Indirect defence via tritrophic interactions

Plant strengtheners enhance parasitoid attraction to herbivore-damaged cotton via qualitative and quantitative changes in induced volatiles

BACKGROUND

Herbivore-damaged plants release a blend of volatile organic compounds (VOCs) that differs from undamaged plants. These induced changes are known to attract the natural enemies of the herbivores and therefore are expected to be important determinants of the effectiveness of biological control in agriculture. One way of boosting this phenomenon is the application of plant strengtheners, which has been shown to enhance parasitoid attraction in maize. It is unclear whether this is also the case for other important crops.

RESULTS

The plant strengtheners BTH [benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester] and laminarin were applied to cotton plants, and the effects on volatile releases and the attraction of three hymenopteran parasitoids, Cotesia marginiventris, Campoletis sonorensis and Microplitis rufiventris, were studied. After treated and untreated plants were induced by real or simulated caterpillar feeding, it was found that BTH treatment increased the attraction of the parasitoids, whereas laminarin had no significant effect. BTH treatment selectively increased the release of two homoterpenes and reduced the emission of indole, the latter of which had been shown to interfere with parasitoid attraction in earlier studies. Canonical variate analyses of the data show that the parasitoid responses were dependent on the quality rather than the quantity of volatile emission in this tritrophic interaction.

CONCLUSION

Overall, these results strengthen the emerging paradigm that induction of plant defences with chemical elicitors such as BTH could provide a sustainable and environmentally friendly strategy for biological control of pests by enhancing the attractiveness of cultivated plants to natural enemies of insect herbivores.

Direct Contact - Sorptive Tape Extraction coupled with Gas Chromatography - Mass Spectrometry to reveal volatile topographical dynamics of lima bean (Phaseolus lunatus L.) upon herbivory by Spodoptera littoralis Boisd

BACKGROUND

The dynamics of plant volatile (PV) emission, and the relationship between damaged area and biosynthesis of bioactive molecules in plant-insect interactions, remain open questions. Direct Contact-Sorptive Tape Extraction (DC-STE) is a sorption sampling technique employing non adhesive polydimethylsiloxane tapes, which are placed in direct contact with a biologically-active surface. DC-STE coupled to Gas Chromatography - Mass Spectrometry (GC-MS) is a non-destructive, high concentration-capacity sampling technique able to detect and allow identification of PVs involved in plant responses to biotic and abiotic stresses. Here we investigated the leaf topographical dynamics of herbivory-induced PV (HIPV) produced by Phaseolus lunatus L. (lima bean) in response to herbivory by larvae of the Mediterranean climbing cutworm (Spodoptera littoralis Boisd.) and mechanical wounding by DC-STE-GC-MS.

RESULTS

Time-course experiments on herbivory wounding caused by larvae (HW), mechanical damage by a pattern wheel (MD), and MD combined with the larvae oral secretions (OS) showed that green leaf volatiles (GLVs) [(E)-2-hexenal, (Z)-3-hexen-1-ol, 1-octen-3-ol, (Z)-3-hexenyl acetate, (Z)-3-hexenyl butyrate] were associated with both MD and HW, whereas monoterpenoids [(E)-β-ocimene], sesquiterpenoids [(E)-nerolidol] and homoterpenes (DMNT and TMTT) were specifically associated with HW. Up-regulation of genes coding for HIPV-related enzymes (Farnesyl Pyrophosphate Synthase, Lipoxygenase, Ocimene Synthase and Terpene Synthase 2) was consistent with HIPV results. GLVs and sesquiterpenoids were produced locally and found to influence their own gene expression in distant tissues, whereas (E)-β-ocimene, TMTT, and DMNT gene expression was limited to wounded areas.

CONCLUSIONS

DC-STE-GC-MS was found to be a reliable method for the topographical evaluation of plant responses to biotic and abiotic stresses, by revealing the differential distribution of different classes of HIPVs. The main advantages of this technique include: a) in vivo sampling; b) reproducible sampling; c) ease of execution; d) simultaneous assays of different leaf portions, and e) preservation of plant material for further "omic" studies. DC-STE-GC-MS is also a low-impact innovative method for in situ PV detection that finds potential applications in sustainable crop management.

Chemical cues modulating electrophysiological and behavioural responses in the parasitic wasp Cotesia sesamiae

A better understanding of the chemical cues employed by parasitoids to locate their herbivore hosts will contribute towards effective exploitation of parasitoids as biological control agents against damaging crop pests. The current study was conducted to establish key bioactive compounds mediating behavioural response in the parasitic wasp Cotesia sesamiae (Cameron, 1906) (Hymenoptera: Braconidae) by employing electrophysiological techniques and behavioural studies. Electroantennogram (EAG) recording with synthetic equivalents of herbivore-induced plant volatiles (HIPVs) using C. sesamiae confirmed that 13 compounds elicited electrophysiological activity. In an olfactometer bioassay, only (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-β-farnesene, and (E,E)-4,8,-trimethyl-1,3,7-tridecatetraene elicited behavioural responses from the parasitoid when tested individually at a natural dose, while (R)-linalool and (E)-caryophyllene were attractive at the highest test dose. Other EAG-active HIPVs were not attractive either at lower or higher concentrations, suggesting their role as blend components rather than functioning as attractants by themselves. In contrast, nine-component synthetic HIPV blends formulated in the same ratio and concentration as in a natural sample was as attractive as the original headspace sample. By providing further insights into maize–stemborer–parasitoid tritrophic interactions, results from this study will help in selecting crop cultivars emitting appropriate HIPVs to attract natural enemies and perhaps repel damaging crop pests.

