard in Germany: Implications for Oak Health

We observed the health status of oak trees in a conservation seed orchard for over twenty years, focusing on characteristic virus-suspected symptoms. The orchard was established in 1992 in Kreuztal, North Rhine-Westphalia (Germany) with 1302 seedlings in 186 clusters. The number of seedlings showing chlorotic ringspots and mottle on leaves has fluctuated annually, but has increased from 3.3% to 12.1% in the last 20 years; the number of affected clusters has risen from 8% to 25.9%. A scientific breakthrough was the identification of a novel virus related to members of the genus Emaravirus in diseased oak by high-throughput sequencing (HTS). Screening of the oak seedlings in three consecutive years, using a newly established virus-specific diagnostic reverse transcription polymerase chain reaction (RT-PCR), confirmed the virus infection and revealed a close to 100% association between the observed leaf symptoms and the novel virus. As no other plant virus could be identified in the HTS-datasets, we assume the novel virus is primarily causing the symptoms. To reliably detect the novel virus in oaks, RT-PCR targeting the viral RNA3 or RNA4 should be applied in routine testing of symptomatic leaf tissue. It was obvious that most groups with virus-infected plants cluster, with only five out of the 42 affected groups being offside, not bordering on other affected groups of plants. There was no clear correlation between the detection of the virus and the overall vitality of the seedlings. There was no relation between seedling performance and presence or absence of viral infection. Forecasts on the future growth behavior of these virus-infected oak trees are therefore not possible.

Climate Disruption of Plant-Microbe Interactions

Interactions between plants and microbes have important influences on evolutionary processes, population dynamics, community structure, and ecosystem function. We review the literature to document how climate change may disrupt these ecological interactions and develop a conceptual framework to integrate the pathways of plant-microbe responses to climate over different scales in space and time. We then create a blueprint to aid generalization that categorizes climate effects into changes in the context dependency of plant-microbe pairs, temporal mismatches and altered feedbacks over time, or spatial mismatches that accompany species range shifts. We pair a new graphical model of how plant-microbe interactions influence resistance to climate change with a statistical approach to predictthe consequences of increasing variability in climate. Finally, we suggest pathways through which plant-microbe interactions can affect resilience during recovery from climate disruption. Throughout, we take a forward-looking perspective, highlighting knowledge gaps and directions for future research.

Climate change and plant virus epidemiology

Changes in global climate driven by anthropogenic activities, especially the burning of fossil fuels and deforestation, have been progressively increasing and are projected to intensify. Increasing concentrations of atmospheric carbon dioxide and temperature will have significant consequences for future food production, quality, distribution and security. The epidemiology of plant viruses will be altered in the future as a result of climate change. Elevated atmospheric carbon dioxide, increased temperature, changes to water availability and more frequent extreme weather events will have direct and indirect effects on plant viruses through changes in hosts and vectors. Predicted climatic changes will affect the distribution and survival of plant viruses and their vectors, which are expected to increase in many geographic regions. Furthermore, climate change can affect the virulence and pathogenicity of plant viruses, consequently increasing the frequency and scale of disease outbreaks. Thus, greater understanding of plant virus epidemiology is needed to better anticipate challenges ahead and to develop effective and robust control strategies that will aid in securing global food production for the future.

Natural variation of Arabidopsis thaliana responses to Cauliflower mosaic virus infection upon water deficit

Plant virus pathogenicity is expected to vary with changes in the abiotic environment that affect plant physiology. Conversely, viruses can alter the host plant response to additional stimuli from antagonism to mutualism depending on the virus, the host plant and the environment. Ecological theory, specifically the CSR framework of plant strategies developed by Grime and collaborators, states that plants cannot simultaneously optimize resistance to both water deficit and pathogens. Here, we investigated the vegetative and reproductive performance of 44 natural accessions of A. thaliana originating from the Iberian Peninsula upon simultaneous exposure to soil water deficit and viral infection by the Cauliflower mosaic virus (CaMV). Following the predictions of Grime's CSR theory, we tested the hypothesis that the ruderal character of a plant genotype is positively related to its tolerance to virus infection regardless of soil water availability. Our results showed that CaMV infection decreased plant vegetative performance and annihilated reproductive success of all accessions. In general, water deficit decreased plant performance, but, despite differences in behavior, ranking of accessions tolerance to CaMV was conserved under water deficit. Ruderality, quantified from leaf traits following a previously published procedure, varied significantly among accessions, and was positively correlated with tolerance to viral infection under both well-watered and water deficit conditions, although the latter to a lesser extent. Also, in accordance with the ruderal character of the accession and previous findings, our results suggest that accession tolerance to CaMV infection is positively correlated with early flowering. Finally, plant survival to CaMV infection increased under water deficit. The complex interactions between plant, virus and abiotic environment are discussed in terms of the variation in plant ecological strategies at the intraspecific level.

