Parasitic modulation of host development by ubiquitin-independent protein degradation.

Certain obligate parasites induce complex and substantial phenotypic changes in their hosts in ways that favor their transmission to other trophic levels. However, the mechanisms underlying these changes remain largely unknown. Here we demonstrate how SAP05 protein effectors from insect-vectored plant pathogenic phytoplasmas take control of several plant developmental processes. These effectors simultaneously prolong the host lifespan and induce witches' broom-like proliferations of leaf and sterile shoots, organs colonized by phytoplasmas and vectors. SAP05 acts by mediating the concurrent degradation of SPL and GATA developmental regulators via a process that relies on hijacking the plant ubiquitin receptor RPN10 independent of substrate ubiquitination. RPN10 is highly conserved among eukaryotes, but SAP05 does not bind insect vector RPN10. A two-amino-acid substitution within plant RPN10 generates a functional variant that is resistant to SAP05 activities. Therefore, one effector protein enables obligate parasitic phytoplasmas to induce a plethora of developmental phenotypes in their hosts.

Molecular Mechanisms Underlying Host Plant Specificity in Aphids.

Aphids are serious pests of agricultural and ornamental plants and important model systems for hemipteran-plant interactions. The long evolutionary history of aphids with their host plants has resulted in a variety of systems that provide insight into the different adaptation strategies of aphids to plants and vice versa. In the past, various plant-aphid interactions have been documented, but lack of functional tools has limited molecular studies on the mechanisms of plant-aphid interactions. Recent technological advances have begun to reveal plant-aphid interactions at the molecular level and to increase our knowledge of the mechanisms of aphid adaptation or specialization to different host plants. In this article, we compile and analyze available information on plant-aphid interactions, discuss the limitations of current knowledge, and argue for new research directions. We advocate for more work that takes advantage of natural systems and recently established molecular techniques to obtain a comprehensive view of plant-aphid interaction mechanisms.

A major-effect genetic locus, ApRVII, controlling resistance against both adapted and non-adapted aphid biotypes in pea.

KEY MESSAGE: A genome-wide association study for pea resistance against a pea-adapted biotype and a non-adapted biotype of the aphid, Acyrthosiphon pisum, identified a genomic region conferring resistance to both biotypes. In a context of reduced insecticide use, the development of cultivars resistant to insect pests is crucial for an integrated pest management. Pea (Pisum sativum) is a crop of major importance among cultivated legumes, for the supply of dietary proteins and nitrogen in low-input cropping systems. However, yields of the pea crop have become unstable due to plant parasites. The pea aphid (Acyrthosiphon pisum) is an insect pest species forming a complex of biotypes, each one adapted to feed on one or a few related legume species. This study aimed to identify resistance to A. pisum and the underlying genetic determinism by examining a collection of 240 pea genotypes. The collection was screened against a pea-adapted biotype and a non-adapted biotype of A. pisum to characterize their resistant phenotype. Partial resistance was observed in some pea genotypes exposed to the pea-adapted biotype. Many pea genotypes were completely resistant to non-adapted biotype, but some exhibited partial susceptibility. A genome-wide association study, using pea exome-capture sequencing data, enabled the identification of the major-effect quantitative trait locus ApRVII on the chromosome 7. ApRVII includes linkage disequilibrium blocks significantly associated with resistance to one or both of the two aphid biotypes studied. Finally, we identified candidate genes underlying ApRVII that are potentially involved in plant-aphid interactions and marker haplotypes linked with aphid resistance. This study sets the ground for the functional characterization of molecular pathways involved in pea defence to the aphids but also is a step forward for breeding aphid-resistant cultivars.

Phytoplasma SAP11 effector destabilization of TCP transcription factors differentially impact development and defence of Arabidopsis versus maize.

