Maria Cristina Digilio

Do interactions between plant roots and the rhizosphere affect parasitoid behaviour

Multitrophic interactions are powerful forces shaping the structure of living communities. Plants encounter a great diversity of organisms in their environment: some of these interactions are beneficial (e.g. symbiotic fungi and insect pollinators) while some are detrimental (e.g. herbivorous insects and pathogenic micro-organisms). Multitrophic interactions between below-ground and above-ground organisms are receiving increasing attention because they may influence plant defences against biotic and abiotic stresses (van Dam et al., 2003). Plant defences can be constitutive or induced (Agrawal et al., 1999; Walling, 2000), and may also be direct (e.g. toxic compounds like glucosinolates in Brassicaceae) or indirect. Indirect defences typically involve the production of volatile semiochemicals that are attractive towards natural enemies of herbivorous insects (Dicke, 1999). These semiochemicals have been referred to as synomones to stress the mutual benefit of the partners involved (Vet & Dicke, 1992). Some of these volatiles are released as a specific response to the attack of a specific herbivore (see Agrawal et al., 1999 and references therein), a feature termed induced indirect defence. Several studies show that the release of induced volatiles is not confined exclusively to the organ attacked but involves all the plant through the circulation of systemic elicitors (Mattiacci et al., 1995; Alborn et al., 1997; Guerrieri et al., 1999; Dicke & Dijkman, 2001). The growing evidence that any colonising organism alters the profile of plant volatiles suggests that this may have intriguing and often unpredictable consequences for the performance of higher trophic levels (Dicke et al., 2003). In this paper we report on the interactions between below-ground interactions and indirect defences. Arbuscular mycorrhizal symbioses are mutualistic interactions between plant roots and soil fungi, and have been reported for more than 80% of higher plants (Smith & Read, 1997). Colonisation by arbuscular mycorrhizal fungi induces resistance or tolerance to a variety of pathogens in tomato and in other plants (Cordier et al., 1996; Trotta et al., 1996; Lingua et al., 2002). These changes are mediated by a variety of mechanisms, including the up-regulation and down-regulation of specific genes (TahiriAlaoui & Antoniw, 1996) that result in localised and systemic responses by the plant. These responses include the synthesis of new isoforms of chitinases and glucanases and the thickening of the cell walls (Azcón-Aguilar et al., 2002; Pozo et al., 2002) that may affect herbivore colonisation. The effects of arbuscular mycorrhizal symbiosis on aboveground herbivores has been investigated with contrasting results (van Dam et al., 2003 and references therein). More recently, the effects of different species of arbuscular mycorrhizal fungi on parasitism rates have been reported (Gange et al., 2003) but this study did not demonstrate a direct link between arbuscular mycorrhizae and attraction of insect parasitoids. In this study we tested the hypothesis that an arbuscular mycorrhizal symbiosis makes tomato plants significantly more attractive towards aphid parasitoids. Fig. 1a shows the multitrophic system used for this study: at the base of the system is tomato (Lycopersicon esculentum Miller) whose roots were colonised by the arbuscular mycorrhizal fungus Glomus mosseae Nicol & Gerd (Gerdemann & Trappe) BEG 12. Although tomato plants are characterised by high levels of constitutive defences (e.g. glandular trichomes, a-tomatine, Kennedy, 2003), induced defence mechanisms are nevertheless important (reviewed in Agrawal et al., 1999 and references therein). The herbivore (the potato aphid, Macrosiphum euphorbiae Thomas) is a key pest of tomato all over the world, causing direct and indirect damage to plants, including Correspondence: Emilio Guerrieri, Istituto per la Protezione delle Piante, CNR, Sez. Portici, Via Università 133, 80055 Portici (NA), Italy. E-mail: guerrieri@ipp.cnr.it Ecological Entomology (2004) 29, 753–756