Interplay between insects and plants: dynamic and complex interactions that have coevolved over millions of years but act in milliseconds

In an environment with changing availability and quality of host plants, phytophagous insects are under selection pressure to find quality hosts. They need to maximize their fitness by locating suitable plants and avoiding unsuitable ones. Thus, they have evolved a finely tuned sensory system, for detection of host cues, and a nervous system, capable of integrating inputs from sensory neurons with a high level of spatio-temporal resolution. Insect responses to cues are not fixed but depend on the context in which they are perceived, the physiological state of the insect, and prior learning experiences. However, there are examples of insects making 'mistakes' and being attracted to poor quality hosts. While insects have evolved ways of finding hosts, plants have been under selection pressure to do precisely the opposite and evade detection or defend themselves when attacked. Once on the plant, insect-associated molecules may trigger or suppress defence depending on whether the plant or the insect is ahead in evolutionary terms. Plant volatile emission is influenced by defence responses induced by insect feeding or oviposition which can attract natural enemies but repel herbivores. Conversely, plant reproductive fitness is increased by attraction of pollinators. Interactions can be altered by other organisms associated with the plant such as other insects, plant pathogens, or mycorrhizal fungi. Plant phenotype is plastic and can be changed by epigenetic factors in adaptation to periods of biotic stress. Space and time play crucial roles in influencing the outcome of interactions between insects and plants.

Multidimensional approaches for studying plant defence against insects: from ecology to omics and synthetic biology

The biggest challenge for modern biology is to integrate multidisciplinary approaches towards understanding the organizational and functional complexity of biological systems at different hierarchies, starting from the subcellular molecular mechanisms (microscopic) to the functional interactions of ecological communities (macroscopic). The plant-insect interaction is a good model for this purpose with the availability of an enormous amount of information at the molecular and the ecosystem levels. Changing global climatic conditions are abruptly resetting plant-insect interactions. Integration of discretely located heterogeneous information from the ecosystem to genes and pathways will be an advantage to understand the complexity of plant-insect interactions. This review will present the recent developments in omics-based high-throughput experimental approaches, with particular emphasis on studying plant defence responses against insect attack. The review highlights the importance of using integrative systems approaches to study plant-insect interactions from the macroscopic to the microscopic level. We analyse the current efforts in generating, integrating and modelling multiomics data to understand plant-insect interaction at a systems level. As a future prospect, we highlight the growing interest in utilizing the synthetic biology platform for engineering insect-resistant plants.

Extrafloral-nectar-based partner manipulation in plant-ant relationships

Plant-ant interactions are generally considered as mutualisms, with both parties gaining benefits from the association. It has recently emerged that some of these mutualistic associations have, however, evolved towards other forms of relationships and, in particular, that plants may manipulate their partner ants to make reciprocation more beneficial, thereby stabilizing the mutualism. Focusing on plants bearing extrafloral nectaries, we review recent studies and address three key questions: (i) how can plants attract potential partners and maintain their services; (ii) are there compounds in extrafloral nectar that could mediate partner manipulation; and (iii) are ants susceptible to such compounds? After reviewing the current knowledge on plant-ant associations, we propose a possible scenario where plant-derived chemicals, such as secondary metabolites, known to have an impact on animal brain, could have evolved in plants to attract and manipulate ant behaviour. This new viewpoint would place plant-animal interaction in a different ecological context, opening new ecological and neurobiological perspectives of drug seeking and use.

Extrafloral nectar at the plant-insect interface: a spotlight on chemical ecology, phenotypic plasticity, and food webs

Plants secrete extrafloral nectar (EFN) as an induced defense against herbivores. EFN contains not only carbohydrates and amino acids but also pathogenesis-related proteins and other protective enzymes, making EFN an exclusive reward. EFN secretion is commonly induced after wounding, likely owing to a jasmonic acid-induced cell wall invertase, and is limited by phloem sucrose availability: Both factors control EFN secretion according to the optimal defense hypothesis. Non-ant EFN consumers include parasitoids, wasps, spiders, mites, bugs, and predatory beetles. Little is known about the relevance of EFN to the nutrition of its consumers and, hence, to the structuring of arthropod communities. The mutualism can be established quickly among noncoevolved (e.g., invasive) species, indicating its easy assembly is due to ecological fitting. Therefore, increasing efforts are directed toward using EFN in biocontrol. However, documentation of the importance of EFN for the communities of plants and arthropods in natural, invasive, and agricultural ecosystems is still limited.

Chloroplast signaling within, between and beyond cells

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.

Differential Recruitment of Camponotus femoratus (Fabricius) Ants in Response to Ant Garden Herbivory

Although several studies have shown that ants can recognize chemical cues from their host plants in ant-plant systems, it is poorly demonstrated in ant gardens (AGs). In this interaction, ant species constantly interact with various epiphyte species. Therefore, it is possible to expect a convergence of chemical signals released by plants that could be acting to ensure that ants are able to recognize and defend epiphyte species frequently associated with AGs. In this study, it was hypothesized that ants recognize and differentiate among chemical stimuli released by AG epiphytes and non-AG epiphytes. We experimentally simulated leaf herbivore damage on three epiphyte species restricted to AGs and a locally abundant understory herb, Piper hispidum, in order to quantify the number of recruited Camponotus femoratus (Fabricius) defenders. When exposed to the AG epiphytes Peperomia macrostachya and Codonanthe uleana leaves, it was observed that the recruitment of C. femoratus workers was, on average, respectively 556% and 246% higher than control. However, the number of ants recruited by the AG epiphyte Markea longiflora or by the non-AG plant did not differ from paper pieces. This indicated that ants could discern between chemicals released by different plants, suggesting that ants can select better plants. These results can be explained by evolutionary process acting on both ants' capability in discerning plants' chemical compounds (innate attraction) or by ants' learning based on the epiphyte frequency in AGs (individual experience). To disentangle an innate behavior, a product of classical coevolutionary process, from an ant's learned behavior, is a complicated but important subject to understand in the evolution of ant-plant mutualisms.

Atmospheric change and induced plant secondary metabolites - are we reshaping the building blocks of multi-trophic interactions?