Phytoplasmas are insect-transmitted bacterial pathogens that colonize a wide range of plant species, including vegetable and cereal crops, and herbaceous and woody ornamentals. Phytoplasma-infected plants often show dramatic symptoms, including proliferation of shoots (witch's brooms), changes in leaf shapes and production of green sterile flowers (phyllody). Aster Yellows phytoplasma Witches' Broom (AY-WB) infects dicots and its effector, secreted AYWB protein 11 (SAP11), was shown to be responsible for the induction of shoot proliferation and leaf shape changes of plants. SAP11 acts by destabilizing TEOSINTE BRANCHED 1-CYCLOIDEA-PROLIFERATING CELL FACTOR (TCP) transcription factors, particularly the class II TCPs of the CYCLOIDEA/TEOSINTE BRANCHED 1 (CYC/TB1) and CINCINNATA (CIN)-TCP clades. SAP11 homologs are also present in phytoplasmas that cause economic yield losses in monocot crops, such as maize, wheat and coconut. Here we show that a SAP11 homolog of Maize Bushy Stunt Phytoplasma (MBSP), which has a range primarily restricted to maize, destabilizes specifically TB1/CYC TCPs. SAP11MBSP and SAP11AYWB both induce axillary branching and SAP11AYWB also alters leaf development of Arabidopsis thaliana and maize. However, only in maize, SAP11MBSP prevents female inflorescence development, phenocopying maize tb1 lines, whereas SAP11AYWB prevents male inflorescence development and induces feminization of tassels. SAP11AYWB promotes fecundity of the AY-WB leafhopper vector on A. thaliana and modulates the expression of A. thaliana leaf defence response genes that are induced by this leafhopper, in contrast to SAP11MBSP. Neither of the SAP11 effectors promote fecundity of AY-WB and MBSP leafhopper vectors on maize. These data provide evidence that class II TCPs have overlapping but also distinct roles in regulating development and defence in a dicot and a monocot plant species that is likely to shape SAP11 effector evolution depending on the phytoplasma host range.

Differential gene expression according to race and host plant in the pea aphid.

Host-race formation in phytophagous insects is thought to provide the opportunity for local adaptation and subsequent ecological speciation. Studying gene expression differences amongst host races may help to identify phenotypes under (or resulting from) divergent selection and their genetic, molecular and physiological bases. The pea aphid (Acyrthosiphon pisum) comprises host races specializing on numerous plants in the Fabaceae and provides a unique system for examining the early stages of diversification along a gradient of genetic and associated adaptive divergence. In this study, we examine transcriptome-wide gene expression both in response to environment and across pea aphid races selected to cover the range of genetic divergence reported in this species complex. We identify changes in expression in response to host plant, indicating the importance of gene expression in aphid-plant interactions. Races can be distinguished on the basis of gene expression, and higher numbers of differentially expressed genes are apparent between more divergent races; these expression differences between host races may result from genetic drift and reproductive isolation and possibly divergent selection. Expression differences related to plant adaptation include a subset of chemosensory and salivary genes. Genes showing expression changes in response to host plant do not make up a large portion of between-race expression differences, providing confirmation of previous studies' findings that genes involved in expression differences between diverging populations or species are not necessarily those showing initial plasticity in the face of environmental change.

Plant-insect interactions under bacterial influence: ecological implications and underlying mechanisms.

Plants and insects have been co-existing for more than 400 million years, leading to intimate and complex relationships. Throughout their own evolutionary history, plants and insects have also established intricate and very diverse relationships with microbial associates. Studies in recent years have revealed plant- or insect-associated microbes to be instrumental in plant-insect interactions, with important implications for plant defences and plant utilization by insects. Microbial communities associated with plants are rich in diversity, and their structure greatly differs between below- and above-ground levels. Microbial communities associated with insect herbivores generally present a lower diversity and can reside in different body parts of their hosts including bacteriocytes, haemolymph, gut, and salivary glands. Acquisition of microbial communities by vertical or horizontal transmission and possible genetic exchanges through lateral transfer could strongly impact on the host insect or plant fitness by conferring adaptations to new habitats. Recent developments in sequencing technologies and molecular tools have dramatically enhanced opportunities to characterize the microbial diversity associated with plants and insects and have unveiled some of the mechanisms by which symbionts modulate plant-insect interactions. Here, we focus on the diversity and ecological consequences of bacterial communities associated with plants and herbivorous insects. We also highlight the known mechanisms by which these microbes interfere with plant-insect interactions. Revealing such mechanisms in model systems under controlled environments but also in more natural ecological settings will help us to understand the evolution of complex multitrophic interactions in which plants, herbivorous insects, and micro-organisms are inserted.