Impact of Abiotic Stresses on Plant Virus Transmission by Aphids

Plants regularly encounter abiotic constraints, and plant response to stress has been a focus of research for decades. Given increasing global temperatures and elevated atmospheric CO2 levels and the occurrence of water stress episodes driven by climate change, plant biochemistry, in particular, plant defence responses, may be altered significantly. Environmental factors also have a wider impact, shaping viral transmission processes that rely on a complex set of interactions between, at least, the pathogen, the vector, and the host plant. This review considers how abiotic stresses influence the transmission and spread of plant viruses by aphid vectors, mainly through changes in host physiology status, and summarizes the latest findings in this research field. The direct effects of climate change and severe weather events that impact the feeding behaviour of insect vectors as well as the major traits (e.g., within-host accumulation, disease severity and transmission) of viral plant pathogens are discussed. Finally, the intrinsic capacity of viruses to react to environmental cues in planta and how this may influence viral transmission efficiency is summarized. The clear interaction between biotic (virus) and abiotic stresses is a risk that must be accounted for when modelling virus epidemiology under scenarios of climate change.

Infection with an asymptomatic virus in rice results in a delayed drought response

Infection of viruses in plants often modifies plant responses to biotic and abiotic stresses. In the present study we examined the effects of Rice tungro spherical virus (RTSV) infection on drought response in rice. RTSV infection delayed the onset of leaf rolling by 1-2 days. During the delay in drought response, plants infected with RTSV showed higher stomatal conductance and less negative leaf water potential under drought than those of uninfected plants, indicating that RTSV-infected leaves were more hydrated. Other growth and physiological traits of plants under drought were not altered by infection with RTSV. An expression analysis of genes for drought response-related transcription factors showed that the expression of OsNAC6 and OsDREB2a was less activated by drought in RTSV-infected plants than in uninfected plants, further suggesting improved water status of the plants due to RTSV infection. RTSV accumulated more in plants under drought than in well-watered plants, indicating the increased susceptibility of rice plants to RTSV infection by drought. Collectively, these results indicated that infection with RTSV can transiently mitigate the influence of drought stress on rice plants by increasing leaf hydration, while drought increased the susceptibility of rice plants to RTSV.

Modelling and manipulation of aphid-mediated spread of non-persistently transmitted viruses

Aphids vector many plant viruses in a non-persistent manner i.e., virus particles bind loosely to the insect mouthparts (stylet). This means that acquisition of virus particles from infected plants, and inoculation of uninfected plants by viruliferous aphids, are rapid processes that require only brief probes of the plant's epidermal cells. Virus infection alters plant biochemistry, which causes changes in emission of volatile organic compounds and altered accumulation of nutrients and defence compounds in host tissues. These virus-induced biochemical changes can influence the migration, settling and feeding behaviours of aphids. Working mainly with cucumber mosaic virus and several potyviruses, a number of research groups have noted that in some plants, virus infection engenders resistance to aphid settling (sometimes accompanied by emission of deceptively attractive volatiles, that can lead to exploratory penetration by aphids without settling). However, in certain other hosts, virus infection renders plants more susceptible to aphid colonisation. It has been suggested that induction of resistance to aphid settling encourages transmission of non-persistently transmitted viruses, while induction of susceptibility to settling retards transmission. However, recent mathematical modelling indicates that both virus-induced effects contribute to epidemic development at different scales. We have also investigated at the molecular level the processes leading to induction, by cucumber mosaic virus, of feeding deterrence versus susceptibility to aphid infestation. Both processes involve complex interactions between specific viral proteins and host factors, resulting in manipulation or suppression of the plant's immune networks.