Aphid-plant interactions: a review

Abstract Aphids are economically important insect pests of agriculture and forest crops. They feed on phloem sap by extremely efficient mouthparts modified into long and flexible stylets. Adaptation to phytophagy is completed by an extremely ductile reproduction system that can alternate biparental and parthenogenetic generations. In order to reach plant phloem, aphids must overcome plant defences, either physically and/or chemically. However, plants respond to aphid attack by activating defence genes that lead to the production of physical barriers and/or chemical toxic compounds (direct resistance). In addition, attacked plants can attract the natural enemies of aphids by releasing specific volatile compounds (indirect resistance). We can take advantage of these different types of resistance in order to enhance the sustainable control of these phytophagous insects. In this review we summarize the main aspects of plant-aphid interactions, focusing on those issues that can have an economic application.

Host regulation effects of ovary fluid and venom of Aphidius ervi (Hymenoptera: Braconidae)

The host regulation effects of venom and ovary fluid of the endophagous braconid Aphidius ervi Haliday on the pea aphid, Acyrthosiphon pisum (Harris), have been studied. Extracts of ovaries and of venom glands were injected into nonparasitized 4th instar pea aphids, both separately and mixed. Aphids treated with parasitoid material died as 4th instars, often showing developmental arrest. In contrast, control aphids that received an injection of Pringles saline solution regularly moulted to the adult stage and reproduced. Venom alone was as effective as the combined extracts in determining developmental arrest and death. Separate heat and protease treatments of these parasitoids reproductive secretions significantly reduced their biological activity, suggesting that the active component(s) involved is a protein(s). SDS-PAGE analysis of haemolymph samples obtained from pea aphids which had received an injection of combined venom and ovary extract revealed an increase of the titre of various proteins, particularly in the 43-47kDa interval, as registered for truly parasitized hosts. This altered protein profile was first detected 48h following injection. Based on this information a tentative physiological model is proposed. The apical tract of host ovarioles, where the germarium and growing oocytes are located, is suggested to be one of the major targets of female parasitoid secretions injected with the egg.

Development and nutrition of the braconid wasp, Aphidius ervi in aposymbiotic host aphids

This study aims at evaluating whether the bacterial endosymbionts of the pea aphid, Acyrthosiphon pisum (Harris), may be of any nutritional importance to the endophagous braconid parasitoid Aphidius ervi Haliday. Aposymbiotic aphids, obtained by rifampicin treatment on artificial diet, were parasitized, and both parasitoid development and major changes in the nitrogen-derived nutrients in the host hemolymph were observed. A. ervi larvae developing in aposymbiotic pea aphids showed a significant delay in reaching the adult stage and the emerging adults showed a 50% weight reduction compared to those emerging from symbiotic aphids. In the hemolymph of parasitized symbiotic aphids, parasitism-specific proteins of approximately 15 and 45 kD were detected, and their level increased on day 5 and 6 after parasitoid oviposition. At that stage, a significant increase in the free amino acid level was observed, with glutamine showing the highest relative abundance. In contrast, the hemolymph of aposymbiotic aphids did not contain the two parasitism-specific proteins and no increase in free amino acids was observed. The results indicate that the bacterial endosymbionts in parasitized pea aphids are of pivotal importance for the development of A. ervi. Aposymbiotic aphids have a lower nutritional suitability, showing a significant reduction in the hemolymph of nitrogen-derived compounds of nutritional importance to the developing parasitoid larva. The possible role of Buchnera in nitrogen recycling is also discussed as a possible strategy for optimizing the utilization of dietary nitrogen by the parasitoid larva. Arch. Insect Biochem. Physiol. 40:53–63, 1999.