The small phytoplasma virulence effector SAP11 contains distinct domains required for nuclear targeting and CIN-TCP binding and destabilization.

Phytoplasmas are insect-transmitted bacterial phytopathogens that secrete virulence effectors and induce changes in the architecture and defense response of their plant hosts. We previously demonstrated that the small (± 10 kDa) virulence effector SAP11 of Aster Yellows phytoplasma strain Witches' Broom (AY-WB) binds and destabilizes Arabidopsis CIN (CINCINNATA) TCP (TEOSINTE-BRANCHED, CYCLOIDEA, PROLIFERATION FACTOR 1 AND 2) transcription factors, resulting in dramatic changes in leaf morphogenesis and increased susceptibility to phytoplasma insect vectors. SAP11 contains a bipartite nuclear localization signal (NLS) that targets this effector to plant cell nuclei. To further understand how SAP11 functions, we assessed the involvement of SAP11 regions in TCP binding and destabilization using a series of mutants. SAP11 mutants lacking the entire N-terminal domain, including the NLS, interacted with TCPs but did not destabilize them. SAP11 mutants lacking the C-terminal domain were impaired in both binding and destabilization of TCPs. These SAP11 mutants did not alter leaf morphogenesis. A SAP11 mutant that did not accumulate in plant nuclei (SAP11ΔNLS-NES) was able to bind and destabilize TCP transcription factors, but instigated weaker changes in leaf morphogenesis than wild-type SAP11. Overall the results suggest that phytoplasma effector SAP11 has a modular organization in which at least three domains are required for efficient CIN-TCP destabilization in plants.

Phytoplasma protein effector SAP11 enhances insect vector reproduction by manipulating plant development and defense hormone biosynthesis.

Phytoplasmas are insect-transmitted phytopathogenic bacteria that can alter plant morphology and the longevity and reproduction rates and behavior of their insect vectors. There are various examples of animal and plant parasites that alter the host phenotype to attract insect vectors, but it is unclear how these parasites accomplish this. We hypothesized that phytoplasmas produce effectors that modulate specific targets in their hosts leading to the changes in plant development and insect performance. Previously, we sequenced and mined the genome of Aster Yellows phytoplasma strain Witches' Broom (AY-WB) and identified 56 candidate effectors. Here, we report that the secreted AY-WB protein 11 (SAP11) effector modulates plant defense responses to the advantage of the AY-WB insect vector Macrosteles quadrilineatus. SAP11 binds and destabilizes Arabidopsis CINCINNATA (CIN)-related TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTORS 1 and 2 (TCP) transcription factors, which control plant development and promote the expression of lipoxygenase (LOX) genes involved in jasmonate (JA) synthesis. Both the Arabidopsis SAP11 lines and AY-WB-infected plants produce less JA on wounding. Furthermore, the AY-WB insect vector produces more offspring on AY-WB-infected plants, SAP11 transgenic lines, and plants impaired in CIN-TCP and JA synthesis. Thus, SAP11-mediated destabilization of CIN-TCPs leads to the down-regulation of LOX2 expression and JA synthesis and an increase in M. quadrilineatus progeny. Phytoplasmas are obligate inhabitants of their plant host and insect vectors, in which the latter transmits AY-WB to a diverse range of plant species. This finding demonstrates that pathogen effectors can reach beyond the pathogen-host interface to modulate a third organism in the biological interaction.

Genomic insights into Spiroplasma endosymbionts that induce male-killing and protective phenotypes in the pea aphid.