Genomic regions on chromosome 5H containing a novel QTL conferring barley yellow dwarf virus-PAV (BYDV-PAV) tolerance in barley

Barley yellow dwarf virus is a widespread disease affecting plant growth and yield in cereal crops including barley. Complete resistance to BYDV encoded by a single gene is lacking in barley. To identify novel resistance genes that can be further utilised in breeding for plant disease resistance, a doubled haploid population originated from a cultivated barley with a known resistance gene and a wild barley was constructed and assessed for barley yellow dwarf tolerance in three trials with two in Tasmania (TAS) and one in Western Australia (WA). We identified two Quantitative trait loci (QTL) in both Tasmanian trials, and four QTL in Western Australian trial. Two QTL from TAS trials were also detected from WA. The QTL on chromosome 3H corresponds to the known major resistance gene Ryd2. The other QTL, Qbyd-5H, represents a potential new resistance locus and contributed 7.0~10.4% of total phenotypic variation in the three trials. It was mapped within the interval of 125.76~139.24 cM of chromosome 5H. Two additional minor effect QTL were identified on chromosome 7H from WA trial, contributing slightly less effect on BYD tolerance. The consistently detected new gene on chromosome 5H will potentially serve as a novel source of tolerance to achieve more sustainable resistance to BYDV in barley.

Evolution and ecology of plant viruses

The discovery of the first non-cellular infectious agent, later determined to be tobacco mosaic virus, paved the way for the field of virology. In the ensuing decades, research focused on discovering and eliminating viral threats to plant and animal health. However, recent conceptual and methodological revolutions have made it clear that viruses are not merely agents of destruction but essential components of global ecosystems. As plants make up over 80% of the biomass on Earth, plant viruses likely have a larger impact on ecosystem stability and function than viruses of other kingdoms. Besides preventing overgrowth of genetically homogeneous plant populations such as crop plants, some plant viruses might also promote the adaptation of their hosts to changing environments. However, estimates of the extent and frequencies of such mutualistic interactions remain controversial. In this Review, we focus on the origins of plant viruses and the evolution of interactions between these viruses and both their hosts and transmission vectors. We also identify currently unknown aspects of plant virus ecology and evolution that are of practical importance and that should be resolvable in the near future through viral metagenomics.

Viruses in the phytobiome

The phytobiome, defined as plants and all the entities that interact with them, is rich in viruses, but with the exception of plant viruses of crop plants, most of the phytobiome viruses remain very understudied. This review focuses on the neglected portions of the phytobiome, including viruses of other microbes interacting with plants, viruses in the soil, viruses of wild plants, and relationships between viruses and the vectors of plant viruses.

Addressing Research Needs in the Field of Plant Virus Ecology by Defining Knowledge Gaps and Developing Wild Dicot Study Systems

Viruses are ubiquitous within all habitats that support cellular life and represent the most important emerging infectious diseases of plants. Despite this, it is only recently that we have begun to describe the ecological roles of plant viruses in unmanaged systems and the influence of ecosystem properties on virus evolution. We now know that wild plants frequently harbor infections by diverse virus species, but much remains to be learned about how viruses influence host traits and how hosts influence virus evolution and vector interactions. To identify knowledge gaps and suggest avenues for alleviating research deficits, we performed a quantitative synthesis of a representative sample of virus ecology literature, developed criteria for expanding the suite of pathosystems serving as models, and applied these criteria through a case study. We found significant gaps in the types of ecological systems studied, which merit more attention. In particular, there is a strong need for a greater diversity of logistically tractable, wild dicot perennial study systems suitable for experimental manipulations of infection status. Based on criteria developed from our quantitative synthesis, we evaluated three California native dicot perennials typically found in Mediterranean-climate plant communities as candidate models: Cucurbita foetidissima (buffalo gourd), Cucurbita palmata (coyote gourd), and Datura wrightii (sacred thorn-apple). We used Illumina sequencing and network analyses to characterize viromes and viral links among species, using samples taken from multiple individuals at two different reserves. We also compared our Illumina workflow with targeted RT-PCR detection assays of varying costs. To make this process accessible to ecologists looking to incorporate virology into existing studies, we describe our approach in detail and discuss advantages and challenges of different protocols. We also provide a bioinformatics workflow based on open-access tools with graphical user interfaces. Our study provides evidence that dicot perennials in xeric habitats support multiple, asymptomatic infections by viruses known to be pathogenic in related crop hosts. Quantifying the impacts of these interactions on plant performance and virus epidemiology in our logistically tractable host systems will provide fundamental information about plant virus ecology outside of crop environments.

Viral Manipulation of Plant Stress Responses and Host Interactions With Insects

Do the alterations in plant defensive signaling and metabolism that occur in susceptible hosts following virus infection serve any purpose beyond directly aiding viruses to replicate and spread? Or indeed, are these modifications to host phenotype purely incidental consequences of virus infection? A growing body of data, in particular from studies of viruses vectored by whiteflies and aphids, indicates that viruses influence the efficiency of their own transmission by insect vectors and facilitate mutualistic relationships between viruses and their insect vectors. Furthermore, it appears that viruses may be able to increase the opportunity for transmission in the long term by providing reward to the host plants that they infect. This may be conditional, for example, by aiding host survival under conditions of drought or cold or, more surprisingly, by helping plants attract beneficial insects such as pollinators. In this chapter, we cover three main areas. First, we describe the molecular-level interactions governing viral manipulation of host plant biology. Second, we review evidence that virus-induced changes in plant phenotype enhance virus transmission. Finally, we discuss how direct and indirect manipulation of insects and plants might impact on the evolution of viruses and their hosts.

