Evidence of the biochemical basis of host virulence in the greenbug aphid, Schizaphis graminum (Homoptera: Aphididae)

Controlling Geminiviruses before Transmission: Prospects

Whitefly (Bemisia tabaci)-transmitted Geminiviruses cause serious diseases of crop plants in tropical and sub-tropical regions. Plants, animals, and their microbial symbionts have evolved complex ways to interact with each other that impact their life cycles. Blocking virus transmission by altering the biology of vector species, such as the whitefly, can be a potential approach to manage these devastating diseases. Virus transmission by insect vectors to plant hosts often involves bacterial endosymbionts. Molecular chaperonins of bacterial endosymbionts bind with virus particles and have a key role in the transmission of Geminiviruses. Hence, devising new approaches to obstruct virus transmission by manipulating bacterial endosymbionts before infection opens new avenues for viral disease control. The exploitation of bacterial endosymbiont within the insect vector would disrupt interactions among viruses, insects, and their bacterial endosymbionts. The study of this cooperating web could potentially decrease virus transmission and possibly represent an effective solution to control viral diseases in crop plants.

AcDCXR Is a Cowpea Aphid Effector With Putative Roles in Altering Host Immunity and Physiology

Cowpea, Vigna unguiculata, is a crop that is essential to semiarid areas of the world like Sub-Sahara Africa. Cowpea is highly susceptible to cowpea aphid, Aphis craccivora, infestation that can lead to major yield losses. Aphids feed on their host plant by inserting their hypodermal needlelike flexible stylets into the plant to reach the phloem sap. During feeding, aphids secrete saliva, containing effector proteins, into the plant to disrupt plant immune responses and alter the physiology of the plant to their own advantage. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was used to identify the salivary proteome of the cowpea aphid. About 150 candidate proteins were identified including diacetyl/L-xylulose reductase (DCXR), a novel enzyme previously unidentified in aphid saliva. DCXR is a member of short-chain dehydrogenases/reductases with dual enzymatic functions in carbohydrate and dicarbonyl metabolism. To assess whether cowpea aphid DCXR (AcDCXR) has similar functions, recombinant AcDCXR was purified and assayed enzymatically. For carbohydrate metabolism, the oxidation of xylitol to xylulose was tested. The dicarbonyl reaction involved the reduction of methylglyoxal, an α-β-dicarbonyl ketoaldehyde, known as an abiotic and biotic stress response molecule causing cytotoxicity at high concentrations. To assess whether cowpea aphids induce methylglyoxal in plants, we measured methylglyoxal levels in both cowpea and pea (Pisum sativum) plants and found them elevated transiently after aphid infestation. Agrobacterium-mediated transient overexpression of AcDCXR in pea resulted in an increase of cowpea aphid fecundity. Taken together, our results indicate that AcDCXR is an effector with a putative ability to generate additional sources of energy to the aphid and to alter plant defense responses. In addition, this work identified methylglyoxal as a potential novel aphid defense metabolite adding to the known repertoire of plant defenses against aphid pests.

Disruption of Chloroplast Function Through Downregulation of Phytoene Desaturase Enhances the Systemic Accumulation of an Aphid-Borne, Phloem-Restricted Virus

Chloroplasts play a central role in pathogen defense in plants. However, most studies explaining the relationship between pathogens and chloroplasts have focused on pathogens that infect mesophyll cells. In contrast, the family Luteoviridae includes RNA viruses that replicate and traffic exclusively in the phloem. Recently, our lab has shown that Potato leafroll virus (PLRV), the type species in the genus Polerovirus, forms an extensive interaction network with chloroplast-localized proteins that is partially dependent on the PLRV capsid readthrough domain (RTD). In this study, we used virus-induced gene silencing to disrupt chloroplast function and assess the effects on PLRV accumulation in two host species. Silencing of phytoene desaturase (PDS), a key enzyme in carotenoid, chlorophyll, and gibberellic acid (GA) biosynthesis, resulted in a substantial increase in the systemic accumulation of PLRV. This increased accumulation was attenuated when plants were infected with a viral mutant that does not express the RTD. Application of GA partially suppressed the increase in virus accumulation in PDS-silenced plants, suggesting that GA signaling also plays a role in limiting PLRV infection. In addition, the fecundity of the aphid vector of PLRV was increased when fed on PDS-silenced plants relative to PLRV-infected plants.

Mechanisms of aphid adaptation to host plant resistance

Host-plant resistant (HPR) crops can play a major role in preventing insect damage, but their durability is limited due to insect adaptation. Research in basal plant resistance provides a framework to investigate adaptation against HPR. Resistance and adaptation are predicted to follow the gene-for-gene and zigzag models of plant defense. These models also highlight the importance of insect effectors, which are small molecules that modulate host plant defense signaling. We highlight research in insect adaptation to plant resistance, and then draw parallels to virulence adaptation. We focus on virulent biotype evolution within the Aphididae, since this group has the highest number of described virulent biotypes. Understanding how virulence occurs will lead to more durable insect management strategies and enhance food production and security.