The endosymbiotic bacteria Spiroplasma (Mollicutes) infect diverse plants and arthropods, and some of which induce male killing, where male hosts are killed during development. Male-killing Spiroplasma strains belong to either the phylogenetically distant Citri-Poulsonii or Ixodetis groups. In Drosophila flies, Spiroplasma poulsonii induces male killing via the Spaid toxin. While Spiroplasma ixodetis infects a wide range of insects and arachnids, little is known about the genetic basis of S. ixodetis-induced male killing. Here, we analyzed the genome of S. ixodetis strains in the pea aphid Acyrthosiphon pisum (Aphididae, Hemiptera). Genome sequencing constructed a complete genome of a male-killing strain, sAp269, consisting of a 1.5 Mb circular chromosome and an 80 Kb plasmid. sAp269 encoded putative virulence factors containing either ankyrin repeat, ovarian tumor-like deubiquitinase, or ribosome inactivating protein domains, but lacked the Spaid toxin. Further comparative genomics of Spiroplasma strains in A. pisum biotypes adapted to different host plants revealed their phylogenetic associations and the diversity of putative virulence factors. Although the mechanisms of S. ixodetis-induced male killing in pea aphids remain elusive, this study underlines the dynamic genome evolution of S. ixodetis and proposes independent acquisition events of male-killing mechanisms in insects.

Advanced backcross QTL analysis and comparative mapping with RIL QTL studies and GWAS provide an overview of QTL and marker haplotype diversity for resistance to Aphanomyces root rot in pea (Pisum sativum).

Aphanomyces euteiches is the most damaging soilborne pea pathogen in France. Breeding of pea resistant varieties combining a diversity of quantitative trait loci (QTL) is a promising strategy considering previous research achievements in dissecting polygenic resistance to A. euteiches. The objective of this study was to provide an overview of the diversity of QTL and marker haplotypes for resistance to A. euteiches, by integrating a novel QTL mapping study in advanced backcross (AB) populations with previous QTL analyses and genome-wide association study (GWAS) using common markers. QTL analysis was performed in two AB populations derived from the cross between the susceptible spring pea variety "Eden" and the two new sources of partial resistance "E11" and "LISA". The two AB populations were genotyped using 993 and 478 single nucleotide polymorphism (SNP) markers, respectively, and phenotyped for resistance to A. euteiches in controlled conditions and in infested fields at two locations. GWAS and QTL mapping previously reported in the pea-Aphanomyces collection and from four recombinant inbred line (RIL) populations, respectively, were updated using a total of 1,850 additional markers, including the markers used in the Eden x E11 and Eden x LISA populations analysis. A total of 29 resistance-associated SNPs and 171 resistance QTL were identified by GWAS and RIL or AB QTL analyses, respectively, which highlighted 10 consistent genetic regions confirming the previously reported QTL. No new consistent resistance QTL was detected from both Eden x E11 and Eden x LISA AB populations. However, a high diversity of resistance haplotypes was identified at 11 linkage disequilibrium (LD) blocks underlying consistent genetic regions, especially in 14 new sources of resistance from the pea-Aphanomyces collection. An accumulation of favorable haplotypes at these 11 blocks was confirmed in the most resistant pea lines of the collection. This study provides new SNP markers and rare haplotypes associated with the diversity of Aphanomyces root rot resistance QTL investigated, which will be useful for QTL pyramiding strategies to increase resistance levels in future pea varieties.

Phytoplasma effector SAP54 induces indeterminate leaf-like flower development in Arabidopsis plants.

Phytoplasmas are insect-transmitted bacterial plant pathogens that cause considerable damage to a diverse range of agricultural crops globally. Symptoms induced in infected plants suggest that these phytopathogens may modulate developmental processes within the plant host. We report herein that Aster Yellows phytoplasma strain Witches' Broom (AY-WB) readily infects the model plant Arabidopsis (Arabidopsis thaliana) ecotype Columbia, inducing symptoms that are characteristic of phytoplasma infection, such as the production of green leaf-like flowers (virescence and phyllody) and increased formation of stems and branches (witches' broom). We found that the majority of genes encoding secreted AY-WB proteins (SAPs), which are candidate effector proteins, are expressed in Arabidopsis and the AY-WB insect vector Macrosteles quadrilineatus (Hemiptera; Cicadellidae). To identify which of these effector proteins induce symptoms of phyllody and virescence, we individually expressed the effector genes in Arabidopsis. From this screen, we have identified a novel AY-WB effector protein, SAP54, that alters floral development, resulting in the production of leaf-like flowers that are similar to those produced by plants infected with this phytoplasma. This study offers novel insight into the effector profile of an insect-transmitted plant pathogen and reports to our knowledge the first example of a microbial pathogen effector protein that targets flower development in a host.