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.

Interactions Between Drought and Plant Genotype Change Epidemiological Traits of Cauliflower mosaic virus

Plants suffer from a broad range of abiotic and biotic stresses that do not occur in isolation but often simultaneously. Productivity of natural and agricultural systems is frequently constrained by water limitation, and the frequency and duration of drought periods will likely increase due to global climate change. In addition, phytoviruses represent highly prevalent biotic threat in wild and cultivated plant species. Several hints support a modification of epidemiological parameters of plant viruses in response to environmental changes but a clear quantification of plant-virus interactions under abiotic stresses is still lacking. Here we report the effects of a water deficit on epidemiological parameters of Cauliflower mosaic virus (CaMV), a non-circulative virus transmitted by aphid vectors, in nine natural accessions of Arabidopsis thaliana with known contrasted responses to water deficit. Plant growth-related traits and virus epidemiological parameters were evaluated in PHENOPSIS, an automated high throughput phenotyping platform. Water deficit had contrasted effects on CaMV transmission rate and viral load among A. thaliana accessions. Under well-watered conditions, transmission rate tended to increase with viral load and with CaMV virulence across accessions. Under water deficit, transmission rate and virulence were negatively correlated. Changes in the rate of transmission under water deficit were not related to changes in viral load. Our results support the idea that optimal virulence of a given virus, as hypothesized under the transmission-virulence trade-off, is highly dependent on the environment and growth traits of the host.

Exploring how viruses enhance plants' resilience to drought and the limits to this form of viral payback

This article comments on: Virulence determines beneficial trade-offs in the response of virus-infected plants to drought via induction of salicylic acid.

Symbiosis: Viruses as Intimate Partners

Viruses must establish an intimate relationship with their hosts and vectors in order to infect, replicate, and disseminate; hence, viruses can be considered as symbionts with their hosts. Symbiotic relationships encompass different lifestyles, including antagonistic (or pathogenic, the most well-studied lifestyle for viruses), commensal (probably the most common lifestyle), and mutualistic (important beneficial partners). Symbiotic relationships can shape the evolution of the partners in a holobiont, and placing viruses in this context provides an important framework for understanding virus-host relationships and virus ecology. Although antagonistic relationships are thought to lead to coevolution, this is not always clear in virus-host interactions, and impacts on evolution may be complex. Commensalism implies a hitchhiking role for viruses-selfish elements just along for the ride. Mutualistic relationships have been described in detail in the past decade, and they reveal how important viruses are in considering host ecology. Ultimately, symbiosis can lead to symbiogenesis, or speciation through fusion, and the presence of large amounts of viral sequence in the genomes of everything from bacteria to humans, including some important functional genes, illustrates the significance of viral symbiogenesis in the evolution of all life on Earth.

Crop-associated virus reduces the rooting depth of non-crop perennial native grass more than non-crop-associated virus with known viral suppressor of RNA silencing (VSR)

