Direct Defense Research Articles

Non-glandular trichomes of Solanum carolinense deter feeding by Manduca sexta caterpillars and cause damage to the gut peritrophic matrix.

Plant trichomes constitute a first line of defence against insect herbivores. The pre- and post-ingestive defensive functions of glandular trichomes are well documented and include direct toxicity, adhesion, antinutrition and defence gene induction. By contrast, the defensive functions of non-glandular trichomes are less well characterized, although these structures are thought to serve as physical barriers that impede herbivore feeding and movement. We experimentally varied the density of stellate non-glandular trichomes in several ways to explore their pre- and post-ingestive effects on herbivores. Larvae of Manduca sexta (Sphingidae) initiated feeding faster and gained more weight on Solanum carolinense (Solanaceae) leaves having lower trichome densities (or experimentally removed trichomes) than on leaves having higher trichome densities. Adding trichomes to artificial diet also deterred feeding and adversely affected caterpillar growth relative to controls. Scanning electron and light microscopy revealed that the ingestion of stellate trichomes by M. sexta caterpillars caused extensive damage to the peritrophic membrane, a gut lining that is essential to digestion and pathogen isolation. These findings suggest that, in addition to acting as a physical barrier to deter feeding, trichomes can inhibit caterpillar growth and development via post-ingestive effects.

Early plant defence against insect attack: involvement of reactive oxygen species in plant responses to insect egg deposition.

Pinus sylvestris responds to insect egg deposition by ROS accumulation linked with reduced activity of the ROS scavenger catalase. Egg mortality in needles with hypersensitive response (HR)-like symptoms is enhanced. Aggressive reactive oxygen species (ROS) play an important role in plant defence against biotic stressors, including herbivorous insects. Plants may even generate ROS in response to insect eggs, thus effectively fighting against future larval herbivory. However, so far nothing is known on how ROS-mediated plant defence against insect eggs is enzymatically regulated. Neither do we know how insects cope with egg-induced plant ROS. We addressed these gaps of knowledge by studying the activities of ROS-related enzymes in Pinus sylvestris deposited with eggs of the herbivorous sawfly Diprion pini. This species cuts a slit into pine needles and inserts its eggs into the needle tissue. About a quarter of egg-deposited needles show chlorotic tissue at the oviposition sites, indicating hypersensitive response-like direct defence responses resulting in reduced larval hatching from eggs. Hydrogen peroxide and peroxidase sensitive staining of sections of egg-deposited pine needles revealed the presence of hydrogen peroxide and peroxidase activity in needle tissue close to the eggs. Activity of ROS-producing NADPH-oxidase did not increase after egg deposition. However, the activity of the ROS-detoxifying enzyme catalase decreased after egg deposition and ovipositional wounding of needles. These results show that local ROS accumulation at the oviposition site is not caused by increased NADPH-oxidase activity, but reduced activity of pine needle catalase may contribute to it. However, our data suggest that pine sawflies can counteract the egg deposition-induced hydrogen peroxide accumulation in pine needles by high catalase activity in their oviduct secretion which is released with the eggs into pine tissue.

Terpenoids in plant and arbuscular mycorrhiza-reinforced defence against herbivorous insects.

Plants, though sessile, employ various strategies to defend themselves against herbivorous insects and convey signals of an impending herbivore attack to other plant(s). Strategies include the production of volatiles that include terpenoids and the formation of symbiotic associations with fungi, such as arbuscular mycorrhiza (AM). This constitutes a two-pronged above-ground/below-ground attack-defence strategy against insect herbivores. Terpenoids represent an important constituent of herbivore-induced plant volatiles that deter herbivores and/or attract their predators. Terpenoids serve as airborne signals that can induce defence responses in systemic undamaged parts of the plant and also prime defence responses in neighbouring plants. Colonization of roots by AM fungi is known to influence secondary metabolism in plants; this includes alteration of the concentration and composition of terpenoids, which can boost both direct and indirect plant defence against herbivorous insects. Enhanced nutrient uptake facilitated by AM, changes in plant morphology and physiology and increased transcription levels of certain genes involved in the terpenoid biosynthesis pathway result in alterations in plant terpenoid profiles. The common mycorrhizal networks of external hyphae have added a dimension to the two-pronged plant defence strategy. These act as conduits to transfer defence signals and terpenoids. Improved understanding of the roles of terpenoids in plant and AM defences against herbivory and of interplant signalling in natural communities has significant implications for sustainable management of pests in agricultural ecosystems.