Aphid Resistance in Pisum Affects the Feeding Behavior of Pea-Adapted and Non-Pea-Adapted Biotypes of Acyrthosiphon pisum Differently.

Resistant genotypes of crops have emerged as an alternative and sustainable solution to pesticide use against pest insects. The resistance depends on the genetic diversity of the host plant and the pest species and can cause an alteration of the insect behavior. The aim of this work was to characterize the resistance level of different Pisum genotypes (one P. fulvum and five P. sativum genotypes) to two biotypes of the aphid Acyrthosiphon pisum, respectively adapted to pea and alfalfa, by measuring the individual aphid weight and analyzing aphid feeding behavior by electropenetrography (EPG). Aphid body mass was influenced by Pisum genotypes reflecting variation in their resistance level. P. fulvum was the most resistant to the A. pisum pea biotype (ArPo28 clone) and showed intermediate resistance to the A. pisum alfalfa biotype (LSR1 clone). The resistance levels of the five P. sativum genotypes to the two aphid biotypes were variable and more pronounced for the alfalfa biotype. EPG data showed that ArPo28 on P. fulvum and LSR1 on all the Pisum genotypes spent shorter time phloem feeding compared to ArPo28 on P. sativum genotypes, indicating that the resistance of Pisum genotypes to non-adapted A. pisum resides in mesophyll and phloem cells. In the meantime, ArPo28 on P. sativum genotypes with a different level of resistance spent a similar length of time phloem feeding, indicating that the quality of phloem sap of the resistance genotypes may not be optimal for the aphid. The study indicated that the resistance of Pisum genotypes to the two A. pisum biotypes involves different genetic factors and mechanisms that affect the aphid differently.

Isolation of Insect Bacteriocytes as a Platform for Transcriptomic Analyses.

Over the past few decades, various techniques have been developed and optimized for the accurate measurement of RNA abundance in cells or tissues. These methods have been instrumental in gaining insight in complex systems such as host-symbiont associations. The pea aphid model has recently emerged as a powerful and experimentally tractable system for studying symbiotic relationships and it is the subject of a growing number of molecular studies. Nevertheless, the lack of standardized protocols for the collection of bacteriocytes, the specialized host cells harboring the symbionts, has limited its use. This chapter provides a simple, step-by-step dissection protocol for the rapid isolation of aphid bacteriocytes. We then describe an adapted protocol for efficient extraction and purification of bacteriocyte RNA that can be used for most downstream transcriptomic analyses.

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-ß-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.

Big Genes, Small Effectors: Pea Aphid Cassette Effector Families Composed From Miniature Exons.

Aphids secrete proteins from their stylets that evidence indicates function similar to pathogen effectors for virulence. Here, we describe two small candidate effector gene families of the pea aphid, Acyrthosiphon pisum, that share highly conserved secretory signal peptide coding regions and divergent non-secretory coding sequences derived from miniature exons. The KQY candidate effector family contains eleven members with additional isoforms, generated by alternative splicing. Pairwise comparisons indicate possible four unique KQY families based on coding regions without the secretory signal region. KQY1a, a representative of the family, is encoded by a 968 bp mRNA and a gene that spans 45.7 kbp of the genome. The locus consists of 37 exons, 33 of which are 15 bp or smaller. Additional KQY members, as well as members of the KHI family, share similar features. Differential expression analyses indicate that the genes are expressed preferentially in salivary glands. Proteomic analysis on salivary glands and saliva revealed 11 KQY members in salivary proteins, and KQY1a was detected in an artificial diet solution after aphid feeding. A single KQY locus and two KHI loci were identified in Myzus persicae, the peach aphid. Of the genes that can be anchored to chromosomes, loci are mostly scattered throughout the genome, except a two-gene region (KQY4/KQY6). We propose that the KQY family expanded in A. pisum through combinatorial assemblies of a common secretory signal cassette and novel coding regions, followed by classical gene duplication and divergence.