As agricultural acreage expanded and came to dominate landscapes across the world, viruses gained opportunities to move between crop and wild native plants. In the Midwestern USA, virus exchange currently occurs between widespread annual Poaceae crops and remnant native perennial prairie grasses now under consideration as bioenergy feedstocks. In this region, the common aphid species Rhopalosiphum padi L. (the bird cherry-oat aphid) transmits several virus species in the family Luteoviridae, including Barley yellow dwarf virus (BYDV-PAV, genus Luteovirus) and Cereal yellow dwarf virus (CYDV-RPV and -RPS, genus Polerovirus). The yellow dwarf virus (YDV) species in these two genera share genetic similarities in their 3'-ends, but diverge in the 5'-regions. Most notably, CYDVs encode a P0 viral suppressor of RNA silencing (VSR) absent in BYDV-PAV. Because BYDV-PAV has been reported more frequently in annual cereals and CYDVs in perennial non-crop grasses, we examine the hypothesis that the viruses' genetic differences reflect different affinities for crop and non-crop hosts. Specifically, we ask (i) whether CYDVs might persist within and affect a native non-crop grass more strongly than BYDV-PAV, on the grounds that the polerovirus VSR could better moderate the defenses of a well-defended perennial, and (ii) whether the opposite pattern of effects might occur in a less defended annual crop. Because previous work found that the VSR of CYDV-RPS possessed greater silencing suppressor efficiency than that of CYDV-RPV, we further explored (iii) whether a novel grass-associated CYDV-RPS isolate would influence a native non-crop grass more strongly than a comparable CYDV-RPV isolate. In growth chamber studies, we found support for this hypothesis: only grass-associated CYDV-RPS stunted the shoots and crowns of Panicum virgatum L. (switchgrass), a perennial native North American prairie grass, whereas crop-associated BYDV-PAV (and coinfection with BYDV-PAV and CYDV-RPS) most stunted annual Avena sativa L. (oats). These findings suggest that some of the diversity in grass-infecting Luteoviridae reflects viral capacity to modulate defenses in different host types. Intriguingly, while all virus treatments also reduced root production in both host species, only crop-associated BYDV-PAV (or co-infection) reduced rooting depths. Such root effects may increase host susceptibility to drought, and indicate that BYDV-PAV pathogenicity is determined by something other than a P0 VSR. These findings contribute to growing evidence that pathogenic crop-associated viruses may harm native species as well as crops. Critical next questions include the extent to which crop-associated selection pressures drive viral pathogenesis.

Vector and nonvector insect feeding reduces subsequent plant susceptibility to virus transmission

The interactions of vector-virus-plant have important ecological and evolutionary implications. While the tripartite interactions have received some attention, little is known about whether vector infestation affects subsequent viral transmission and infection. Working with the whitefly Bemisia tabaci, begomovirus and tobacco/tomato, we demonstrate that pre-infestation of plants by the whitefly vector reduced subsequent plant susceptibility to viral transmission. Pre-infestation by the cotton bollworm, a nonvector of the virus, likewise repressed subsequent viral transmission. The two types of insects, with piercing and chewing mouthparts, respectively, activated different plant signaling pathways in the interactions. Whitefly pre-infestation activated the salicylic acid (SA) signaling pathway, leading to deposition of callose that inhibited begomovirus replication/movement. Although cotton bollworm infestation elicited the jasmonic acid (JA) defense pathway and was beneficial to virus replication, the pre-infested plants repelled whiteflies from feeding and so decreased virus transmission. Experiments using a pharmaceutical approach with plant hormones or a genetic approach using hormone transgenic or mutant plants further showed that SA played a negative but JA played a positive role in begomovirus infection. These novel findings indicate that both vector and nonvector insect feeding of a plant may have substantial negative consequences for ensuing viral transmission and infection.

Plant Water Stress Affects Interactions Between an Invasive and a Naturalized Aphid Species on Cereal Crops

In cereal cropping systems of the Pacific Northwestern United States (PNW), climate change is projected to increase the frequency of drought during summer months, which could increase water stress for crop plants. Yet, it remains uncertain how interactions between herbivore species are affected by drought stress. Here, interactions between two cereal aphids present in PNW cereal systems, Metopolophium festucae (Theobald) subsp. cerealium (a newly invasive species) and Rhopalosiphum padi L. (a naturalized species), were tested relative to wheat water stress. When aphids were confined in leaf cages on wheat, asymmetrical facilitation occurred; per capita fecundity of R. padi was increased by 46% when M. festucae cerealium was also present, compared to when only R. padi was present. Imposed water stress did not influence this interaction. When aphids were confined on whole wheat plants, asymmetrical competition occurred; cocolonization inhibited M. festucae cerealium population growth but did not affect R. padi population growth. Under conditions of plant water stress, however, the inhibitory effect of R. padi on M. festucae cerealium was not observed. We conclude that beneficial effects of cocolonization on R. padi are due to a localized plant response to M. festucae cerealium feeding, and that cocolonization of plants is likely to suppress M. festucae cerealium populations under ample water conditions, but not when plants are water stressed. This suggests that plant responses to water stress alter the outcome of competition between herbivore species, with implications for the structure of pest communities on wheat during periods of drought.