Characterization and antiviral activity of a newly identified defensin-like peptide, HEdefensin, in the hard tick Haemaphysalis longicornis

Tick defensins are antimicrobial peptides that play a major role in the innate immunity of ticks by providing a direct antimicrobial defense. In this study, we identified and characterized a defensin-like encoding gene, HEdefensin, from the expressed sequence tags (EST) database of hemolymph from the hard tick Haemaphysalis longicornis. Expression of the gene in whole adult ticks and in different organs was upregulated during blood feeding, though not after Langat virus (LGTV) challenge. A synthetic HEdefensin peptide demonstrated significant virucidal activity against LGTV but not against an adenovirus in co-incubation virucidal assays. Moreover, the RNAi-mediated gene silencing of HEdefensin did not significantly affect the virus titer as compared to the control group. The data reported here have established the in vitro virucidal activity of the peptide against LGTV. However, its role in the innate antiviral immunity of H. longicornis remains to be explored, and further studies are needed to fully evaluate the potential biological activities of the peptide against bacteria, fungi or parasites.

Quantification of the plant terpenome: predicted versus actual emission potentials

Plant essential oils are complex mixtures of volatile organic compounds, which play indispensable roles in communication, defense, and adaptive evolution. The complete chemical library produced by a plant is referred to as its terpenome. The potential biological information stored in essential oil composition data can provide an insight into the silent language of plants, as well as roles of terpene emissions in direct and indirect defense, and for playing a crucial role in adaptive evolution. In this work, we have attempted to measure the plant terpenome from a global perspective. One way of measuring the terpenome is to observe and record actual emissions in natural conditions, and this has been in practice for over a century through variously evolving methods of comprehensive GC–MS and HPLC. An alternative method is a knowledge-based prediction of the terpenome, and this method has gained popularity in recent years, with the advent of large-scale genome sequencing technologies. Over the past decade, our laboratory has been involved in compilation and investigation of the plant terpenome using both these methods and this has offered us the opportunity to compare and contrast data from actual and potential emissions, in order to better understand the terpenome and its roles in primary, secondary and adaptive metabolism. We have used emission data in conjunction with genomic data in order to understand how a plant creates the so-called final terpenome, specific to itself, and whether or not plants tap the complete potential for terpene biosynthesis at their disposal according to their genomes. For measuring actual emissions, we have used EssOilDB (the ESSential OIL DataBase), the largest contextual web resource for phytochemicals and for measuring the total plant potential for emissions, we have used TERZYME, an automated algorithm for identification and analysis of genes and proteins involved in isoprenoid biosynthesis.

Resilience and robustness of IPM in protected horticulture in the face of potential invasive pests

Recently, various approaches for the biological control of major pests in protected crops have successfully been adopted. These successes have primarily occurred via the selection, release and/or conservation of generalist native predators that naturally colonize Mediterranean crops that are highly adapted to local environmental conditions. These generalist predators have resulted very effective in controlling key pests; as a result, pesticide use has been reduced considerably. In addition, because these predators are typically highly polyphagous, horticultural crops have become more resilient to newly emerging pests, which are preyed upon by these predators. Possibilities to further strengthen pest management in horticultural crops have recently arisen, such as the use of beneficial microbes (BM) to induce plant resilience. In this review, we used a tomato crop as an example to present two new, highly interrelated plant defense induction strategies: the use of Miridae zoophytophagous predators and the use of Trichoderma spp. microbial biological control agents. Both the mirid predators and Trichoderma symbiotic fungi can activate direct and indirect plant defense responses, which may increase the robustness of pest management against invading organisms. The practical use of these new approaches is discussed.