Differential Expression of Candidate Salivary Effector Genes in Pea Aphid Biotypes With Distinct Host Plant Specificity.

Effector proteins play crucial roles in determining the outcome of various plant-parasite interactions. Aphids inject salivary effector proteins into plants to facilitate phloem feeding, but some proteins might trigger defense responses in certain plants. The pea aphid, Acyrthosiphon pisum, forms multiple biotypes, and each biotype is specialized to feed on a small number of closely related legume species. Interestingly, all the previously identified biotypes can feed on Vicia faba; hence, it serves as a universal host plant of A. pisum. We hypothesized that the salivary effector proteins have a key role in determining the compatibility between specific host species and A. pisum biotypes and that each biotype produces saliva containing a specific mixture of effector proteins due to differential expression of encoding genes. As the first step to address these hypotheses, we conducted two sets of RNA-seq experiments. RNA-seq analysis of dissected salivary glands (SGs) from reference alfalfa- and pea-specialized A. pisum lines revealed common and line-specific repertoires of candidate salivary effector genes. Based on the results, we created an extended catalogue of A. pisum salivary effector candidates. Next, we used aphid head samples, which contain SGs, to examine biotype-specific expression patterns of candidate salivary genes. RNA-seq analysis of head samples of alfalfa- and pea-specialized biotypes, each represented by three genetically distinct aphid lines reared on either a universal or specific host plant, showed that a majority of the candidate salivary effector genes was expressed in both biotypes at a similar level. Nonetheless, we identified small sets of genes that were differentially regulated in a biotype-specific manner. Little host plant effect (universal vs. specific) was observed on the expression of candidate salivary genes. Analysis of previously obtained genome re-sequenced data of the two biotypes revealed the copy number variations that might explain the differential expression of some candidate salivary genes. In addition, at least four candidate effector genes that were present in the alfalfa biotype but might not be encoded in the pea biotype were identified. This work sets the stage for future functional characterization of candidate genes potentially involved in the determination of plant specificity of pea aphid biotypes.

Characterization of two fungal lipoxygenases expressed in Aspergillus oryzae.

Two fungal lipoxygenase genes were cloned from a rice pathogen, Magnaporthe salvinii, and the take-all fungus, Gaeumannomyces graminis var. tritici, and successfully expressed in Aspergillus oryzae in secreted form. The lipoxygenases expressed, termed MLOX and GLOX, were purified and characterized to evaluate suitability for industrial applications. Both enzymes were active broadly at pH 4-11 and had optimum temperatures around 60 °C, but they were largely different in substrate specificity. Where MLOX was active broadly on arachidonic acid, EPA and DHA, and even on derivatives of fatty acids, such as methyl linoleate or linoleoyl alcohol, GLOX was more specific to linoleic acid and linolenic acid. The most remarkable difference between the two fungal LOXs was the positional and stereo-specificity of oxygenation reactions on polyunsaturated fatty acids. When using linoleic acid as the substrate, the product of MLOX was 9S-hydroperoxy-(E,Z)-octadecadienoic acid (9S(E,Z)-HPODE), on the other hand, the product of GLOX was 13R(E,Z)-HPODE. The enzymes were evaluated for a couple of potential applications and found to be effective on bleaching colored compounds such as carotenoids.

Fast Evolution and Lineage-Specific Gene Family Expansions of Aphid Salivary Effectors Driven by Interactions with Host-Plants.