Water deficit enhances the transmission of plant viruses by insect vectors

Drought is a major threat to crop production worldwide and is accentuated by global warming. Plant responses to this abiotic stress involve physiological changes overlapping, at least partially, the defense pathways elicited both by viruses and their herbivore vectors. Recently, a number of theoretical and empirical studies anticipated the influence of climate changes on vector-borne viruses of plants and animals, mainly addressing the effects on the virus itself or on the vector population dynamics, and inferring possible consequences on virus transmission. Here, we directly assess the effect of a severe water deficit on the efficiency of aphid-transmission of the Cauliflower mosaic virus (CaMV) or the Turnip mosaic virus (TuMV). For both viruses, our results demonstrate that the rate of vector-transmission is significantly increased from water-deprived source plants: CaMV transmission reproducibly increased by 34% and that of TuMV by 100%. In both cases, the enhanced transmission rate could not be explained by a higher virus accumulation, suggesting a more complex drought-induced process that remains to be elucidated. The evidence that infected plants subjected to drought are much better virus sources for insect vectors may have extensive consequences for viral epidemiology, and should be investigated in a wide range of plant-virus-vector systems.

The evolution of parasitic and mutualistic plant-virus symbioses through transmission-virulence trade-offs

Virus-plant interactions range from parasitism to mutualism. Viruses have been shown to increase fecundity of infected plants in comparison with uninfected plants under certain environmental conditions. Increased fecundity of infected plants may benefit both the plant and the virus as seed transmission is one of the main virus transmission pathways, in addition to vector transmission. Trade-offs between vertical (seed) and horizontal (vector) transmission pathways may involve virulence, defined here as decreased fecundity in infected plants. To better understand plant-virus symbiosis evolution, we explore the ecological and evolutionary interplay of virus transmission modes when infection can lead to an increase in plant fecundity. We consider two possible trade-offs: vertical seed transmission vs infected plant fecundity, and horizontal vector transmission vs infected plant fecundity (virulence). Through mathematical models and numerical simulations, we show (1) that a trade-off between virulence and vertical transmission can lead to virus extinction during the course of evolution, (2) that evolutionary branching can occur with subsequent coexistence of mutualistic and parasitic virus strains, and (3) that mutualism can out-compete parasitism in the long-run. In passing, we show that ecological bi-stability is possible in a very simple discrete-time epidemic model. Possible extensions of this study include the evolution of conditional (environment-dependent) mutualism in plant viruses.

The Effects of Bean Leafroll Virus on Life History Traits and Host Selection Behavior of Specialized Pea Aphid (Acyrthosiphon pisum, Hemiptera: Aphididae) Genotypes

Intraspecific specialization by insect herbivores on different host plant species contributes to the formation of genetically distinct "host races," but the effects of plant virus infection on interactions between specialized herbivores and their host plants have barely been investigated. Using three genetically and phenotypically divergent pea aphid clones (Acyrthosiphon pisum L.) adapted to either pea (Pisum sativum L.) or alfalfa (Medicago sativa L.), we tested how infection of these hosts by an insect-borne phytovirus (Bean leafroll virus; BLRV) affects aphid performance and preference. Four important findings emerged: 1) mean aphid survival rate and intrinsic rate of population growth (Rm) were increased by 15% and 14%, respectively, for aphids feeding on plants infected with BLRV; 2) 34% of variance in survival rate was attributable to clone × host plant interactions; 3) a three-way aphid clone × host plant species × virus treatment significantly affected intrinsic rates of population growth; and 4) each clone exhibited a preference for either pea or alfalfa when choosing between noninfected host plants, but for two of the three clones tested these preferences were modestly reduced when selecting among virus-infected host plants. Our studies show that colonizing BLRV-infected hosts increased A. pisum survival and rates of population growth, confirming that the virus benefits A. pisum. BLRV transmission affected aphid discrimination of host plant species in a genotype-specific fashion, and we detected three unique "virus-association phenotypes," with potential consequences for patterns of host plant use by aphid populations and crop virus epidemiology.

Whitefly interactions with plants

Whiteflies are important pests of many crops worldwide. They are polyphagous and effectively feed on phloem sap using mouthparts modified into long, flexible stylets. Plants respond to whitefly attack by activating defense genes leading to production of toxic compounds. To reach plant phloem and survive on host plants, whiteflies secret effectors in the saliva to regulate plant responses and activate detoxification system to cope with plant defenses. Additionally, whitefly-transmitted viruses may exert substantial effects on host plants and in turn the performance of whiteflies. Understanding the interactions between whiteflies and host plants will promote the development of novel strategies for controlling whiteflies. Here, we summarize the genetics, molecular genetics and genomics of the whitefly's interactions with plants.