At least for the foreseeable future, atmospheric concentrations of greenhouse gases - particularly carbon dioxide (CO₂) and ozone (O₃) - are projected to rise inexorably. Recent studies have begun to unveil the complex nature of how these gases modulate the expression of plant signaling hormones, the defensive chemistries produced, and the responses of the myriad trophic interactions involving plant pathogens as well as insect herbivores and their natural enemies. Given the ubiquity of complex trophic interactions in both natural and managed systems, it is crucial that we understand how CO₂ and O₃ interact with defense signaling hormones of plants and their consequences for their trophic associates if we are to adapt to, and even mitigate, the effects of climate change.

Plant methyl salicylate induces defense responses in the rhizobacterium Bacillus subtilis

Bacillus subtilis is a rhizobacterium that promotes plant growth and health. Cultivation of B. subtilis with an uprooted weed on solid medium produced pleat-like architectures on colonies near the plant. To test whether plants emit signals that affect B. subtilis colony morphology, we examined the effect of plant-related compounds on colony morphology. Bacillus subtilis formed mucoid colonies specifically in response to methyl salicylate, which is a plant-defense signal released in response to pathogen infection. Methyl salicylate induced mucoid colony formation by stimulating poly-γ-glutamic acid biosynthesis, which formed enclosing capsules that protected the cells from exposure to antimicrobial compounds. Poly-γ-glutamic acid synthesis depended on the DegS-DegU two-component regulatory system, which activated DegSU-dependent gene transcription in response to methyl salicylate. Bacillus subtilis did not induce plant methyl salicylate production, indicating that the most probable source of methyl salicylate in the rhizosphere is pathogen-infected plants. Methyl salicylate induced B. subtilis biosynthesis of the antibiotics bacilysin and fengycin, the latter of which exhibited inhibitory activity against the plant pathogenic fungus Fusarium oxysporum. We propose that B. subtilis may sense plants under pathogen attack via methyl salicylate, and express defense responses that protect both B. subtilis and host plants in the rhizosphere.

Homogalacturonan-modifying enzymes: structure, expression, and roles in plants

Understanding the changes affecting the plant cell wall is a key element in addressing its functional role in plant growth and in the response to stress. Pectins, which are the main constituents of the primary cell wall in dicot species, play a central role in the control of cellular adhesion and thereby of the rheological properties of the wall. This is likely to be a major determinant of plant growth. How the discrete changes in pectin structure are mediated is thus a key issue in our understanding of plant development and plant responses to changes in the environment. In particular, understanding the remodelling of homogalacturonan (HG), the most abundant pectic polymer, by specific enzymes is a current challenge in addressing its fundamental role. HG, a polymer that can be methylesterified or acetylated, can be modified by HGMEs (HG-modifying enzymes) which all belong to large multigenic families in all species sequenced to date. In particular, both the degrees of substitution (methylesterification and/or acetylation) and polymerization can be controlled by specific enzymes such as pectin methylesterases (PMEs), pectin acetylesterases (PAEs), polygalacturonases (PGs), or pectate lyases-like (PLLs). Major advances in the biochemical and functional characterization of these enzymes have been made over the last 10 years. This review aims to provide a comprehensive, up to date summary of the recent data concerning the structure, regulation, and function of these fascinating enzymes in plant development and in response to biotic stresses.

Jasmonate-dependent induction of polyphenol oxidase activity in tomato foliage is important for defense against Spodoptera exigua but not against Manduca sexta

BACKGROUND

Jasmonates are involved in plant defense, participating in the timely induction of defense responses against insect herbivores from different feeding guilds and with different degrees of host specialization. It is less clear to what extent the induction of plant defense is controlled by different members of the jasmonate family and how specificity of the response is achieved. Using transgenic plants blocked in jasmonic acid (JA) biosynthesis, we previously showed that JA is required for the formation of glandular trichomes and trichome-borne metabolites as constitutive defense traits in tomato, affecting oviposition and feeding behavior of the specialist Manduca sexta. In contrast, JA was not required for the local induction of defense gene expression after wounding. In JA-deficient plants, the JA precursor oxophytodienoic acid (OPDA) substituted as a regulator of defense gene expression maintaining considerable resistance against M. sexta larvae. In this study, we investigate the contribution of JA and OPDA to defense against the generalist herbivore Spodoptera exigua.

RESULTS

S. exigua preferred JA-deficient over wild-type tomato plants as a host for both oviposition and feeding. Feeding preference for JA-deficient plants was caused by constitutively reduced levels of repellent terpenes. Growth and development of the larvae, on the other hand, were controlled by additional JA-dependent defense traits, including the JA-mediated induction of foliar polyphenol oxidase (PPO) activity. PPO induction was more pronounced after S. exigua herbivory as compared to mechanical wounding or M. sexta feeding. The difference was attributed to an elicitor exclusively present in S. exigua oral secretions.

CONCLUSIONS

The behavior of M. sexta and S. exigua during oviposition and feeding is controlled by constitutive JA/JA-Ile-dependent defense traits involving mono- and sesquiterpenes in both species, and cis-3-hexenal as an additional chemical cue for M. sexta. The requirement of jasmonates for resistance of tomato plants against caterpillar feeding differs for the two species. While the OPDA-mediated induction of local defense is sufficient to restrict growth and development of M. sexta larvae in absence of JA/JA-Ile, defense against S. exigua relied on additional JA/JA-Ile dependent factors, including the induction of foliar polyphenol oxidase activity in response to S. exigua oral secretions.

Eugenol synthase genes in floral scent variation in Gymnadenia species

Floral signaling, especially through floral scent, is often highly complex, and little is known about the molecular mechanisms and evolutionary causes of this complexity. In this study, we focused on the evolution of "floral scent genes" and the associated changes in their functions in three closely related orchid species of the genus Gymnadenia. We developed a benchmark repertoire of 2,571 expressed sequence tags (ESTs) in Gymnadenia odoratissima. For the functional characterization and evolutionary analysis, we focused on eugenol synthase, as eugenol is a widespread and important scent compound. We obtained complete coding complementary DNAs (cDNAs) of two copies of putative eugenol synthase genes in each of the three species. The proteins encoded by these cDNAs were characterized by expression and testing for activity in Escherichia coli. While G. odoratissima and Gymnadenia conopsea enzymes were found to catalyze the formation of eugenol only, the Gymnadenia densiflora proteins synthesize eugenol, as well as a smaller amount of isoeugenol. Finally, we showed that the eugenol and isoeugenol producing gene copies of G. densiflora are evolutionarily derived from the ancestral genes of the other species producing only eugenol. The evolutionary switch from production of one to two compounds evolved under relaxed purifying selection. In conclusion, our study shows the molecular bases of eugenol and isoeugenol production and suggests that an evolutionary transition in a single gene can lead to an increased complexity in floral scent emitted by plants.