Aphid specialism as an example of ecological-evolutionary divergence

Debate still continues around the definition of generalism and specialism in nature. To some, generalism is equated solely with polyphagy, but this cannot be readily divorced from other essential biological factors, such as morphology, behaviour, genetics, biochemistry, chemistry and ecology, including chemical ecology. Viewed in this light, and accepting that when living organisms evolve to fill new ecological-evolutionary niches, this is the primal act of specialisation, then perhaps all living organisms are specialist in the broadest sense. To illustrate the levels of specialisation that may be found in a group of animals, we here provide an overview of those displayed by a subfamily of hemipteran insects, the Aphididae, which comprises some 1600 species/subspecies in Europe alone and whose members are specialised in a variety of lifestyle traits. These include life cycle, host adaptation, dispersal and migration, associations with bacterial symbionts (in turn related to host adaptation and resistance to hymenopterous wasp parasitoids), mutualisms with ants, and resistance to insecticides. As with polyphagy, these traits cannot easily be separated from one another, but rather, are interconnected, often highly so, which makes the Aphididae a fascinating animal group to study, providing an informative, perhaps unique, model to illustrate the complexities of defining generalism versus specialism.

Host Plants Indirectly Influence Plant Virus Transmission by Altering Gut Cysteine Protease Activity of Aphid Vectors

The green peach aphid, Myzus persicae, is a vector of the Potato leafroll virus (PLRV, Luteoviridae), transmitted exclusively by aphids in a circulative manner. PLRV transmission efficiency was significantly reduced when a clonal lineage of M. persicae was reared on turnip as compared with the weed physalis, and this was a transient effect caused by a host-switch response. A trend of higher PLRV titer in physalis-reared aphids as compared with turnip-reared aphids was observed at 24 h and 72 h after virus acquisition. The major difference in the proteomes of these aphids was the up-regulation of predicted lysosomal enzymes, in particular the cysteine protease cathepsin B (cathB), in aphids reared on turnip. The aphid midgut is the site of PLRV acquisition, and cathB and PLRV localization were starkly different in midguts of the aphids reared on the two host plants. In viruliferous aphids that were reared on turnip, there was near complete colocalization of cathB and PLRV at the cell membranes, which was not observed in physalis-reared aphids. Chemical inhibition of cathB restored the ability of aphids reared on turnip to transmit PLRV in a dose-dependent manner, showing that the increased activity of cathB and other cysteine proteases at the cell membrane indirectly decreased virus transmission by aphids. Understanding how the host plant influences virus transmission by aphids is critical for growers to manage the spread of virus among field crops.

Virulent Diuraphis noxia Aphids Over-Express Calcium Signaling Proteins to Overcome Defenses of Aphid-Resistant Wheat Plants

The Russian wheat aphid, Diuraphis noxia, an invasive phytotoxic pest of wheat, Triticum aestivum, and barley, Hordeum vulgare, causes huge economic losses in Africa, South America, and North America. Most acceptable and ecologically beneficial aphid management strategies include selection and breeding of D. noxia-resistant varieties, and numerous D. noxia resistance genes have been identified in T. aestivum and H. vulgare. North American D. noxia biotype 1 is avirulent to T. aestivum varieties possessing Dn4 or Dn7 genes, while biotype 2 is virulent to Dn4 and avirulent to Dn7. The current investigation utilized next-generation RNAseq technology to reveal that biotype 2 over expresses proteins involved in calcium signaling, which activates phosphoinositide (PI) metabolism. Calcium signaling proteins comprised 36% of all transcripts identified in the two D. noxia biotypes. Depending on plant resistance gene-aphid biotype interaction, additional transcript groups included those involved in tissue growth; defense and stress response; zinc ion and related cofactor binding; and apoptosis. Activation of enzymes involved in PI metabolism by D. noxia biotype 2 aphids allows depletion of plant calcium that normally blocks aphid feeding sites in phloem sieve elements and enables successful, continuous feeding on plants resistant to avirulent biotype 1. Inhibition of the key enzyme phospholipase C significantly reduced biotype 2 salivation into phloem and phloem sap ingestion.

Whole Genome Sequence of the Soybean Aphid Endosymbiont Buchnera aphidicola and Genetic Differentiation among Biotype-Specific Strains

Endosymbiosis with microorganisms is common in insects, with more than 10% of species requiring the metabolic capabilities of intracellular bacteria for their nutrient acquisition. Aphids harbor an obligate mutualism with the vertically transferred endosymbiont, Buchnera aphidicola, which produces key nutrients lacking in the aphid's phloem-based diet that are necessary for normal development and reproduction. It is thought that, in some groups of insects, bacterial symbionts may play key roles in biotype evolution against host-plant resistance. The genome of Buchnera has been sequenced in several aphid strains but little genomic data is currently available for the soybean aphid (Aphis glycines), one of the most important pests of soybean in North America. In this study, DNA sequencing was used to assemble and annotate the genome sequence of the Buchnera A. glycines strain and to reconstruct phylogenetic relationships among different strains. In addition, we identified several fixed Buchnera SNPs between Aphis glycines biotypes that were avirulent or virulent to a soybean aphid resistance gene (Rag1). The results of this study describe the genetic and evolutionary relationships of the Buchnera A. glycines strain, and begin to define the roles of an aphid symbiont in host-plant resistance.