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

Viruses may induce changes in plant hosts and vectors to enhance their transmission. The white-backed planthopper (WBPH) and brown planthopper (BPH) are vectors of Southern rice black-streaked dwarf virus (SRBSDV) and Rice ragged stunt virus (RRSV), respectively, which cause serious rice diseases. We herein describe the effects of SRBSDV and RRSV infections on host-selection behaviour of vector and non-vector planthoppers at different disease stages. The Y-tube olfactometer choice and free-choice tests indicated that SRBSDV and RRSV infections altered the attractiveness of rice plants to vector and non-vector planthoppers. The attractiveness was mainly mediated by rice volatiles, and varied with disease progression. The attractiveness of the SRBSDV- or RRSV-infected rice plants to the virus-free WBPHs or BPHs initially decreased, then increased, and finally decreased again. For the viruliferous WBPHs and BPHs, SRBSDV or RRSV infection increased the attractiveness of plants more for the non-vector than for the vector planthoppers. Furthermore, we observed that the attractiveness of infected plants to planthoppers was positively correlated with the virus titres. The titre effects were greater for virus-free than for viruliferous planthoppers. Down-regulated defence genes OsAOS1, OsICS, and OsACS2 and up-regulated volatile-biosynthesis genes OsLIS, OsCAS, and OsHPL3 expression in infected plants may influence their attractiveness.

Manipulation of hosts and vectors by plant viruses and impact of the environment

The effect of environmental factors on the efficiency of plant virus transmission is extremely difficult to predict, because they obviously impact concomitantly multiple steps of the complex three-way plant-virus-vector interaction. This review summarizes the diversity of the relationship between plants, viruses and insect vectors, and highlights the numerous phases of this process that can be altered by the virus in ways that can potentially enhance its transmission success. Many of the reported cases are often considered to be possible viral manipulations acting through modifications of the physiology of the host plant, indirectly reaching to the insect vector. Plants are extremely responsive to environmental fluctuations and so interferences with these putative viral manipulations are highly expected. The role of environmental factors in plant virus transmission can thus be envisaged solely in the context of this complexity. It is only briefly evoked here because this field of research is in its infancy and currently suffers from an impressive lack of experimental data.

Effects of pathogens on sensory-mediated interactions between plants and insect vectors

Vector-borne plant pathogens frequently alter host-plant quality and associated plant cues in ways that influence vector recruitment and pathogen acquisition. Furthermore, following acquisition by the vector, pathogens may influence subsequent vector behavior either directly or via effects on the host plant. Given that such effects have significant implications for pathogen acquisition and inoculation, selection might be expected to favor patterns of pathogen effects on host-vector interactions that are conducive to transmission. Consequently, we might also expect to observe broad similarity in the effects of pathogens sharing similar modes of transmission. Here we discuss some specific hypotheses arising from these expectations and the implications of recent empirical findings. On the whole, this evidence is consistent with the expectation that pathogen effects on host-vector interactions are often (though not always) adaptive with respect to transmission.

Water Stress Modulates Soybean Aphid Performance, Feeding Behavior, and Virus Transmission in Soybean

Little is known about how water stress including drought and flooding modifies the ability of plants to resist simultaneous attack by insect feeding and transmission of insect-vectored pathogen. We analyzed insect population growth, feeding behaviors, virus transmission, and plant amino acid profiles and defense gene expression to characterize mechanisms underlying the interaction between water stress, soybean aphid and aphid-transmitted, Soybean mosaic virus, on soybean plants. Population growth of non-viruliferous aphids was reduced under drought stress and saturation, likely because the aphids spent less time feeding from the sieve element on these plants compared to well-watered plants. Water stress did not impact population growth of viruliferous aphids. However, virus incidence and transmission rate was lowest under drought stress and highest under saturated conditions since viruliferous aphids took the greatest amount time to puncture cells and transmit the virus under saturated conditions and lowest time under drought stress. Petiole exudates from drought-stressed plants had the highest level of total free amino acids including asparagine and valine that are critical for aphid performance. Aphids did not benefit from improved phloem sap quality as indicated by their lower densities on drought-stressed plants. Saturation, on the other hand, resulted in low amino acid content compared to all of the other treatments. Drought and saturation had significant and opposing effects on expression of marker genes involved in abscisic acid (ABA) signaling. Drought alone significantly increased expression of ABA marker genes, which likely led to suppression of salicylic acid (SA)- and jasmonic acid (JA)-related genes. In contrast, ABA marker genes were down-regulated under saturation, while expression of SA- and JA-related genes was up-regulated. We propose that the apparent antagonism between ABA and SA/JA signaling pathways contributed to an increase in aphid densities under drought and their decrease under saturation. Taken together, our findings suggests that plant responses to water stress is complex involving changes in phloem amino acid composition and signaling pathways, which can impact aphid populations and virus transmission.