Host-locating response by the aphid parasitoid Aphidius ervi to tomato plant volatiles

Abstract The blend of volatile compounds emitted by tomato plants (Solanum lycopersicum) infested with the potato aphid (Macrosiphum euphorbiae) has been studied comparatively with undamaged plants and aphids themselves. Aphid-infested plants were significantly more attractive towards Aphidius ervi than undamaged plants and aphids themselves. Oriented response towards host-damaged plant, from which aphids were removed just before running the bioassay, did not differ from that recorded for infested plants. Collection of the volatiles and analysis by gas chromatography revealed only quantitative differences between uninfested and aphid-infested plants. Nine compounds, α-pinene, (Z)-3-hexen-1-ol, α-phellandrene, limonene, (E)-β-ocimene, p-cymene, methyl salicylate, (E)-β-caryophyllene and an unknown compound, were emitted at higher levels from aphid-infested plants than from undamaged control plants, whilst no differences were noted for hexanal, 6-methyl-5-hepten-2-one, and humulene (=α-caryophyllene). Synthetic standards of these compounds were tested in wind tunnel bioassays and all elicited a significant increase in oriented flight and landings on the target by the aphid parasitoid Aphidius ervi. (E)-β-caryophyllene resulted the most attractive towards female wasps. These results corroborate the hypothesis that the volatiles produced by the plant in response to aphid attack derive from both jasmonic and salicylic acid pathways, and are exploited by A. ervi as olfactory cues to locate its hosts.

Transcriptomic and proteomic analysis of a compatible tomato-aphid interaction reveals a predominant salicylic acid-dependent plant response

BackgroundAphids are among the most destructive pests in temperate climates, causing significant damage on several crops including tomato. We carried out a transcriptomic and proteomic study to get insights into the molecular mechanisms and dynamics of the tomato response to the Macrosyphum euphorbiae aphid.ResultsThe time course analysis of aphid infestation indicated a complex, dynamic pattern of gene expression. Several biological functions were affected and genes related to the stress and defence response were the most represented. The Gene Ontology categories of the differentially expressed genes (899) and identified proteins (57) indicated that the tomato response is characterized by an increased oxidative stress accompanied by the production of proteins involved in the detoxification of oxygen radicals. Aphids elicit a defense reaction based on the cross-communication of different hormone-related signaling pathways such as those related to the salicylic acid (SA), jasmonic acid (JA), ethylene and brassinosteroids. Among them, the SA-signaling pathway and stress-responsive SA-dependent genes play a dominant role. Furthermore, tomato response is characterized by a reduced accumulation of photosynthetic proteins and a modification of the expression of various cell wall related genes.ConclusionsOur work allowed a more comprehensive understanding of the signaling events and the defense dynamics of the tomato response to aphids in a compatible interaction and, based on experimental data, a model of the tomato–aphid molecular interaction was proposed. Considering the rapid advancement of tomato genomics, this information will be important for the development of new protection strategies.

Insecticide activity of Mediterranean essential oils

Abstract Vapours of essential oils, extracted from 12 Mediterranean plants of the families Lamiaceae, Verbenaceae and Apiaceae, were assayed for insecticide activity against the aphid pests Acyrthosiphon pisum (Harris) and Myzus persicae (Sulzer). Different doses were applied, starting from 2 µl/l air and halving the dose until no activity was registered. Anise, fennel and basil essential oils resulted in high mortality, even applied at low doses. Activity was dose-dependent. The occurrence of phytotoxicity following the application of some essential oil is discussed. In spite of the well-known drawbacks owing to phytotoxicity, the application of essential oils in the control of pests on plants appears feasible.

Larval anatomy and structure of absorbing epithelia in the aphid parasitoid Aphidius ervi Haliday (Hymenoptera, Braconidae)