Microbiology of sugar-rich environments: diversity, ecology and system constraints

Microbial habitats that contain an excess of carbohydrate in the form of sugar are widespread in the microbial biosphere. Depending on the type of sugar, prevailing water activity and other substances present, sugar-rich environments can be highly dynamic or relatively stable, osmotically stressful, and/or destabilizing for macromolecular systems, and can thereby strongly impact the microbial ecology. Here, we review the microbiology of different high-sugar habitats, including their microbial diversity and physicochemical parameters, which act to impact microbial community assembly and constrain the ecosystem. Saturated sugar beet juice and floral nectar are used as case studies to explore the differences between the microbial ecologies of low and higher water-activity habitats respectively. Nectar is a paradigm of an open, dynamic and biodiverse habitat populated by many microbial taxa, often yeasts and bacteria such as, amongst many others, Metschnikowia spp. and Acinetobacter spp., respectively. By contrast, thick juice is a relatively stable, species-poor habitat and is typically dominated by a single, xerotolerant bacterium (Tetragenococcus halophilus). A number of high-sugar habitats contain chaotropic solutes (e.g. ethyl acetate, phenols, ethanol, fructose and glycerol) and hydrophobic stressors (e.g. ethyl octanoate, hexane, octanol and isoamyl acetate), all of which can induce chaotropicity-mediated stresses that inhibit or prevent multiplication of microbes. Additionally, temperature, pH, nutrition, microbial dispersion and habitat history can determine or constrain the microbiology of high-sugar milieux. Findings are discussed in relation to a number of unanswered scientific questions.

Metabolic engineering of volatile isoprenoids in plants and microbes

The chemical properties and diversity of volatile isoprenoids lends them to a broad variety of biological roles. It also lends them to a host of biotechnological applications, both by taking advantage of their natural functions and by using them as industrial chemicals/chemical feedstocks. Natural functions include roles as insect attractants and repellents, abiotic stress protectants in pathogen defense, etc. Industrial applications include use as pharmaceuticals, flavours, fragrances, fuels, fuel additives, etc. Here we will examine the ways in which researchers have so far found to exploit volatile isoprenoids using biotechnology. Production and/or modification of volatiles using metabolic engineering in both plants and microorganisms are reviewed, including engineering through both mevalonate and methylerythritol diphosphate pathways. Recent advances are illustrated using several case studies (herbivores and bodyguards, isoprene, and monoterpene production in microbes). Systems and synthetic biology tools with particular utility for metabolic engineering are also reviewed. Finally, we discuss the practical realities of various applications in modern biotechnology, explore possible future applications, and examine the challenges of moving these technologies forward so that they can deliver tangible benefits. While this review focuses on volatile isoprenoids, many of the engineering approaches described here are also applicable to non-isoprenoid volatiles and to non-volatile isoprenoids.

Direct and indirect chemical defences against insects in a multitrophic framework

Plant secondary metabolites play an important role in mediating interactions with insect herbivores and their natural enemies. Metabolites stored in plant tissues are usually investigated in relation to herbivore behaviour and performance (direct defence), whereas volatile metabolites are often studied in relation to natural enemy attraction (indirect defence). However, so-called direct and indirect defences may also affect the behaviour and performance of the herbivore's natural enemies and the natural enemy's prey or hosts, respectively. This suggests that the distinction between these defence strategies may not be as black and white as is often portrayed in the literature. The ecological costs associated with direct and indirect chemical defence are often poorly understood. Chemical defence traits are often studied in two-species interactions in highly simplified experiments. However, in nature, plants and insects are often engaged in mutualistic interactions with microbes that may also affect plant secondary chemistry. Moreover, plants are challenged by threats above- and belowground and herbivory may have consequences for plant-insect multitrophic interactions in the alternative compartment mediated by changes in plant secondary chemistry. These additional associations further increase the complexity of interaction networks. Consequently, the effect of a putative defence trait may be under- or overestimated when other interactions are not considered.

Insect attraction to herbivore-induced beech volatiles under different forest management regimes

Insect herbivore enemies such as parasitoids and predators are important in controlling herbivore pests. From agricultural systems we know that land-use intensification can negatively impact biological control as an important ecosystem service. The aim of our study was to investigate the importance of management regime for natural enemy pressure and biological control possibilities in forests dominated by European beech. We hypothesize that the volatile blend released from herbivore-infested beech trees functions as a signal, attracting parasitoids and herbivore enemies. Furthermore, we hypothesize that forest management regime influences the composition of species attracted by these herbivore-induced beech volatiles. We installed flight-interception traps next to Lymantria dispar caterpillar-infested young beech trees releasing herbivore-induced volatiles and next to non-infested control trees. Significantly more parasitoids were captured next to caterpillar-infested trees compared to non-infested controls, irrespective of forest type. However, the composition of the trophic guilds in the traps did vary in response to forest management regime. While the proportion of chewing insects was highest in non-managed forests, the proportion of sucking insects peaked in forests with low management and of parasitoids in young, highly managed, forest stands. Neither the number of naturally occurring beech saplings nor herbivory levels in the proximity of our experiment affected the abundance and diversity of parasitoids caught. Our data show that herbivore-induced beech volatiles attract herbivore enemies under field conditions. They further suggest that differences in the structural complexity of forests as a consequence of management regime only play a minor role in parasitoid activity and thus in indirect tree defense.