From resource to female defence: the impact of roosting ecology on a bat's mating strategy.

With their extraordinary species richness and diversity in ecological traits and social systems, bats are a promising taxon for testing socio-ecological hypotheses in order to get new insights into the evolution of animal social systems. Regarding its roosting habits, proboscis bats form an extreme by occupying sites which are usually completely exposed to daylight (e.g. tree trunks, vines or rocks). This is accompanied by morphological and behavioural adaptations to remain cryptic in exposed day roosts. With long-term behavioural observations and genetic parentage analyses of individually marked proboscis bats, we assessed its social dispersion and male mating strategy during day and night. Our results reveal nocturnal male territoriality—a strategy which most closely resembles a resource-defence polygyny that is frequent also in other tropical bats. Its contrasting clumped social dispersion during the day is likely to be the result of strong selection for crypsis in exposed roosts and is accompanied by direct female defence in addition to male territoriality. To the best of our knowledge, such contrasting male mating strategies within a single day–night cycle have not been described in a vertebrate species so far and illustrate a possible evolutionary trajectory from resource-defence to female-defence strategy by small ecologically driven evolutionary steps.

Elevated O3 increases volatile organic compounds via jasmonic acid pathway that promote the preference of parasitoid Encarsia formosa for tomato plants

The elevated atmospheric O3 level may change the interactions of plants and insects, which potentially affects direct and indirect plant defences. However, the underlying mechanism of the impact of elevated O3 on indirect plant defence, namely the efficacy of natural enemies, is unclear. Here we tested a hypothesis that linked the effects of elevated O3 and whitefly herbivory on tomato volatile releases mediated by the jasmonic acid (JA) pathway with the preferences of parasitoid Encarsia formosa for two different tomato genotypes (wild-type (Wt) and JA-deficient genotype (spr2)). The O3 and whitefly herbivory significantly increased the production of volatile organic compounds (VOCs), including monoterpenes and green leaf volatiles (GLVs). The Wt plants released higher volatile levels, particularly monoterpenes, than did the spr2 plants. In Y-tube tests, limonene and Z-3-hexanol played key roles in the attraction of E. formosa. Moreover, regardless of plant genotype, the two plant genotypes were preferred by adult E. formosa under the O3 and O3+ herbivory treatments. Our results suggest that under elevated O3, the activation of the JA pathway significantly up-regulates the emission rates of volatiles, through which the efficacy of natural enemy might be promoted.

Trading direct for indirect defense? Phytochrome B inactivation in tomato attenuates direct anti-herbivore defenses whilst enhancing volatile-mediated attraction of predators.

Under conditions of competition for light, which lead to the inactivation of the photoreceptor phytochrome B (phyB), the growth of shade-intolerant plants is promoted and the accumulation of direct anti-herbivore defenses is down-regulated. Little is known about the effects of phyB on emissions of volatile organic compounds (VOCs), which play a major role as informational cues in indirect defense. We investigated the effects of phyB on direct and indirect defenses in tomato (Solanum lycopersicum) using two complementary approaches to inactivate phyB: illumination with a low red to far-red ratio, simulating competition, and mutation of the two PHYB genes present in the tomato genome. Inactivation of phyB resulted in low levels of constitutive defenses and down-regulation of direct defenses induced by methyl jasmonate (MeJA). Interestingly, phyB inactivation also had large effects on the blends of VOCs induced by MeJA. Moreover, in two-choice bioassays using MeJA-induced plants, the predatory mirid bug Macrolophus pygmaeus preferred VOCs from plants in which phyB was inactivated over VOCs from control plants. These results suggest that, in addition to repressing direct defense, phyB inactivation has consequences for VOC-mediated tritrophic interactions in canopies, presumably attracting predators to less defended plants, where they are likely to find more abundant prey.

Prospects of herbivore egg-killing plant defenses for sustainable crop protection.