The present work describes Aphidius ervi Haliday (Hymenoptera, Braconidae) larval anatomy and development, focusing on time-related changes of body structure and cell ultrastructure, especially of the epithelial layers involved in nutrient absorption. Newly hatched 1st instar larvae of A. ervi are characterised by gut absence and a compact cluster of cells makes up their body. As the parasitoid larva develops, the central undifferentiated cell mass becomes hollowed out, leading to the formation of gut anlage. This suggests that absorption of nutrients at that stage may take place through the body surface, as more directly demonstrated by the occurrence on the epidermis of proteins associated with transepithelial transport, such as Na(+)/K(+)-ATPase and alkaline phosphatase (ALP). Second instar larvae show the presence of the gut with a well-differentiated brush border and a peritrophic membrane. Gut cells are filled by masses of glycogen granules and lipid droplets. The tracheal system starts to be visible. The haemocoel becomes evident in late 2nd instar, and contains large silk glands. Mature 3rd instar larvae are typically hymenopteriform. The midgut accounts for most of the body volume and is actively involved in nutrient absorption, as indicated by the well developed brush border and by the presence of Na(+)/K(+)-ATPase and ALP on the basolateral and luminal membrane respectively. At this stage, large lipid droplets have gradually replaced the cellular glycogen stores in the midgut cells. The tracheae are completely differentiated, but their internal lumen still contains fibrillar material, suggesting that they are not functional as long as host fluids bath the parasitoid larva. In late 3rd instar larvae, silk glands, structurally similar to Malpighian tubules, show a very intense vesicular traffic toward the internal lumen, which, eventually, results in being filled by secretion products, suggesting the possible recycling of metabolic waste products during mummy formation.

Molecular and chemical mechanisms involved in aphid resistance in cultivated tomato

*An integrated approach has been used to obtain an understanding of the molecular and chemical mechanisms underlying resistance to aphids in cherry-like tomato (Solanum lycopersicum) landraces from the Campania region (southern Italy). The aphid-parasitoid system Macrosiphum euphorbiae-Aphidius ervi was used to describe the levels of resistance against aphids in two tomato accessions (AN5, AN7) exhibiting high yield and quality traits and lacking the tomato Mi gene. *Aphid development and reproduction, flight response by the aphid parasitoid A. ervi, gas chromatography-mass spectrometry headspace analysis of plant volatile organic compounds and transcriptional analysis of aphid responsive genes were performed on selected tomato accessions and on a susceptible commercial variety (M82). *When compared with the cultivated variety, M82, AN5 and AN7 showed a significant reduction of M. euphorbiae fitness, the release of larger amounts of specific volatile organic compounds that are attractive to the aphid parasitoid A. ervi, a constitutively higher level of expression of plant defence genes and differential enhancement of plant indirect resistance induced by aphid feeding. *These results provide new insights on how local selection can offer the possibility of the development of innovative genetic strategies to increase tomato resistance against aphids.

Prosystemin Overexpression in Tomato Enhances Resistance to Different Biotic Stresses by Activating Genes of Multiple Signaling Pathways.

Systemin is a signal peptide that promotes the response to wounding and herbivore attack in tomato. This 18-amino acid peptide is released from a larger precursor, prosystemin. To study the role of systemin as a modulator of defense signaling, we generated tomato (Solanum lycopersicum) transgenic plants that overexpress the prosystemin cDNA. We carried out a transcriptomic analysis comparing two different transgenic events with the untransformed control. The Gene Ontology categories of the 503 differentially expressed genes indicated that several biological functions were affected. Systemin promotes the expression of an array of defense genes that are dependent on different signaling pathways and it downregulates genes connected with carbon fixation and carbohydrate metabolism. These alterations present a degree of overlap with the response programs that are classically associated to pathogen defense or abiotic stress protection, implying that end products of the systemin signaling pathway may be more diverse than expected. We show also that the observed transcriptional modifications have a relevant functional outcome, since transgenic lines were more resistant against very different biotic stressors such as aphids (Macrosiphum euphorbiae), phytopathogenic fungi (Botrytis cinerea and Alternaria alternata) and phytophagous larvae (Spodoptera littoralis). Our work demonstrated that in tomato the modulation of a single gene is sufficient to provide a wide resistance against stress by boosting endogenous defense pathways. Overall, the data provided evidence that the systemin peptide might serve as DAMP signal in tomato, acting as a broad indicator of tissue integrity.