Future Scenarios for Plant Virus Pathogens as Climate Change Progresses

Knowledge of how climate change is likely to influence future virus disease epidemics in cultivated plants and natural vegetation is of great importance to both global food security and natural ecosystems. However, obtaining such knowledge is hampered by the complex effects of climate alterations on the behavior of diverse types of vectors and the ease by which previously unknown viruses can emerge. A review written in 2011 provided a comprehensive analysis of available data on the effects of climate change on virus disease epidemics worldwide. This review summarizes its findings and those of two earlier climate change reviews and focuses on describing research published on the subject since 2011. It describes the likely effects of the full range of direct and indirect climate change parameters on hosts, viruses and vectors, virus control prospects, and the many information gaps and deficiencies. Recently, there has been encouraging progress in understanding the likely effects of some climate change parameters, especially over the effects of elevated CO2, temperature, and rainfall-related parameters, upon a small number of important plant viruses and several key insect vectors, especially aphids. However, much more research needs to be done to prepare for an era of (i) increasingly severe virus epidemics and (ii) increasing difficulties in controlling them, so as to mitigate their detrimental effects on future global food security and plant biodiversity.

Virus infection mediates the effects of elevated CO2 on plants and vectors

Atmospheric carbon dioxide (CO₂) concentration has increased significantly and is projected to double by 2100. To increase current food production levels, understanding how pests and diseases respond to future climate driven by increasing CO₂ is imperative. We investigated the effects of elevated CO₂ (eCO₂) on the interactions among wheat (cv. Yitpi), Barley yellow dwarf virus and an important pest and virus vector, the bird cherry-oat aphid (Rhopalosiphum padi), by examining aphid life history, feeding behavior and plant physiology and biochemistry. Our results showed for the first time that virus infection can mediate effects of eCO₂ on plants and pathogen vectors. Changes in plant N concentration influenced aphid life history and behavior, and N concentration was affected by virus infection under eCO₂. We observed a reduction in aphid population size and increased feeding damage on noninfected plants under eCO₂ but no changes to population and feeding on virus-infected plants irrespective of CO₂ treatment. We expect potentially lower future aphid populations on noninfected plants but no change or increased aphid populations on virus-infected plants therefore subsequent virus spread. Our findings underscore the complexity of interactions between plants, insects and viruses under future climate with implications for plant disease epidemiology and crop production.

Interactions between Metopolophium festucae cerealium (Hemiptera: Aphididae) and Barley yellow dwarf virus (BYDV-PAV)

Interactions between an invasive aphid, Metopolophium festucae (Theobald) subsp. cerealium, and Barley yellow dwarf virus (BYDV-PAV) were studied under laboratory conditions. M. festucae cerealium is an economic pest of wheat and barley that has recently been found in high population densities in wheat in the Pacific Northwest of the United States. BYDV-PAV is the most prevalent and injurious species of BYDV worldwide and in the Pacific Northwest. Although M. festucae sensu stricto (Theobald 1917) has been reported previously as a vector of some BYDV isolates, there is no confirmed transmission of BYDV by M. festucae cerealium. Two experiments examined the ability of M. festucae cerealium to transmit BYDV-PAV. The first used single aphids caged to indicator plants of a BYDV-susceptible winter wheat cultivar and the second used multiple aphids on each test plant. M. festucae cerealium did not transmit BYDV-PAV in either experiment, whereas transmission by a known BYDV vector, Rhopalosiphum padi L., was consistently high (≥ 93%). A third experiment compared the intrinsic growth rate, days until first reproduction and daily reproduction by M. festucae cerealium on sham-inoculated and BYDV-PAV-infected wheat, but detected no differences. The findings are reviewed in light published data on M. festucae species, BYDV transmission, and the potential pest status of this new invading aphid.

Plant Virus Diversity and Evolution

Historically, the majority of plant virology focused on agricultural systems. Recent efforts have expanded our knowledge of the true diversity of plant viruses by studying those viruses that infect wild, undomesticated plants. Those efforts have provided answers to basic ecological questions regarding viruses in the wild, and insights into evolutionary questions, regarding the origins of viruses. While much work has been done, we have merely scratched the surface of the diversity that is estimated to exist. In this chapter we discuss the state of our knowledge of virus diversity, both in agricultural systems as well as in native wild systems, the border between these two systems and how viruses adapt and move across this border into an artificial, domesticated environment. We look at how this diversity has affected our outlook on viruses as a whole, shifting our past view of viruses as purely antagonistic entities of destruction to one where viruses are in a mutually beneficial relationship with their hosts. Additionally, we discuss the current work that plant virology has put forth regarding the evolutionary mechanisms, the life histories, and the deep evolution of viruses.