Due to a growing demand of food production worldwide, new strategies are suggested to allow for sustainable production of food with minimal effects on natural resources. A promising alternative to the application of chemical pesticides is the implementation of crops resistant to insect pests. Plants produce compounds that are harmful to a wide range of attackers, including insect pests; thus, exploitation of their natural defense system can be the key for the development of pest‐resistant crops. Interestingly, some plants possess a unique first line of defense that eliminates the enemy before it becomes destructive: egg‐killing. Insect eggs can trigger (1) direct defenses, mostly including plant cell tissue growth or cell death that lead to eggs desiccating, being crushed or falling off the plant or (2) indirect defenses, plant chemical cues recruiting natural enemies that kill the egg or hatching larvae (parasitoids). The consequences of plant responses to eggs are that insect larvae do not hatch or that they are impeded in development, and damage to the plant is reduced. Here, we provide an overview on the ubiquity and evolutionary history of egg‐killing traits within the plant kingdom including crops. Up to now, little is known on the mechanisms and on the genetic basis of egg‐killing traits. Making use of egg‐killing defense traits in crops is a promising new way to sustainably reduce losses of crop yield. We provide suggestions for new breeding strategies to grow egg‐killing crops and improve biological control.

Spatial differences in (Z)-3-hexen-1-ol production preferentially reduces Spodoptera litura larva attack on the young leaves of Nicotiana benthamiana

Plants synthesize specialized metabolites which possess extremely important ecological functions including direct defense, indirect defense, and signaling. The optimal defense theory (ODT) proposes that defensive metabolites are preferentially allocated to the tissues with high fitness value or in locations that are easily injured. In our present study, using the model plant Nicotiana benthamiana, we found that direct defense of N. benthamiana against Spodoptera litura (Fabricius) larvae showed spatial differences in the sites producing defensive chemicals. The upper leaves possessed significantly stronger direct defense ability than the middle and lower leaves. Interestingly, the strong defense ability of the upper leaves was not due to occurrences of well-known defensive metabolites such as nicotine and chlorogenic acid. After damage, the middle and lower leaves emitted higher amounts of (Z)-3-hexen-1-ol than the upper leaves, which could both attract larvae and significantly increase the amount of middle and lower leaf eaten by the larvae. The spatial difference in (Z)-3-hexen-1-ol emission may be due to spatial differences in expression of lipoxygenase (NbLOX2), which is responsible for the formation and emission of (Z)-3-hexen-1-ol. This study provided new insight into ODT, showing that plants effectively protect easily injured tissues through reduction in concentration of herbivore-feeding stimulant in the tissues.

Behavioral sensitivity of Myzus persicae to volatile isoprenoids in plant tissues

AbstractVolatile isoprenoids (VIPs) are low‐molecular isoprene‐derived compounds that are produced and emitted by plants as a protection against and in response to various biotic and abiotic stresses. They also participate in direct and indirect plant defense against herbivores. Foliar VIPs may accumulate in mesophyll cells, which makes them detectable for foraging aphids. In this study, probing behavior of the peach potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), was monitored using the electrical penetration graph (EPG) technique, following the exogenous application of camphene, α‐ and β‐ionones, and linalool to its host plant Brassica rapa subsp. pekinensis (Lour.) Hanelt (Brassicaceae). None of the VIPs studied affected M. persicae propensity to probe: aphids spent at least 75% of the experimental time penetrating plant tissues. The application of α‐ionone and linalool did not alter M. persicae behavior relative to control individuals. In contrast, on camphene‐ and β‐ionone‐treated plants, the proportion of non‐probing relative to other stylet activities was high, the success rate in reaching sieve elements and feeding was low, and the proportion of salivation in phloem phase was high. The aphid reluctance to continue probing and feeding due to the exogenously applied VIPs may be considered as means for the limitation of transmission of semi‐persistent and persistent viruses.

Induction of Direct or Priming Resistance against Botrytis cinerea in Strawberries by β-Aminobutyric Acid and Their Effects on Sucrose Metabolism.