Reciprocal crosstalk between jasmonate and salicylate defence-signalling pathways modulates plant volatile emission and herbivore host-selection behaviour

The jasmonic acid (JA) and salicylic acid (SA) signalling pathways, which mediate induced plant defence responses, can express negative crosstalk. Limited knowledge is available on the effects of this crosstalk on host-plant selection behaviour of herbivores. We report on temporal and dosage effects of such crosstalk on host preference and oviposition-site selection behaviour of the herbivorous spider mite Tetranychus urticae towards Lima bean (Phaseolus lunatus) plants, including underlying mechanisms. Behavioural observations reveal a dynamic temporal response of mites to single or combined applications of JA and SA to the plant, including attraction and repellence, and an antagonistic interaction between SA- and JA-mediated plant responses. Dose-response experiments show that concentrations of 0.001mM and higher of one phytohormone can neutralize the repellent effect of a 1mM application of the other phytohormone on herbivore behaviour. Moreover, antagonism between the two signal-transduction pathways affects phytohormone-induced volatile emission. Our multidisciplinary study reveals the dynamic plant phenotype that is modulated by subtle changes in relative phytohormonal titres and consequences for the dynamic host-plant selection by an herbivore. The longer-term effects on plant-herbivore interactions deserve further investigation.

Elevated O₃ enhances the attraction of whitefly-infested tomato plants to Encarsia formosa

We experimentally examined the effects of elevated O₃ and whitefly herbivory on tomato volatiles, feeding and oviposition preferences of whiteflies and behavioural responses of Encarsia formosa to these emissions on two tomato genotypes, a wild-type (Wt) and a jasmonic acid (JA) defence-enhanced genotype (JA-OE, 35S). The O₃ level and whitefly herbivory significantly increased the total amount of volatile organic compounds (VOCs), monoterpenes, green leaf volatiles (GLVs), and aldehyde volatiles produced by tomato plants. The 35S plants released higher amount of total VOCs and monoterpene volatiles than Wt plants under O₃+herbivory treatments. The feeding and oviposition bioassays showed that control plants were preferred by adult whiteflies whereas the 35S plants were not preferred by whiteflies. In the Y-tube tests, O₃+herbivory treatment genotypes were preferred by adult E. Formosa. The 35S plants were preferred by adult E. formosa under O₃, herbivory and O₃+herbivory treatments. Our results demonstrated that elevated O₃ and whitefly herbivory significantly increased tomato volatiles, which attracted E. formosa and reduced whitefly feeding. The 35S plants had a higher resistance to B. tabaci than Wt plant. Such changes suggest that the direct and indirect defences of resistant genotypes, such as 35S, could strengthen as the atmospheric O₃ concentration increases.

Multiple ant species tending lac insect Kerria yunnanensis (Hemiptera: Kerriidae) provide asymmetric protection against parasitoids

This study investigated the effects of ant attendance on the parasitoid community and parasitism of lac insect Kerria yunnanensis aggregations in Yunnan province, China. We manipulated ant attendance to establish three treatments: (1) ant exclusion; (2) low ant attendance by several ant species; and (3) high ant attendance by Crematogaster macaoensis. Five parasitoid species were collected, with two species contributing 82.7 and 13.2% of total abundance respectively. Total parasitoid abundance was lowest in the February sample when K. yunnanensis was in its younger life stage, being significantly lower in the ant exclusion treatment. In April, all three treatments had significantly different parasitoid abundances, being highest in the ant exclusion treatment and the lowest in the high ant attendance treatment. When ants were present, there were strong negative relationships between total parasitoid abundance and ant abundance, with the relationships being dependent upon the ant species composition and abundance. The patterns of total parasitoid abundance were driven by the two most abundant parasitoid species. Parasitoid species richness did not differ among treatments or between sample times, however, multivariate analysis confirmed that overall parasitoid community structure differed significantly among treatments and between sample times, with the high ant attendance treatment differing most from the other two treatments. Interestingly the absence of ants did not result in increased parasitism from four of the five parasitoids. Ants in lac insect farming systems have a clear role for agricultural pest management. A full understanding of the asymmetric abilities of ants to influence parasitoid communities, and affect parasitism of hosts will require further experimental manipulation to assess the relative roles of 1) the abundance of each individual ant species on parasitoid access to hosts, 2) competition among parasitoids, and 3) the interaction between the first two factors.

Ecological interactions shape the dynamics of seed predation in Acrocomia aculeata (Arecaceae)

Background

The complex network of direct and indirect relationships determines not only the species abundances but also the community characteristics such as diversity and stability. In this context, seed predation is a direct interaction that affects the reproductive success of the plant. For Acrocomia aculeata, the seed predation by Pachymerus cardo and Speciomerus revoili in post-dispersal may destroy more than 70% of the propagules and is influenced by the herbivory of the fruits during pre-dispersal. Fruits of plants with a higher level of herbivory during pre-dispersal are less attacked by predators in post-dispersal. We proposed a hypothesis that describes this interaction as an indirect defense mediated by fungi in a multitrophic interaction. As explanations, we proposed the predictions: i) injuries caused by herbivores in the fruits of A. aculeata favor fungal colonization and ii) the colonization of A. acuelata fruit by decomposing fungi reduces the selection of the egg-laying site by predator.

Methodology/Principal Findings

For prediction (i), differences in the fungal colonization in fruits with an intact or damaged epicarp were evaluated in fruits exposed in the field. For prediction (ii), we performed fruit observations in the field to determine the number of eggs of P. cardo and/or S. revoili per fruit and the amount of fungal colonization in the fruits. In another experiment, in the laboratory, we use P. cardo females in a triple-choice protocol. Each insect to choose one of the three options: healthy fruits, fruits with fungus, or an empty pot. The proposed hypothesis was corroborated. Fruits with injuries in the epicarp had a higher fungal colonization, and fruits colonized by fungi were less attractive for egg-laying by seed predators.

Conclusion/Significance

This study emphasizes the importance of exploring the networks of interactions between multitrophic systems to understand the dynamics and maintenance of natural populations.