Effector proteins play crucial roles in plant-parasite interactions by suppressing plant defenses and hijacking plant physiological responses to facilitate parasite invasion and propagation. Although effector proteins have been characterized in many microbial plant pathogens, their nature and role in adaptation to host plants are largely unknown in insect herbivores. Aphids rely on salivary effector proteins injected into the host plants to promote phloem sap uptake. Therefore, gaining insight into the repertoire and evolution of aphid effectors is key to unveiling the mechanisms responsible for aphid virulence and host plant specialization. With this aim in mind, we assembled catalogues of putative effectors in the legume specialist aphid, Acyrthosiphon pisum, using transcriptomics and proteomics approaches. We identified 3,603 candidate effector genes predicted to be expressed in A. pisum salivary glands (SGs), and 740 of which displayed up-regulated expression in SGs in comparison to the alimentary tract. A search for orthologs in 17 arthropod genomes revealed that SG-up-regulated effector candidates of A. pisum are enriched in aphid-specific genes and tend to evolve faster compared with the whole gene set. We also found that a large fraction of proteins detected in the A. pisum saliva belonged to three gene families, of which certain members show evidence consistent with positive selection. Overall, this comprehensive analysis suggests that the large repertoire of effector candidates in A. pisum constitutes a source of novelties promoting plant adaptation to legumes.

Consequences of coinfection with protective symbionts on the host phenotype and symbiont titres in the pea aphid system.

Symbiotic associations between microbes and insects are widespread, and it is frequent that several symbionts share the same host individual. Hence, interactions can occur between these symbionts, influencing their respective abundance within the host with consequences on its phenotype. Here, we investigate the effects of multiple infections in the pea aphid, Acyrthosiphon pisum, which is the host of an obligatory and several facultative symbionts. In particular, we study the influence of a coinfection with 2 protective symbionts: Hamiltonella defensa, which confers protection against parasitoids, and Rickettsiella viridis, which provides protection against fungal pathogens and predators. The effects of Hamiltonella-Rickettsiella coinfection on the respective abundance of the symbionts, host fitness and efficacy of enemy protection were studied. Asymmetrical interactions between the 2 protective symbionts have been found: when they coinfect the same aphid individuals, the Rickettsiella infection affected Hamiltonella abundance within hosts but not the Hamiltonella-mediated protective phenotype while the Hamiltonella infection negatively influences the Rickettsiella-mediated protective phenotype but not its abundance. Harboring the 2 protective symbionts also reduced the survival and fecundity of host individuals. Overall, this work highlights the effects of multiple infections on symbiont abundances and host traits that are likely to impact the maintenance of the symbiotic associations in natural habitats.

Rapid transcriptional plasticity of duplicated gene clusters enables a clonally reproducing aphid to colonise diverse plant species.

BACKGROUND: The prevailing paradigm of host-parasite evolution is that arms races lead to increasing specialisation via genetic adaptation. Insect herbivores are no exception and the majority have evolved to colonise a small number of closely related host species. Remarkably, the green peach aphid, Myzus persicae, colonises plant species across 40 families and single M. persicae clonal lineages can colonise distantly related plants. This remarkable ability makes M. persicae a highly destructive pest of many important crop species. RESULTS: To investigate the exceptional phenotypic plasticity of M. persicae, we sequenced the M. persicae genome and assessed how one clonal lineage responds to host plant species of different families. We show that genetically identical individuals are able to colonise distantly related host species through the differential regulation of genes belonging to aphid-expanded gene families. Multigene clusters collectively upregulate in single aphids within two days upon host switch. Furthermore, we demonstrate the functional significance of this rapid transcriptional change using RNA interference (RNAi)-mediated knock-down of genes belonging to the cathepsin B gene family. Knock-down of cathepsin B genes reduced aphid fitness, but only on the host that induced upregulation of these genes. CONCLUSIONS: Previous research has focused on the role of genetic adaptation of parasites to their hosts. Here we show that the generalist aphid pest M. persicae is able to colonise diverse host plant species in the absence of genetic specialisation. This is achieved through rapid transcriptional plasticity of genes that have duplicated during aphid evolution.