Potato leafroll virus structural proteins manipulate overlapping, yet distinct protein interaction networks during infection

Potato leafroll virus (PLRV) produces a readthrough protein (RTP) via translational readthrough of the coat protein amber stop codon. The RTP functions as a structural component of the virion and as a nonincorporated protein in concert with numerous insect and plant proteins to regulate virus movement/transmission and tissue tropism. Affinity purification coupled to quantitative MS was used to generate protein interaction networks for a PLRV mutant that is unable to produce the read through domain (RTD) and compared to the known wild-type PLRV protein interaction network. By quantifying differences in the protein interaction networks, we identified four distinct classes of PLRV-plant interactions: those plant and nonstructural viral proteins interacting with assembled coat protein (category I); plant proteins in complex with both coat protein and RTD (category II); plant proteins in complex with the RTD (category III); and plant proteins that had higher affinity for virions lacking the RTD (category IV). Proteins identified as interacting with the RTD are potential candidates for regulating viral processes that are mediated by the RTP such as phloem retention and systemic movement and can potentially be useful targets for the development of strategies to prevent infection and/or viral transmission of Luteoviridae species that infect important crop species.

Insights into the polerovirus-plant interactome revealed by coimmunoprecipitation and mass spectrometry

Identification of host proteins interacting with the aphidborne Potato leafroll virus (PLRV) from the genus Polerovirus, family Luteoviridae, is a critical step toward understanding how PLRV and related viruses infect plants. However, the tight spatial distribution of PLRV to phloem tissues poses challenges. A polyclonal antibody raised against purified PLRV virions was used to coimmunoprecipitate virus-host protein complexes from Nicotiana benthamiana tissue inoculated with an infectious PLRV cDNA clone using Agrobacterium tumefaciens. A. tumefaciens-mediated delivery of PLRV enabled infection and production of assembled, insect-transmissible virus in most leaf cells, overcoming the dynamic range constraint posed by a systemically infected host. Isolated protein complexes were characterized using high-resolution mass spectrometry and consisted of host proteins interacting directly or indirectly with virions, as well as the nonincorporated readthrough protein (RTP) and three phosphorylated positional isomers of the RTP. A bioinformatics analysis using ClueGO and STRING showed that plant proteins in the PLRV protein interaction network regulate key biochemical processes, including carbon fixation, amino acid biosynthesis, ion transport, protein folding, and trafficking.

Is there a role for symbiotic bacteria in plant virus transmission by insects?

During the process of circulative plant virus transmission by insect vectors, viruses interact with different insect vector tissues prior to transmission to a new host plant. An area of intense debate in the field is whether bacterial symbionts of insect vectors are involved in the virus transmission process. We critically review the literature in this area and present a simple model that can be used to quantitatively settle the debate. The simple model determines whether the symbiont is involved in virus transmission and determines what fraction of the pathogen transmission phenotype is contributed by the symbiont. The model is general and can be applied to any vector-pathogen-symbiont interactions.

Circulative, "nonpropagative" virus transmission: an orchestra of virus-, insect-, and plant-derived instruments

Species of plant viruses within the Luteoviridae, Geminiviridae, and Nanoviridae are transmitted by phloem-feeding insects in a circulative, nonpropagative manner. The precise route of virus movement through the vector can differ across and within virus families, but these viruses all share many biological, biochemical, and ecological features. All share temporal and spatial constraints with respect to transmission efficiency. The viruses also induce physiological changes in their plant hosts resulting in behavioral changes in the insects that optimize the transmission of virus to new hosts. Virus proteins interact with insect, endosymbiont, and plant proteins to orchestrate, directly and indirectly, virus movement in insects and plants to facilitate transmission. Knowledge of these complex interactions allows for the development of new tools to reduce or prevent transmission, to quickly identify important vector populations, and to improve the management of these economically important viruses affecting agricultural and natural plant populations.

Evidence for lysine acetylation in the coat protein of a polerovirus

Virions of the RPV strain of Cereal yellow dwarf virus-RPV were purified from infected oat tissue and analysed by MS. Two conserved residues, K147 and K181, in the virus coat protein, were confidently identified to contain epsilon-N-acetyl groups. While no functional data are available for K147, K181 lies within an interfacial region critical for virion assembly and stability. The signature immonium ion at m/z 126.0919 demonstrated the presence of N-acetyllysine, and the sequence fragment ions enabled an unambiguous assignment of the epsilon-N-acetyl modification on K181. We hypothesize that selection favours acetylation of K181 in a fraction of coat protein monomers to stabilize the capsid by promoting intermonomer salt bridge formation.