Emilio Guerrieri

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

Induction and Systemic Release of Herbivore-Induced Plant Volatiles Mediating In-Flight Orientation of Aphidius ervi

In-flight orientation of the braconid Aphidius ervi in response to volatiles released from broad bean plants infested by the pea aphid, Acyrthosiphon pisum, was studied in a no-choice wind-tunnel bioassay. The role of aphid infestation level and duration, systemic production of volatiles by “insect-free” parts of the plant, and the specificity of aphid-induced volatiles on the flight behavior of the foraging female parasitoids were investigated. The upper insect-free part of a three-leaved broad bean plant, which was basally infested by a population of 40 A. pisum, released synomones detectable by A. ervi females after at least 48–72 hr of infestation, resulting in both significant increases in oriented flights and landings on the source compared with uninfested control plants. This suggests that volatiles involved in host-location by A. ervi are systemically released by broad bean plants either in response to circulation of aphid saliva, circulation of saliva-induced bioactive elicitors, or circulation of the synomones themselves. Air entrainment extracts of volatiles collected from a broad bean plant infested by the nonhost Aphis fabae or an uninfested broad bean plant elicited few oriented flights and landing responses by female parasitoids. These extracts were significantly less attractive than extracts collected from a broad bean plant infested by the host A. pisum, indicating the specificity of synomones elicited by different aphid species on the same plant species.

Volatiles from whitefly-infested plants elicit a host-locating response in the parasitoid, Encarsia formosa.

The blend of volatile compounds emitted by bean plants (Phaseolus vulgaris) infested with greenhouse whitefly (Trialeurodes vaporariorum) has been studied comparatively with undamaged plants and whiteflies themselves. Collection of the volatiles and analysis by gas chromatography revealed more than 20 compounds produced by plants infested with whitefly. Of these, 4 compounds, (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, 3-octanone, and one unidentified compound were emitted at higher levels than from the undamaged control plants. Synthetic (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, or 3-octanone all elicited a significant increase in oriented flight and landing on the source by the parasitoid, Encarsia formosa, in wind tunnel bioassays. Two-component mixtures of the compounds and the three-component mixture all elicited a similar or, in most cases, a better response by the parasitoid, the most effective being a mixture of (Z)-3-hexen-1-ol and 3-octanone. These results demonstrate that E. formosa uses volatiles from the plant-host complex as olfactory cues for host location.

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.

Systemin regulates both systemic and volatile signaling in tomato plants.

The prevailing reaction of plants to pest attack is the activation of various defense mechanisms. In tomato, several studies indicate that an 18 amino acid (aa) peptide, called systemin, is a primary signal for the systemic induction of direct resistance against plant-chewing pests, and that the transgenic expression of the prosystemin gene (encoding the 200 aa systemin precursor) activates genes involved in the plant response to herbivores. By using a combination of behavioral, chemical, and gene expression analyses, we report that systemin enhances the production of bioactive volatile compounds, increases plant attractivity towards parasitiod wasps, and activates genes involved in volatile production. Our data imply that systemin is involved in the systemic activation of indirect defense in tomato, and we conclude that a single gene controls the systemic activation of coordinated and associated responses against pests.

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.

Electrophysiological and behavioural responses of Aphidius ervi (Hymenoptera: Braconidae) to tomato plant volatiles

Flight responses of the aphid parasitoid Aphidius ervi to tomato volatiles have recently demonstrated that different plant stresses can lead to increases in attractiveness for this parasitoid. For example, infestation of tomato plants by the aphid Macrosiphum euphorbiae results in the overexpression of defensive genes, as well as the release of volatile compounds that attract aphid parasitoids. Here, we determine which of the induced compounds elicit a significant electrophysiological response from parasitoid antennae. Compounds shown to be detected at the antennal level were then tested at a range of doses in a wind tunnel assay. A significant electroantennogram response was demonstrated for three compounds, (8S,9R)-(E)-caryophyllene, methyl salicylate, and (Z)-3-hexen-1-ol, over four concentrations. These compounds proved to be significantly attractive in the wind tunnel at a rate not always proportionally dependent upon the dose. The practical implications of these findings are discussed in the framework of sustainable control for pest aphids in agriculture.

Insights on the Impact of Arbuscular Mycorrhizal Symbiosis on Tomato Tolerance to Water Stress

Arbuscular mycorrhizal symbiosis can improve tolerance to severe water stress conditions in tomato plants. Arbuscular mycorrhizal (AM) fungi, which form symbioses with the roots of the most important crop species, are usually considered biofertilizers, whose exploitation could represent a promising avenue for the development in the future of a more sustainable next-generation agriculture. The best understood function in symbiosis is an improvement in plant mineral nutrient acquisition, as exchange for carbon compounds derived from the photosynthetic process: this can enhance host growth and tolerance to environmental stresses, such as water stress (WS). However, physiological and molecular mechanisms occurring in arbuscular mycorrhiza-colonized plants and directly involved in the mitigation of WS effects need to be further investigated. The main goal of this work is to verify the potential impact of AM symbiosis on the plant response to WS. To this aim, the effect of two AM fungi (Funneliformis mosseae and Rhizophagus intraradices) on tomato (Solanum lycopersicum) under the WS condition was studied. A combined approach, involving ecophysiological, morphometric, biochemical, and molecular analyses, has been used to highlight the mechanisms involved in plant response to WS during AM symbiosis. Gene expression analyses focused on a set of target genes putatively involved in the plant response to drought, and in parallel, we considered the expression changes induced by the imposed stress on a group of fungal genes playing a key role in the water-transport process. Taken together, the results show that AM symbiosis positively affects the tolerance to WS in tomato, with a different plant response depending on the AM fungi species involved.

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.

Root symbionts: Powerful drivers of plant above- and belowground indirect defenses

Soil microbial mutualists of plants, including mycorrhizal fungi, non‐mycorrhizal fungi and plant growth promoting rhizobacteria, have been typically characterized for increasing nutrient acquisition and plant growth. More recently, soil microbes have also been shown to increase direct plant defense against above‐ and belowground herbivores. Plants, however, do not only rely on direct defenses when attacked, but they can also recruit pest antagonists such as predators and parasitoids, both above and belowground, mainly via the release of volatile organic compounds (i.e., indirect defenses). In this review, we illustrate the main features and effects of soil microbial mutualists of plants on plant indirect defenses and discuss possible applications within the framework of sustainable crop protection against root‐ and shoot‐feeding arthropod pests. We indicate the main knowledge gaps and the future challenges to be addressed in the study and application of these multifaceted interactions.