Current issues in the evolutionary ecology of ant-plant symbioses

Ant-plant symbioses involve plants that provide hollow structures specialized for housing ants and often food to ants. In return, the inhabiting ants protect plants against herbivores and sometimes provide them with nutrients. Here, we review recent advances in ant-plant symbioses, focusing on three areas. First, the nutritional ecology of plant-ants, which is based not only on plant-derived food rewards, but also on inputs from other symbiotic partners, in particular fungi and possibly bacteria. Food and protection are the most important 'currencies' exchanged between partners and they drive the nature and evolution of the relationships. Secondly, studies of conflict and cooperation in ant-plant symbioses have contributed key insights into the evolution and maintenance of mutualism, particularly how partner-mediated feedbacks affect the specificity and stability of mutualisms. There is little evidence that mutualistic ants or plants are under selection to cheat, but the costs and benefits of ant-plant interactions do vary with environmental factors, making them vulnerable to natural or anthropogenic environmental change. Thus, thirdly, ant-plant symbioses should be considered good models for investigating the effects of global change on the outcome of mutualistic interactions.

Mutualistic ants as an indirect defence against leaf pathogens

Mutualistic ants are commonly considered as an efficient indirect defence against herbivores. Nevertheless, their indirect protective role against plant pathogens has been scarcely investigated. We compared the protective role against pathogens of two different ant partners, a mutualistic and a parasitic ant, on the host plant Acacia hindsii (Fabaceae). The epiphytic bacterial community on leaves was evaluated in the presence and absence of both ant partners by cultivation and by 454 pyrosequencing of the 16S rRNA gene. Pathogen-inflicted leaf damage, epiphytic bacterial abundance (colony-forming units) and number of operational taxonomic units (OTUs) were significantly higher in plants inhabited by parasitic ants than in plants inhabited by mutualistic ants. Unifrac unweighted and weighted principal component analyses showed that the bacterial community composition on leaves changed significantly when mutualistic ants were removed from plants or when plants were inhabited by parasitic ants. Direct mechanisms provided by ant-associated bacteria would contribute to the protective role against pathogens. The results suggest that the indirect defence of mutualistic ants also covers the protection from bacterial plant pathogens. Our findings highlight the importance of considering bacterial partners in ant-plant defensive mutualisms, which can contribute significantly to ant-mediated protection from plant pathogens.

The contribution of genetic and genomic approaches to plant domestication studies

The application of genomic approaches to the phenomenon of plant domestication promises a better understanding of the origins of agriculture, but also of the way plant genomes in general are organized and expressed. Building on earlier genetic research, more detailed information has become available on the organization of genetic diversity at the genome level and the effects of gene flow on diversity in different regions of the genome. In addition, putative domestication genes have been identified through population genomics approaches (selective sweeps or divergence scanning). Further information has been obtained on the origin of domestication syndrome mutations and the dispersal and adaptation of crops after domestication. For the future, increasingly multidisciplinary approaches using combinations of genomics and other approaches will prevail.

The evolution of floral deception in Epipactis veratrifolia (Orchidaceae): from indirect defense to pollination

BACKGROUND

It is estimated that floral deception has evolved in at least 7500 species of angiosperms, of which two thirds are orchids. Epipactis veratrifolia (Orchidaceae) is a model system of aphid mimicry as aphidophagous hoverflies lay eggs on false brood sites on their flowers. To understand the evolutionary ecology of floral deception, we investigated the pollination biology of E. veratrifolia across 10 populations in the Eastern Himalayas. We reconstructed the phylogeny of Epipactis and mapped the known pollination systems of previously studied species onto the tree.

RESULTS

Some inflorescences of E. veratrifolia were so infested with aphids while they were still in bud that the some larvae of hoverflies developed to the third instar while flower buds opened. This indicated that adult female hoverflies were partly rewarded for oviposition. Although flowers failed to secrete nectar, they mimicked both alarm pheromones and aphid coloring of to attract female hoverflies as their exclusive pollinators. Phylogenetic mapping indicate that pollination by aphidophagous hoverflies is likely an ancestral condition in the genus Epipactis. We suggest that the biological interaction of aphid (prey), orchid (primary producer) and hoverfly (predator) may represent an intermediate stage between mutualism and deception in the evolution of pollination-by-deceit in E. veratrifolia.

CONCLUSIONS

Our analyses indicate that this intermediate stage may be used as a model system to interpret the origin of oviposition (brood site) mimicry in Epipactis. We propose the hypothesis that some deceptive pollination systems evolved directly from earlier (partly) mutualistic systems that maintained the fidelity of the original pollinator(s) even though rewards (nectar/ brood site) were lost.

Damaged-self recognition in common bean (Phaseolus vulgaris) shows taxonomic specificity and triggers signaling via reactive oxygen species (ROS)

Plants require reliable mechanisms to detect injury. Danger signals or "damage-associated molecular patterns" (DAMPs) are released from stressed host cells and allow injury detection independently of enemy-derived molecules. We studied the response of common bean (Phaseolus vulgaris) to the application of leaf homogenate as a source of DAMPs and measured the production of reactive oxygen species (ROS) as an early response and the secretion of extrafloral nectar (EFN) as a jasmonic acid (JA)-dependent late response. We observed a strong taxonomic signal in the response to different leaf homogenates. ROS formation and EFN secretion were highly correlated and responded most strongly to leaf homogenates produced using the same cultivar or closely related accessions, less to a distantly related cultivar of common bean or each of the two congeneric species, P. lunatus and P. coccineus, and not at all to homogenates prepared from species in different genera, not even when using other Fabaceae. Interestingly, leaf homogenates also reduced the infection by the bacterial pathogen, Pseudomonas syringae, when they were applied directly before challenging, although the same homogenates exhibited no direct in vitro inhibitory effect in the bacterium. We conclude that ROS signaling is associated to the induction of EFN secretion and that the specific blend of DAMPs that are released from damaged cells allows the plant to distinguish the "damaged-self" from the damaged "non-self." The very early responses of plants to DAMPs can trigger resistance to both, herbivores and pathogens, which should be adaptive because injury facilitates infection, independently of its causal reason.