The specific forms of disease resistance induced by β-aminobutyric acid (BABA) and their impacts on sucrose metabolism of postharvest strawberries were determined in the present research. Treatment with 10-500 mmol L(-1) BABA inhibited the Botrytis cinerea infection, possibly directly by suppressing the fungus growth and indirectly by triggering disease resistance. Moreover, BABA-induced resistance against B. cinerea infection in strawberries was associated with either one of two mechanisms, depending upon the concentration used: BABA at concentrations higher than 100 mmol L(-1) directly induced the defense response, including a H2O2 burst, modulation of the expression of PR genes, including FaPR1, FaChi3, Faβglu, and FaPAL, and increased activities of chitinase, β-1,3-glucanase, and PAL, whereas BABA at 10 mmol L(-1) activated a priming response because the BABA-treated fruits exhibited an increased capacity to express molecular defense only when the fruits were inoculated with B. cinerea. Activation of the priming defense appeared almost as effective against B. cinerea as inducing direct defense. However, the primed strawberries maintained higher activities of SS synthesis and SPS and SPP enzymes) and lower level of SS cleavage during the incubation; these activities contributed to higher sucrose, fructose, and glucose contents, sweetness index, and sensory scores compared to fruits exhibiting the direct defense. Thus, it is plausible that the priming defense, which can be activated by BABA at relatively low concentrations, represents an optimal strategy for combining the advantages of enhanced disease protection and soluble sugar accumulation.

The simultaneous inducibility of phytochemicals related to plant direct and indirect defences against herbivores is stronger at low elevation

Summary Ecological theory indicates that warmer and more stable climates should result in stronger biotic interactions. Therefore, plant species growing at lower elevations and experiencing greater herbivore pressure should invest in higher levels of defences than those at higher elevations. Nonetheless, there are a number of studies that have found no effect of elevational gradients on plant defensive traits. Several factors might explain the lack of consistency for the altitude–defence relationships, including (i) the reduction of all defensive traits into one measure of resistance; (ii) not considering plant defence as the simultaneous expression of several defensive traits; and (iii) not considering the relative influence of biotic (e.g. herbivory) and abiotic (e.g. climate and soil conditions) factors associated with the ecological gradient. Here, we present a comprehensive test of the effects of elevation and its associated biotic and abiotic factors on the individual and simultaneous expression of constitutive direct and indirect defences and their inducibility (i.e. expression of defences after herbivore attack). Specifically, we estimated climatic and soil variables and measured herbivore damage and constitutive and jasmonic acid‐induced glucosinolate levels in the leaves as a proxy for direct defences, and volatile emission as a proxy for indirect defences in 16 Cardamine species naturally growing along the steep elevational gradient of the Alps. Within a phylogenetic comparative framework, we found that species growing at lower elevations invested more in the simultaneous inducibility of both direct and indirect defences, whereas species growing at higher elevations invested more in constitutive direct defences. Although we found strong elevational gradients in herbivory and climatic and soil variables, these biotic and abiotic factors only partially explained elevational patterns in plant defences. Synthesis. These results highlight that the complex regulation of multiple defence traits strongly vary across elevational gradients and build towards a better understanding of the multiple mechanisms underlying trait evolution and species interactions along ecological gradients.

COI1-Regulated Hydroxylation of Jasmonoyl-L-isoleucine Impairs Nicotiana attenuata's Resistance to the Generalist Herbivore Spodoptera litura.

The phytohormone jasmonoyl-L-isoleucine (JA-Ile) is well-known as the key signaling molecule that elicits plant defense responses after insect herbivory. Oxidation, which is catalyzed by the cytochrome P450s of the CYP94 family, is thought to be one of the main catabolic pathways of JA-Ile. In this study, we identified four CYP94B3 homologues in the wild tobacco plant Nicotiana attenuata. Individually silencing the four homologues revealed that NaCYP94B3 like-1 and NaCYP94B3 like-2, but not NaCYP94B3 like-3 and NaCYP94B3 like-4, are involved in the C-12-hydroxylation of JA-Ile. Simultaneously silencing three of the NaCYP94B3 like genes, NaCYP94B3 like-1, -2, and -4, in the VIGS-NaCYP94B3s plants doubled herbivory-induced JA-Ile levels and greatly enhanced plant resistance to the generalist insect herbivore, Spodoptera litura. The poor larval performance was strongly correlated with the high concentrations of several JA-Ile-dependent direct defense metabolites in VIGS-NaCYP94B3s plants. Furthermore, we show that the abundance of 12-hydroxy-JA-Ile was dependent on JA-Ile levels as well as COI1, the receptor of JA-Ile. COI1 appeared to transcriptionally control NaCYP94B3 like-1 and -2 and thus regulates the catabolism of its own ligand molecule, JA-Ile. These results highlight the important role of JA-Ile degradation in jasmonate homeostasis and provide new insight into the feedback regulation of JA-Ile catabolism. Given that silencing these CYP94 genes did not detectably alter plant growth and highly increased plant defense levels, we propose that CYP94B3 genes can be potential targets for genetic improvement of herbivore-resistant crops.

Cytokinin production by Pseudomonas fluorescens G20-18 determines biocontrol activity against Pseudomonas syringae in Arabidopsis.

Plant beneficial microbes mediate biocontrol of diseases by interfering with pathogens or via strengthening the host. Although phytohormones, including cytokinins, are known to regulate plant development and physiology as well as plant immunity, their production by microorganisms has not been considered as a biocontrol mechanism. Here we identify the ability of Pseudomonas fluorescens G20-18 to efficiently control P. syringae infection in Arabidopsis, allowing maintenance of tissue integrity and ultimately biomass yield. Microbial cytokinin production was identified as a key determinant for this biocontrol effect on the hemibiotrophic bacterial pathogen. While cytokinin-deficient loss-of-function mutants of G20-18 exhibit impaired biocontrol, functional complementation with cytokinin biosynthetic genes restores cytokinin-mediated biocontrol, which is correlated with differential cytokinin levels in planta. Arabidopsis mutant analyses revealed the necessity of functional plant cytokinin perception and salicylic acid-dependent defence signalling for this biocontrol mechanism. These results demonstrate microbial cytokinin production as a novel microbe-based, hormone-mediated concept of biocontrol. This mechanism provides a basis to potentially develop novel, integrated plant protection strategies combining promotion of growth, a favourable physiological status and activation of fine-tuned direct defence and abiotic stress resilience.

PHYSIOLOGICAL EFFECTS OF RESVERATROL AND COUMARIC ACID ON TWO MAJOR GROUNDNUT PESTS AND THEIR EGG PARASITOID BEHAVIOR.

Groundnut, Arachis hypogea L., is one of the plant species that synthesizes phenolic compounds, resveratrol and coumaric acid. They are induced as a defense strategy in plant in response to feeding lepidopterans. The present study investigated the role of resveratrol and coumaric acid in producing antiherbivore effects as a direct defense against two major groundnut pests, Spodoptera litura F. and Amsacta albistriga W., and in indirect defense by attracting the egg parasitoid Trichogramma chilonis Ishii under laboratory conditions. The phenolic compounds deterred the feeding of both pests and caused reduction in the larval weights in a dose-dependent manner in leaf disk bioassays. Antioxidant mechanisms of larvae fed with these phenols were measured by estimating the activities of superoxide dismutase (SOD), ascorbate peroxidase (APOX), and catalase (CAT). Enzyme activities increased significantly in experimental larvae, more so in resveratrol-treated than in coumaric acid treated larvae. Feeding larvae with resveratrol and coumaric acid resulted in enhanced activities of detoxifying enzymes, carboxyl esterase (EST), and glutathione-S-transferase (GST) in the midgut tissues of both species, indicating the toxic nature of these compounds. Trichogramma chilonis was more attracted toward coumaric acid treatments in Y-olfactometer tests than to resveratrol. This study contributes to the understanding of the roles of resveratrol and coumaric acid in direct as well as indirect defense, we infer a possible utilization of these compounds in alternate measures of groundnut pest control in future.

Herbivore intoxication as a potential primary function of an inducible volatile plant signal

SummaryPlants release herbivore‐induced volatiles (HIPVs), which can be used as cues by plants, herbivores and natural enemies. Theory predicts thatHIPVs may initially have evolved because of their direct benefits for the emitter and were subsequently adopted as infochemicals.Here, we investigated the potential direct benefits of indole, a majorHIPVconstituent of many plant species and a key defence priming signal in maize. We used indole‐deficient maize mutants and synthetic indole at physiologically relevant doses to document the impact of the volatile on the generalist herbivoreSpodoptera littoralis.Our experiments demonstrate that indole directly decreases food consumption, plant damage and survival ofS. littoraliscaterpillars. Surprisingly, exposure to volatile indole increased caterpillar growth. Furthermore, we show thatS. littoraliscaterpillars and adults consistently avoid indole‐producing plants in olfactometer experiments, feeding assays and oviposition trials.Synthesis. Together, these results provide a potential evolutionary trajectory by which the release of aHIPVas a direct defence precedes its use as a cue by herbivores and an alert signal by plants. Furthermore, our experiments show that the effects of a plant secondary metabolite on weight gain and food consumption can diverge in a counterintuitive manner, which implies that larval growth can be a poor proxy for herbivore fitness and plant resistance.

Comparing the attraction of two parasitoids to herbivore-induced volatiles of maize and its wild ancestors, the teosintes

Artificial selection of crop plants for desired traits such as increased yield and improved seed or fruit quality has been hypothesized to have had a cost for other potentially useful traits, including resistance to herbivores. Besides direct defences, such as the production of toxins, plants may also indirectly protect themselves by emitting volatile organic compounds (VOCs) that attract the natural enemies of herbivores. Parasitoid wasps are known to use these VOCs to localize hosts for their offspring. However, domestication and selective breeding of crop plants have reportedly led to the loss of such signals. The aim of this study was to identify possible differences in the attraction of parasitoid wasps by modern maize and its wild ancestors, the teosintes. In a six-arm olfactometer, we compared the capacity of teosintes and maize to attract the parasitoid wasps Cotesia marginiventris (Hymenoptera: Braconidae) and Campoletis sonorensis (Hymenoptera: Ichneumonidae). We studied the attractiveness of plants in which VOC emission was induced by the application of artificial damage and caterpillar regurgitant, as well as the attractiveness of extracts of volatiles that we collected from plants exposed to herbivory. C. sonorensis did not distinguish between the odours of maize and teosintes, whereas C. marginiventris showed a significant preference for the odours of teosintes over the odours of maize. The fact that we obtained very similar results with extracts of collected volatiles implies that we could use these extracts to identify the key compounds that are responsible for wasp attraction. Restoring and/or enhancing such key parasitoid attractants in cultivated plants could be an effective way to increase natural pest control.

Insect Herbivory-Elicited GABA Accumulation in Plants is a Wound-Induced, Direct, Systemic, and Jasmonate-Independent Defense Response.

The non-proteinogenic amino acid γ-aminobutyric acid (GABA) is present in all organisms analyzed so far. In invertebrates GABA acts as a neurotransmitter; in plants different functions are under discussion. Among others, its involvement in abiotic stress reactions and as a defensive compound against feeding insects is suggested. GABA is synthesized from glutamate by glutamate decarboxylases and degraded by GABA-transaminases. Here, in Arabidopsis thaliana, gad1/2 double mutants showing reduced GABA concentrations as well as GABA-enriched triple mutants (gad1/2 x pop2-5) were generated and employed for a systematic study of GABA induction, accumulation and related effects in Arabidopsis leaves upon herbivory. The results demonstrate that GABA accumulation is stimulated by insect feeding-like wounding by a robotic caterpillar, MecWorm, as well as by real insect (Spodoptera littoralis) herbivory. Higher GABA levels in both plant tissue and artificial dietary supplements in turn affect the performance of feeding larvae. GABA enrichment occurs not only in the challenged but also in adjacent leaf. This induced response is neither dependent on herbivore defense-related phytohormones, jasmonates, nor is jasmonate induction dependent on the presence of GABA. Thus, in Arabidopsis the rapid accumulation of GABA very likely represents a general, direct and systemic defense reaction against insect herbivores.