Plant interactions with multiple insect herbivores: from community to genes

Every plant is a member of a complex insect community that consists of tens to hundreds of species that belong to different trophic levels. The dynamics of this community are critically influenced by the plant, which mediates interactions between community members that can occur on the plant simultaneously or at different times. Herbivory results in changes in the plant's morphological or chemical phenotype that affect interactions with subsequently arriving herbivores. Changes in the plant's phenotype are mediated by molecular processes such as phytohormonal signaling networks and transcriptomic rearrangements that are initiated by oral secretions of the herbivore. Processes at different levels of biological complexity occur at timescales ranging from minutes to years. In this review, we address plant-mediated interactions with multiple species of the associated insect community and their effects on community dynamics, and link these to the mechanistic effects that multiple attacks have on plant phenotypes.

Relationships between plant diversity and the abundance and α-diversity of predatory ground beetles (Coleoptera: Carabidae) in a mature Asian temperate forest ecosystem

A positive relationship between plant diversity and both abundance and diversity of predatory arthropods is postulated by the Enemies Hypothesis, a central ecological top-down control hypothesis. It has been supported by experimental studies and investigations of agricultural and grassland ecosystems, while evidence from more complex mature forest ecosystems is limited. Our study was conducted on Changbai Mountain in one of the last remaining large pristine temperate forest environments in China. We used predatory ground beetles (Coleoptera: Carabidae) as target taxon to establish the relationship between phytodiversity and their activity abundance and diversity. Results showed that elevation was the only variable included in both models predicting carabid activity abundance and α-diversity. Shrub diversity was negatively and herb diversity positively correlated with beetle abundance, while shrub diversity was positively correlated with beetle α-diversity. Within the different forest types, a negative relationship between plant diversity and carabid activity abundance was observed, which stands in direct contrast to the Enemies Hypothesis. Furthermore, plant species density did not predict carabid α-diversity. In addition, the density of herbs, which is commonly believed to influence carabid movement, had little impact on the beetle activity abundance recorded on Changbai Mountain. Our study indicates that in a relatively large and heterogeneous mature forest area, relationships between plant and carabid diversity are driven by variations in environmental factors linked with altitudinal change. In addition, traditional top-down control theories that are suitable in explaining diversity patterns in ecosystems of low diversity appear to play a much less pronounced role in highly complex forest ecosystems.

Loss of specificity: native but not invasive populations of Triadica sebifera vary in tolerance to different herbivores

During introduction, invasive plants can be released from specialist herbivores, but may retain generalist herbivores and encounter novel enemies. For fast-growing invasive plants, tolerance of herbivory via compensatory regrowth may be an important defense against generalist herbivory, but it is unclear whether tolerance responses are specifically induced by different herbivores and whether specificity differs among native and invasive plant populations. We conducted a greenhouse experiment to examine the variation among native and invasive populations of Chinese tallow tree, Triadica sebifera, in their specificity of tolerance responses to herbivores by exposing plants to herbivory from either one of two generalist caterpillars occurring in the introduced range of Triadica. Simultaneously, we measured the specificity of another defensive trait, extrafloral nectar (EFN) production, to detect potential tradeoffs between resistance and tolerance of herbivores. Invasive populations had higher aboveground biomass tolerance than native populations, and responded non-specifically to either herbivore, while native populations had significantly different and specific aboveground biomass responses to the two herbivores. Both caterpillar species similarly induced EFN in native and invasive populations. Plant tolerance and EFN were positively correlated or had no relationship and biomass in control and herbivore-damaged plants was positively correlated, suggesting little costs of tolerance. Relationships among these vegetative traits depended on herbivore type, suggesting that some defense traits may have positive associations with growth-related processes that are differently induced by herbivores. Importantly, loss of specificity in invasive populations indicates subtle evolutionary changes in defenses in invasive plants that may relate to and enhance their invasive success.

How can we exploit above-belowground interactions to assist in addressing the challenges of food security?

Can above-belowground interactions help address issues of food security? We address this question in this manuscript, and review the intersection of above-belowground interactions and food security. We propose that above-belowground interactions could address two strategies identified by Godfray etal. (2010): reducing the Yield Gap, and Increasing Production Limits. In particular, to minimize the difference between potential and realized production (The Yield Gap) above-belowground interactions could be manipulated to reduce losses to pests and increase crop growth (and therefore yields). To Increase Production Limits we propose two mechanisms: utilizing intercropping (which uses multiple aspects of above-belowground interactions) and breeding for traits that promote beneficial above-belowground interactions, as well as breeding mutualistic organisms to improve their provided benefit. As a result, if they are managed correctly, there is great potential for above-belowground interactions to contribute to food security.

Tomato below ground-above ground interactions: Trichoderma longibrachiatum affects the performance of Macrosiphum euphorbiae and its natural antagonists

Below ground and above ground plant-insect-microorganism interactions are complex and regulate most of the developmental responses of important crop plants such as tomato. We investigated the influence of root colonization by a nonmycorrhizal plant-growth-promoting fungus on direct and indirect defenses of tomato plant against aphids. The multitrophic system included the plant Solanum lycopersicum ('San Marzano nano'), the root-associated biocontrol fungus Trichoderma longibrachiatum strain MK1, the aphid Macrosiphum euphorbiae (a tomato pest), the aphid parasitoid Aphidius ervi, and the aphid predator Macrolophus pygmaeus. Laboratory bioassays were performed to assess the effect of T. longibrachiatum MK1, interacting with the tomato plant, on quantity and quality of volatile organic compounds (VOC) released by tomato plant, aphid development and reproduction, parasitoid behavior, and predator behavior and development. When compared with the uncolonized controls, plants whose roots were colonized by T. longibrachiatum MK1 showed quantitative differences in the release of specific VOC, better aphid population growth indices, a higher attractiveness toward the aphid parasitoid and the aphid predator, and a quicker development of aphid predator. These findings support the development of novel strategies of integrated control of aphid pests. The species-specific or strain-specific characteristics of these below ground-above ground interactions remain to be assessed.

PTR-MS in Italy: a multipurpose sensor with applications in environmental, agri-food and health science

Proton Transfer Reaction Mass Spectrometry (PTR-MS) has evolved in the last decade as a fast and high sensitivity sensor for the real-time monitoring of volatile compounds. Its applications range from environmental sciences to medical sciences, from food technology to bioprocess monitoring. Italian scientists and institutions participated from the very beginning in fundamental and applied research aiming at exploiting the potentialities of this technique and providing relevant methodological advances and new fundamental indications. In this review we describe this activity on the basis of the available literature. The Italian scientific community has been active mostly in food science and technology, plant physiology and environmental studies and also pioneered the applications of the recently released PTR-ToF-MS (Proton Transfer Reaction-Time of Flight-Mass Spectrometry) in food science and in plant physiology. In the very last years new results related to bioprocess monitoring and health science have been published as well. PTR-MS data analysis, particularly in the case of the ToF based version, and the application of advanced chemometrics and data mining are also aspects characterising the activity of the Italian community.

Extrafloral nectaries in neotropical Gentianaceae: occurrence, distribution patterns, and anatomical characterization

PREMISE OF THE STUDY

Extrafloral nectaries (EFNs) are structures that secrete nectar and protect plants against herbivores and pathogens. In Gentianaceae, these structures have been described in species of Calolisianthus, Fagraea, and Anthocleista and are important morphological markers for taxonomic and phylogenetic studies. To establish a foundation for further studies, we investigated the occurrence, distribution patterns, and anatomy of EFNs on leaves of 27 species belonging to 13 genera and three tribes of neotropical Gentianaceae. •

METHODS

Leaf samples were diaphanized, stained with basic fuchsin, and mounted in glycerinated gelatin. Cross sections were obtained from material embedded in methacrylate or paraffin, stained, and mounted in Permount. Polysaccharides were histochemically stained with periodic acid-Schiff stain. Samples were also examined with scanning electron microscopy. •

KEY RESULTS

Unusual EFNs, visible only with light microscopy, were formed of modified epidermal cells. Each EFN consisted of 2-5 secretory cells encircling a central cell. The EFNs varied in size and in the shape and arrangement of the adjacent cells surrounding the secretory cells. EFNs occurred in all analyzed species as isolated units distributed throughout the leaf blade or as aggregates; aggregates were generally visible to the naked eye. Based on their occurrence as aggregates or isolated units and on their location on the leaf blade, six distribution patterns were identified. •

CONCLUSIONS

This is the first comprehensive study of EFNs on the leaves of neotropical Gentianaceae. The data suggested that NEFs evolved from isolated units for NEFs in aggregates. The results represent a new source of data for future ecological, systematic, and phylogenetic studies in Gentianaceae.

Specificity of extrafloral nectar induction by herbivores differs among native and invasive populations of tallow tree

BACKGROUND AND AIMS

Invasive plants can be released from specialist herbivores and encounter novel generalists in their introduced ranges, leading to variation in defence among native and invasive populations. However, few studies have examined how constitutive and induced indirect defences change during plant invasion, especially during the juvenile stage.

METHODS

Constitutive extrafloral nectar (EFN) production of native and invasive populations of juvenile tallow tree (Triadica sebifera) were compared, and leaf clipping, and damage by a native specialist (Noctuid) and two native generalist caterpillars (Noctuid and Limacodid) were used to examine inducible EFN production.

KEY RESULTS

Plants from introduced populations had more leaves producing constitutive EFN than did native populations, but the content of soluble solids of EFN did not differ. Herbivores induced EFN production more than simulated herbivory. The specialist (Noctuid) induced more EFN than either generalist for native populations. The content of soluble solids in EFN was higher (2·1 times), with the specialist vs. the generalists causing the stronger response for native populations, but the specialist response was always comparable with the generalist responses for invasive populations.

CONCLUSIONS

These results suggest that constitutive and induced indirect defences are retained in juvenile plants of invasive populations even during plant establishment, perhaps due to generalist herbivory in the introduced range. However, responses specific to a specialist herbivore may be reduced in the introduced range where specialists are absent. This decreased defence may benefit specialist insects that are introduced for classical biological control of invasive plants.

Seedling-herbivore interactions: insights into plant defence and regeneration patterns

BACKGROUND

Herbivores have the power to shape plant evolutionary trajectories, influence the structure and function of vegetation, devastate entire crops, or halt the spread of invasive weeds, and as a consequence, research into plant-herbivore interactions is pivotal to our understanding of plant ecology and evolution. However, the causes and consequences of seedling herbivory have received remarkably little attention, despite the fact that plants tend to be most susceptible to herbivory during establishment, and this damage can alter community composition and structure.

SCOPE

In this Viewpoint article we review why herbivory during early plant ontogeny is important and in so doing introduce an Annals of Botany Special Issue that draws together the latest work on the topic. In a synthesis of the existing literature and a collection of new studies, we examine several linked issues. These include the development and expression of seedling defences and patterns of selection by herbivores, and how seedling selection affects plant establishment and community structure. We then examine how disruption of the seedling-herbivore interaction might affect normal patterns of plant community establishment and discuss how an understanding of patterns of seedling herbivory can aid our attempts to restore semi-natural vegetation. We finish by outlining a number of areas where more research is required. These include a need for a deeper consideration of how endogenous and exogenous factors determine investment in seedling defence, particularly for the very youngest plants, and a better understanding of the phylogenetic and biogeographical patterns of seedling defence. There is also much still be to be done on the mechanisms of seedling selection by herbivores, particularly with respect to the possible involvement of volatile cues. These inter-related issues together inform our understanding of how seedling herbivory affects plant regeneration at a time when anthropogenic change is likely to disrupt this long-established, but all-too-often ignored interaction.

Drought and root herbivory interact to alter the response of above-ground parasitoids to aphid infested plants and associated plant volatile signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/below-ground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40-55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and below-ground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought.