Extrafloral nectar production of the ant-associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid

Emerging Trends in Ant-Pollinator Conflict in Extrafloral Nectary-Bearing Plants

The net outcomes of mutualisms are mediated by the trade-offs between the costs and benefits provided by both partners. Our review proposes the existence of a trade-off in ant protection mutualisms between the benefits generated by the ants' protection against the attack of herbivores and the losses caused by the disruption of pollination processes, which are commonly not quantified. This trade-off has important implications for understanding the evolution of extrafloral nectaries (EFNs), an adaptation that has repeatedly evolved throughout the flowering plant clade. We propose that the outcome of this trade-off is contingent on the specific traits of the organisms involved. We provide evidence that the protective mutualisms between ants and plants mediated by EFNs have optimal protective ant partners, represented by the optimum point of the balance between positive effects on plant protection and negative effects on pollination process. Our review also provides important details about a potential synergism of EFN functionality; that is, these structures can attract ants to protect against herbivores and/or distract them from flowers so as not to disrupt pollination processes. Finally, we argue that generalizations regarding how ants impact plants should be made with caution since ants' effects on plants vary with the identity of the ant species in their overall net outcome.

Spider-Plant Interaction: The Role of Extrafloral Nectaries in Spider Attraction and Their Influence on Plant Herbivory and Reproduction

Spiders, abundant and diverse arthropods which occur in vegetation, have received little attention in studies investigating spider-plant interactions, especially in plants which have extrafloral nectaries (EFNs). This study examines whether spiders attracted to EFNs on the plant Heteropterys pteropetala (Malpighiaceae) function as biological protectors, mitigating leaf herbivory and positively impacting plant fitness, through manipulative experiments. Spiders are attracted to EFNs because, in addition to consuming the resource offered by these structures, they also consume the herbivores that are attracted by the nectar. At the same time, we documented the reproductive phenology of the plant studied and the abundance of spiders over time. Our results revealed that the plant's reproductive period begins in December with the emergence of flower buds and ends in April with the production of samarids, fruits which are morphologically adapted for wind dispersal, aligning with the peak abundance of spiders. Furthermore, our results demonstrated that spiders are attracted to plants that exude EFNs, resulting in a positive impact on reducing leaf area loss but with a neutral effect on protecting reproductive structures. By revealing the protective function of spiders' vegetative structures on plants, this research highlights the ecological importance of elucidating the dynamics between spiders and plants, contributing to a deeper understanding of ecosystems.

Dear neighbor: Trees with extrafloral nectaries facilitate defense and growth of adjacent undefended trees

Plant diversity can increase productivity. One mechanism behind this biodiversity effect is facilitation, which is when one species increases the performance of another species. Plants with extrafloral nectaries (EFNs) establish defense mutualisms with ants. However, whether EFN plants facilitate defense of neighboring non-EFN plants is unknown. Synthesizing data on ants, herbivores, leaf damage, and defense traits from a forest biodiversity experiment, we show that trees growing adjacent to EFN trees had higher ant biomass and species richness and lower caterpillar biomass than conspecific controls without EFN-bearing neighbors. Concurrently, the composition of defense traits in non-EFN trees changed. Thus, when non-EFN trees benefit from lower herbivore loads as a result of ants spilling over from EFN tree neighbors, this may allow relatively reduced resource allocation to defense in the former, potentially explaining the higher growth of those trees. Via this mutualist-mediated facilitation, promoting EFN trees in tropical reforestation could foster carbon capture and multiple other ecosystem functions.

Symbiotic ant traits produce differential host-plant carbon and water dynamics in a multi-species mutualism

Cooperative interactions may frequently be reinforced by "partner fidelity feedback," in which high- or low-quality partners drive positive feedbacks with high or low benefits for the host, respectively. Benefits of plant-animal mutualisms for plants have been quantified almost universally in terms of growth or reproduction, but these are only two of many sinks to which a host-plant allocates its resources. By investigating how partners to host-plants impact two fundamental plant resources, carbon and water, we can better characterize plant-partner fidelity and understand how plant-partner mutualisms may be modulated by resource dynamics. In Laikipia, Kenya, four ant species compete for Acacia drepanolobium host-plants. These ants differ in multiple traits, from nectar consumption to host-plant protection. Using a 5-year ant removal experiment, we compared carbon fixation, leaf water status, and stem non-structural carbohydrate concentrations for adult ant-plants with and without ant partners. Removal treatments showed that the ants differentially mediate tree carbon and/or water resources. All three ant species known to be aggressive against herbivores were linked to benefits for host-plant resources, but only the two species that defend but do not prune the host, Crematogaster mimosae and Tetraponera penzigi, increased tree carbon fixation. Of these two species, only the nectivore C. mimosae increased tree simple sugars. Crematogaster nigriceps, which defends the tree but also castrates flowers and prunes meristems, was linked only to lower tree water stress approximated by pre-dawn leaf water potential. In contrast to those defensive ants, Crematogaster sjostedti, a poor defender that displaces other ants, was linked to lower tree carbon fixation. Comparing the effects of the four ant species across control trees suggests that differential ant occupancy drives substantial differences in carbon and water supply among host trees. Our results highlight that ant partners can positively or negatively impact carbon and/or water relations for their host-plant, and we discuss the likelihood that carbon- and water-related partner fidelity feedback loops occur across ant-plant mutualisms.

Mutualist- and antagonist-mediated selection contribute to trait diversification of flowers

Flowers are generally short-lived, and they all face a multidimensional challenge because they have to attract mutualists, compel them to vector pollen with minimal investment in rewards, and repel floral enemies during this short time window. Their displays are under complex selection, either consistent or conflicting, to maximize reproductive fitness under heterogeneous environments. The phenological or morphological mismatches between flowers and visitors will influence interspecific competition, resource access, mating success and, ultimately, population and community dynamics. To better understand the effects of the plant visitors on floral traits, it is necessary to determine the functional significance of specific floral traits for the visitors; how plants respond to both mutualists and antagonists through adaptive changes; and to evaluate the net fitness effects of biological mutualisms and antagonism on plants. In this review, we bring together insights from fields as diverse as floral biology, insect behavioral responses, and evolutionary biology to explain the processes and patterns of floral diversity evolution. Then, we discuss the ecological significance of plant responses to mutualists and antagonists from a community perspective, and propose a set of research questions that can guide the research field to integrate studies of plant defense and reproduction.

Disentangling the effects of jasmonate and tissue loss on the sex allocation of an annual plant

Selection through pollinators plays a major role in the evolution of reproductive traits. However, herbivory can also induce changes in plant sexual expression and sexual systems, potentially influencing conditions governing transitions between sexual systems. Previous work has shown that herbivory has a strong effect on sex allocation in the wind-pollinated annual plant Mercurialis annua, likely via responses to resource loss. It is also known that many plants respond to herbivory by inducing signaling, and endogenous responses to it, via the plant hormone jasmonate. Here, we attempt to uncouple the effects of herbivory on sex allocation in M. annua through resource limitation (tissue loss) versus plant responses to jasmonate hormone signaling. We used a two-factorial experiment with four treatment combinations: control, herbivory (25% chronic tissue loss), jasmonate, and combined herbivory and jasmonate. We estimated the effects of tissue loss and defense-inducing hormones on reproductive allocation, male reproductive effort, and sex allocation. Tissue loss caused plants to reduce their male reproductive effort, resulting in changes in total sex allocation. However, application of jasmonate after herbivory reversed its effect on male investment. Our results show that herbivory has consequences on plant sex expression and sex allocation, and that defense-related hormones such as jasmonate can buffer the impacts. We discuss the physiological mechanisms that might underpin the effects of herbivory on sex allocation, and their potential implications for the evolution of plant sexual systems.

Interactions With Plant Defences Isolate Sympatric Populations of an Herbivorous Mite

Host plant specialisation can promote evolutionary divergence between herbivore populations associated with different plant species. While the mechanisms by which specialist species exploit their hosts have been studied widely across taxa, less is known about the mechanisms that allow intraspecific variants to arise and to be maintained across spatial and temporal scales. To understand whether adaptations to plant defences against herbivory contribute to the co-existence of genetically distinct populations of an herbivore, we investigate the interaction between honeysuckle (Lonicera periclymenum) and sympatric specialist and generalist populations of the spider mite Tetranychus urticae. We found that mite folivory induces the production of sticky droplets on honeysuckle, which have a defensive role: they increase mite mortality directly, and potentially indirectly by increasing the arrestment of a predator. We show that droplet induction and the preference to feed on honeysuckle depend on mite genotype, where the generalist avoids this host and the specialist suppresses droplet production. These traits are heritable and dominant in F1 hybrids between generalists and specialists. Selection pressure from honeysuckle and differences in host preference likely reduce the opportunity of mating encounters on this host. We propose that the interplay between selection from host plant defences and ecological barriers to hybridisation contribute to the persistence of genetically distinct populations of a single species in sympatry.

Evolutionary Ecology of Plant-Arthropod Interactions in Light of the "Omics" Sciences: A Broad Guide

Aboveground plant-arthropod interactions are typically complex, involving herbivores, predators, pollinators, and various other guilds that can strongly affect plant fitness, directly or indirectly, and individually, synergistically, or antagonistically. However, little is known about how ongoing natural selection by these interacting guilds shapes the evolution of plants, i.e., how they affect the differential survival and reproduction of genotypes due to differences in phenotypes in an environment. Recent technological advances, including next-generation sequencing, metabolomics, and gene-editing technologies along with traditional experimental approaches (e.g., quantitative genetics experiments), have enabled far more comprehensive exploration of the genes and traits involved in complex ecological interactions. Connecting different levels of biological organization (genes to communities) will enhance the understanding of evolutionary interactions in complex communities, but this requires a multidisciplinary approach. Here, we review traditional and modern methods and concepts, then highlight future avenues for studying the evolution of plant-arthropod interactions (e.g., plant-herbivore-pollinator interactions). Besides promoting a fundamental understanding of plant-associated arthropod communities' genetic background and evolution, such knowledge can also help address many current global environmental challenges.

Species specific plant‐mediated effects between herbivores converge at high damage intensity

Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant‐mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaf‐feeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root‐feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant‐mediated effects on herbivores. In particular, effects are species‐specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant‐mediated interactions between herbivores, which advances our understanding of the structure and composition of herbivore communities and terrestrial food webs.

Chinese Cabbage Changes Its Release of Volatiles to Defend against Spodoptera litura

Simple Summary

Biological control is an important direction for pest control in the future, and chemical ecology is an indispensable part of biological control. Therefore, we tested the selection of Spodoptera litura and parasitic wasps on the volatiles of different treatments of cabbage and collected and analyzed the volatiles of different treatments of cabbage. This study found that cabbage was fed by Spodoptera litura to produce volatiles to avoid Spodoptera litura while also attracting Microplitis similis. As a result, some compounds were found to be related to the behavior of Spodoptera litura and Microplitis similis. These results provide a theoretical basis for searching for biological control resources and chemical control.

Abstract

Plants respond to herbivorous insect attacks by releasing volatiles that directly harm the herbivore or that indirectly harm the herbivore by attracting its natural enemies. Although the larvae of Spodoptera litura (the tobacco cutworm) are known to induce the release of host plant volatiles, the effects of such volatiles on host location by S. litura and by the parasitoid Microplitis similis, a natural enemy of S. litura larvae, are poorly understood. Here, we found that both the regurgitate of S. litura larvae and S. litura-infested cabbage leaves attracted M. similis. S. litura had a reduced preference for cabbage plants that had been infested with S. litura for 24 or 48 h. M. similis selection of plants was positively correlated with the release of limonene; linalool and hexadecane, and was negatively correlated with the release of (E)-2-hexenal and 1-Butene, 4-isothiocyanato. S. litura selection of plants was positively correlated with the release of (E)-2-hexenal, 1-Butene, 4-isothiocyanato, and decanal, and was negatively correlated with the release of limonene, nonanal, hexadecane, heptadecane, and octadecane. Our results indicate that host plant volatiles can regulate the behavior of S. litura and M. similis.

Extrafloral nectar production induced by simulated herbivory does not improve ant bodyguard attendance and ultimately plant defence

Highly competitive and aggressive ant species are efficient bodyguards that monopolize the more attractive plants bearing extrafloral nectaries. Given that herbivory often increases the quality of extrafloral nectar, we hypothesized that plants damaged by herbivory would be more prone to interact with high-quality ant bodyguards and be better defended against herbivores. We performed an experiment with Chamaecrista nictitans plants. We induced anti-herbivore responses by applying jasmonic acid to a group of plants while keeping another group unmanaged. We measured extrafloral nectar production, censused ants visiting extrafloral nectaries and, subsequently, added herbivore mimics to measure the efficiency of ant anti-herbivore defence in both conditions. Induction increased the volume of extrafloral nectar and the mass of sugar per nectary without affecting the sugar concentration or the patterns of plant attendance and defence by ants. Thus, we found no evidence that defence-induced C. nictitans plants are more prone to interact with high-quality bodyguards or to receive better anti-herbivore defence. These findings highlight that increases in extrafloral nectar production are not always rewarded with increases in the biotic defences; instead, these rewards might be dependent on the traits of the nectar induced by herbivory events and/or on the ecological context in which the interaction is embedded. Consequently, herbivory might increase the costs of this induced biotic defence to plants bearing extrafloral nectaries when the induced defence does not increase the attractiveness of the plants to ants.

Individual and interactive effects of chronic anthropogenic disturbance and rainfall on taxonomic, functional and phylogenetic composition and diversity of extrafloral nectary-bearing plants in Brazilian Caatinga

Chronic anthropogenic disturbance (CAD) and climate change represent two of the major threats to biodiversity globally, but their combined effects are not well understood. Here we investigate the individual and interactive effects of increasing CAD and decreasing rainfall on the composition and taxonomic (TD), functional (FD) and phylogenetic diversity (PD) of plants possessing extrafloral nectaries (EFNs) in semi-arid Brazilian Caatinga. EFNs attract ants that protect plants against insect herbivore attack and are extremely prevalent in the Caatinga flora. EFN-bearing plants were censused along gradients of disturbance and rainfall in Catimbau National Park in north-eastern Brazil. We recorded a total of 2243 individuals belonging to 21 species. Taxonomic and functional composition varied along the rainfall gradient, but not along the disturbance gradient. There was a significant interaction between increasing disturbance and decreasing rainfall, with CAD leading to decreased TD, FD and PD in the most arid areas, and to increased TD, FD and PD in the wettest areas. We found a strong phylogenetic signal in the EFN traits we analysed, which explains the strong matching between patterns of FD and PD along the environmental gradients. The interactive effects of disturbance and rainfall revealed by our study indicate that the decreased rainfall forecast for Caatinga under climate change will increase the sensitivity of EFN-bearing plants to anthropogenic disturbance. This has important implications for the availability of a key food resource, which would likely have cascading effects on higher trophic levels.

Jasmonic acid biosynthesis by fungi: derivatives, first evidence on biochemical pathways and culture conditions for production

Jasmonic acid (JA) and its derivatives called jasmonates (JAs) are lipid-derived signalling molecules that are produced by plants and certain fungi. Beside this function, JAs have a great variety of applications in flavours and fragrances production. In addition, they may have a high potential in agriculture. JAs protect plants against infections. Although there is much information on the biosynthesis and function of JA concerning plants, knowledge on these aspects is still scarce for fungi. Taking into account the practical importance of JAs, the objective of this review is to summarize knowledge on the occurrence of JAs from fungal culture media, their biosynthetic pathways and the culture conditions for optimal JA production as an alternative source for the production of these valuable metabolites.

Human-Mediated Land Use Change Drives Intraspecific Plant Trait Variation

In the Anthropocene, more than three quarters of ice-free land has experienced some form of human-driven habitat modification, with agriculture dominating 40% of the Earth's surface. This land use change alters the quality, availability, and configuration of habitat resources, affecting the community composition of plants and insects, as well as their interactions with each other. Landscapes dominated by agriculture are known to support a lower abundance and diversity of pollinators and frequently larger populations of key herbivore pests. In turn, insect communities subsidized by agriculture may spill into remaining natural habitats with consequences for wild plants persisting in (semi) natural habitats. Adaptive responses by wild plants may allow them to persist in highly modified landscapes; yet how landscape-mediated variation in insect communities affects wild plant traits related to reproduction and defense remains largely unknown. We synthesize the evidence for plant trait changes across land use gradients and propose potential mechanisms by which landscape-mediated changes in insect communities may be driving these trait changes. Further, we present results from a common garden experiment on three wild Brassica species demonstrating variation in both defensive and reproductive traits along an agricultural land use gradient. Our framework illustrates the potential for plant adaptation under land use change and predicts how defense and reproduction trait expression may shift in low diversity landscapes. We highlight areas of future research into plant population and community effects of land use change.

Genetic and evolution analysis of extrafloral nectary in cotton

Extrafloral nectaries are a defence trait that plays important roles in plant-animal interactions. Gossypium species are characterized by cellular grooves in leaf midribs that secret large amounts of nectar. Here, with a panel of 215 G. arboreum accessions, we compared extrafloral nectaries to nectariless accessions to identify a region of Chr12 that showed strong differentiation and overlapped with signals from GWAS of nectaries. Fine mapping of an F2 population identified GaNEC1, encoding a PB1 domain-containing protein, as a positive regulator of nectary formation. An InDel, encoding a five amino acid deletion, together with a nonsynonymous substitution, was predicted to cause 3D structural changes in GaNEC1 protein that could confer the nectariless phenotype. mRNA-Seq analysis showed that JA-related genes are up-regulated and cell wall-related genes are down-regulated in the nectary. Silencing of GaNEC1 led to a smaller size of foliar nectary phenotype. Metabolomics analysis identified more than 400 metabolites in nectar, including expected saccharides and amino acids. The identification of GaNEC1 helps establish the network regulating nectary formation and nectar secretion, and has implications for understanding the production of secondary metabolites in nectar. Our results will deepen our understanding of plant-mutualism co-evolution and interactions, and will enable utilization of a plant defence trait in cotton breeding efforts.

Herbivore-specific plant volatiles prime neighboring plants for nonspecific defense responses

Plants produce species-specific herbivore-induced plant volatiles (HIPVs) after damage. We tested the hypothesis that herbivore-specific HIPVs prime neighboring plants to induce defenses specific to the priming herbivore. Since Manduca sexta (specialist) and Heliothis virescens (generalist) herbivory induced unique HIPV profiles in Nicotiana benthamiana, we used these HIPVs to prime receiver plants for defense responses to simulated herbivory (mechanical wounding and herbivore regurgitant application). Jasmonic acid (JA) accumulations and emitted volatile profiles were monitored as representative defense responses since JA is the major plant hormone involved in wound and defense signaling and HIPVs have been implicated as signals in tritrophic interactions. Herbivore species-specific HIPVs primed neighboring plants, which produced 2 to 4 times more volatiles and JA after simulated herbivory when compared to similarly treated constitutive volatile-exposed plants. However, HIPV-exposed plants accumulated similar amounts of volatiles and JA independent of the combination of priming or challenging herbivore. Furthermore, volatile profiles emitted by primed plants depended only on the challenging herbivore species but not on the species-specific HIPV profile of damaged emitter plants. This suggests that feeding by either herbivore species primed neighboring plants for increased HIPV emissions specific to the subsequently attacking herbivore and is probably controlled by JA.

Potential Impacts of Translocation of Neonicotinoid Insecticides to Cotton (Gossypium hirsutum (Malvales: Malvaceae)) Extrafloral Nectar on Parasitoids

Neonicotinoid seed treatments are frequently used in cotton (Gossypium hirsutum L. [Malvales: Malvaceae]) production to provide protection against early-season herbivory. However, there is little known about how these applications affect extrafloral nectar (EFN), an important food resource for arthropod natural enemies. Using enzyme-linked immunosorbent assays, we found that neonicotinoids were translocated to the EFN of clothianidin- and imidacloprid-treated, greenhouse-grown cotton plants at concentrations of 77.3 ± 17.3 and 122.6 ± 11.5 ppb, respectively. We did not find differences in the quantity of EFN produced by neonicotinoid-treated cotton plants compared to untreated controls, either constitutively or after mechanical damage. Metabolomic analysis of sugars and amino acids from treated and untreated plants did not detect differences in overall composition of EFN. In bioassays, female Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae) parasitoid wasps that fed on EFN from untreated, clothianidin-treated, or imidacloprid-treated plants demonstrated no difference in mortality or parasitization success. We also conducted acute toxicity assays for C. marginiventris fed on honey spiked with clothianidin and imidacloprid and established LC50 values for male and female wasps. Although LC50 values were substantially higher than neonicotinoid concentrations detected in EFN, caution should be used when translating these results to the field where other stressors could alter the effects of neonicotinoids. Moreover, there are a wide range of possible sublethal impacts of neonicotinoids, none of which were explored here. Our results suggest that EFN is a potential route of exposure of neonicotinoids to beneficial insects and that further field-based studies are warranted.

Evaluation of Macaranga tanarius as a biomass feedstock for fermentable sugars production

Macaranga tanarius is a fast-growing tree species that could be potentially utilized as a biomass feedstock for biorefinery. The average productivity of M. tanarius biomass was estimated to be ~19.2 ton/ha if the above-ground biomass is harvested bi-annually. Different pretreatment approaches were investigated to increase the enzymatic digestibility of foliage and woody biomass. The results indicated that no pretreatment was required for the foliage biomass while sequential acid/alkali pretreatment was necessary for the woody biomass before enzymatic hydrolysis. For the woody biomass, the delignification was 34.5% after sequential dilute acid/alkali pretreatment. The reducing sugar yields from enzymatic hydrolysis of foliage and pretreated woody biomass were 0.31 and 0.42 g/g dry biomass, respectively. The results also showed that both hydrolysates were fermentable by lactic acid bacteria. Overall, the results suggested that M. tanarius could be a potential feedstock for biorefinery based on the findings and processes derived from this study.

Gall-Colonizing Ants and Their Role as Plant Defenders: From 'Bad Job' to 'Useful Service'

Galls are neoformed structures on host plant tissues caused by the attack of insects or other organisms. They support different communities of specialized parasitic insects (the gall inducers), and can also provide refuge to other insects, such as moths, beetles and ants, referred to as secondary occupants. This study focuses on galls induced by the oak gall wasp Andricus quercustozae and secondarily colonized by ants in a mixed oak forest. A field survey and two experiments were carried out to a) study ant (species-specific) preferences for different features of the galls, b) describe differences in gall architecture due to ant activity, c) analyse the effects of the presence of gall-dwelling ants on plant health. The results show that there are differences between ant species in gall colonization and in the alteration of gall opening and inner structure. We verified that gall-dwelling ants protect their host plants efficiently, offering them an indirect defence mechanism against enemies (predators and pathogens). The data suggest a new paradigm in ant–plant relationships mediated by the presence of galls on the plants whose ecological and evolutionary implications are discussed.

Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton

The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number of studies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap, here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum). We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant. Single events of 25% and 50% leaf damage did not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of  leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar production was stronger than systemic induction, whereas the reverse pattern was observed for phenolics. Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.

Induction of the sticky plant defense syndrome in wild tobacco

Many plants engage in protective mutualisms, offering resources such as extrafloral nectar and shelters to predatory arthropods in exchange for protection against herbivores. Recent work indicates that sticky plants catch small insects and provide this carrion to predators who defend the plants against herbivores. In this study, we investigated whether wild tobacco, Nicotiana attenuata, fits this sticky plant defense syndrome that has been described for other sticky plants. We developed a bioassay for stickiness involving the number of flies that adhered to flowers, the stickiest tissues. In surveys conducted over three field seasons at four sites, we found that the number of carrion that adhered to a plant was positively correlated with the number of predators that we observed foraging over its surfaces. The number of predators was positively correlated with the number of seed capsules that the plant produced, a measure of lifetime female reproductive success. Structural equation modeling indicated strong support for the causal path linking carrion numbers to predator numbers to capsule production. We investigated whether stickiness was an inducible trait and examined two potential cues. We found that experimental clipping of rosette leaves induced greater stickiness, although clipping of neighboring sagebrush leaves did not. Damage to leaf tissue is likely to be a more reliable predictor of risk than is damage to a neighboring plant. The sticky plant defense syndrome is a widespread protective mutualism; its strength and ecological relevance can adjust as risk of herbivory changes.

Testing the Distraction Hypothesis: Do extrafloral nectaries reduce ant-pollinator conflict?

Ant guards protect plants from herbivores, but can also hinder pollination by damaging reproductive structures and/or repelling pollinators. Natural selection should favour the evolution of plant traits that deter ants from visiting flowers during anthesis, without waiving their defensive services. The Distraction Hypothesis posits that rewarding ants with extrafloral nectar could reduce their visitation of flowers, reducing ant-pollinator conflict while retaining protection of other structures.We characterised the proportion of flowers occupied by ants and the number of ants per flower in a Mexican ant-plant, Turnera velutina. We clogged extrafloral nectaries on field plants and observed the effects on patrolling ants, pollinators and ants inside flowers, and quantified the effects on plant fitness. Based on the Distraction Hypothesis, we predicted that preventing extrafloral nectar secretion should result in fewer ants active at extrafloral nectaries, more ants inside flowers and a higher proportion of flowers occupied by ants, leading to ant-pollinator conflict, with reduced pollinator visitation and reduced plant fitness.Overall ant activity inside flowers was low. Preventing extrafloral nectar secretion through clogging reduced the number of ants patrolling extrafloral nectaries, significantly increased the proportion of flowers occupied by ants from 6.1% to 9.7%, and reduced plant reproductive output through a 12% increase in the probability of fruit abortion. No change in the numbers of ants or pollinators inside flowers was observed. This is the first support for the Distraction Hypothesis obtained under field conditions, showing ecological and plant fitness benefits of the distracting function of extrafloral nectar during anthesis. Synthesis. Our study provides the first field experimental support for the Distraction Hypothesis, suggesting that extrafloral nectaries located close to flowers may bribe ants away from reproductive structures during the crucial pollination period, reducing the probability of ant occupation of flowers, reducing ant-pollinator conflict and increasing plant reproductive success.

An integrated transcriptomics and metabolomics analysis of the Cucurbita pepo nectary implicates key modules of primary metabolism involved in nectar synthesis and secretion

Nectar is the main reward that flowers offer to pollinators to entice repeated visitation. Cucurbita pepo (squash) is an excellent model for studying nectar biology, as it has large nectaries that produce large volumes of nectar relative to most other species. Squash is also monoecious, having both female and male flowers on the same plant, which allows comparative analyses of nectary function in one individual. Here, we report the nectary transcriptomes from both female and male nectaries at four stages of floral maturation. Analysis of these transcriptomes and subsequent confirmatory experiments revealed a metabolic progression in nectaries leading from starch synthesis to starch degradation and to sucrose biosynthesis. These results are consistent with previously published models of nectar secretion and also suggest how a sucrose-rich nectar can be synthesized and secreted in the absence of active transport across the plasma membrane. Nontargeted metabolomic analyses of nectars also confidently identified 40 metabolites in both female and male nectars, with some displaying preferential accumulation in nectar of either male or female flowers. Cumulatively, this study identified gene targets for reverse genetics approaches to study nectary function, as well as previously unreported nectar metabolites that may function in plant-biotic interactions.

From dispersal to predation: A global synthesis of ant-seed interactions

Ant-seed interactions take several forms, including dispersal, predation, and parasitism, whereby ants consume seed appendages without dispersal of seeds. We hypothesized that these interaction outcomes could be predicted by ant and plant traits and habitat, with outcomes falling along a gradient of cost and benefit to the plant. To test this hypothesis, we conducted a global literature review and classified over 6,000 pairs of ant-seed interactions from 753 studies across six continents. Linear models showed that seed and ant size, habitat, and dispersal syndrome were the most consistent predictors. Predation was less likely than parasitism and seed dispersal among myrmecochorous plants. A classification tree of the predicted outcomes from linear models revealed that dispersal and predation formed distinct categories based on habitat, ant size, and dispersal mode, with parasitism outcomes forming a distinct subgroup of predation based on seed size and shape. Multiple correspondence analysis indicated some combinations of ant genera and plant families were strongly associated with particular outcomes, whereas other ant-seed combinations were much more variable. Taken together, these results demonstrate that ant and plant traits are important overall predictors of potential seed fates in different habitat types.

Okinawan Subtropical Plants as a Promising Resource for Novel Chemical Treasury

The Okinawa Islands are a crescent-shaped archipelago and their natural forests hold a huge variety of unique subtropical plants with relatively high endemism. We have performed phytochemical study on Okinawan subtropical plants for many years. In this review, we describe our recent research progress on the isolation of new compounds and their various bioactivities.

Induction and relaxation of extrafloral nectaries in response to simulated herbivory in young Mallotus japonicus plants

The disadvantage of induced defenses compared with constitutive defenses is the time during which a plant is vulnerable to herbivory before activation. There is obvious importance in determining the costs and benefits of induced defenses. Some plants produce extrafloral nectaries (EFNs), which attract ants that protect against herbivores, and induce EFNs and extrafloral nectar in response to leaf damage. To understand induction of indirect defense by ants, we investigated the induction and relaxation of extrafloral nectar secretion and EFN formation after artificial leaf damage in young Mallotus japonicus. Plants were grown under control or leaf damage conditions a greenhouse or in the field. Following artificial leaf damage, we assessed secretion of extrafloral nectar and the number of ant workers on plants. We measured the number of EFNs on each of seven leaves produced after leaf damage. Extrafloral nectar secretion was induced within 1 day following leaf damage, resulting in the attraction of numerous ant workers, and the extrafloral nectar secretion decreased to initial levels after 7 days. The number of EFNs was largest on the first leaf and smallest on the sixth leaf produced after leaf damage, but the total number of EFNs did not differ between treatments. Thus, M. japonicus rapidly induces extrafloral nectar secretion after leaf damage, followed by relaxation. Furthermore, following induction of EFNs on newly produced leaves, it may decrease the cost of induction by reducing the number of EFNs on leaves produced later.

Risk of herbivore attack and heritability of ontogenetic trajectories in plant defense

Ontogeny has been identified as a main source of variation in the expression of plant phenotypes. However, there is limited information on the mechanisms behind the evolution of ontogenetic trajectories in plant defense. We explored if risk of attack, herbivore damage, heritability, and phenotypic plasticity can promote or constrain the evolutionary potential of ontogenetic trajectories in three defensive traits. We exposed 20 genotypes of Turnera velutina to contrasting environments (shadehouse and field plots), and measured the cyanogenic potential, trichome density, and sugar content in extrafloral nectar in seedlings, juveniles and reproductive plants. We also assessed risk of attack through oviposition preferences, and quantified herbivore damage in the field. We estimated genetic variance, broad sense heritability, and evolvability of the defensive traits at each ontogenetic stage, and of the ontogenetic trajectories themselves. For plants growing in the shadehouse, we found genetic variation and broad sense heritability for cyanogenic potential in seedlings, and for trichome density at all ontogenetic stages. Genetic variation and heritability of ontogenetic trajectories was detected for trichome density only. These genetic pre-requisites for evolution, however, were not detected in the field, suggesting that environmental variation and phenotypic plastic responses mask any heritable variation. Finally, ontogenetic trajectories were found to be plastic, differing between shadehouse and field conditions for the same genetic families. Overall, we provide support for the idea that changes in herbivore pressure can be a mechanism behind the evolution of ontogenetic trajectories. This evolutionary potential, however, can be constrained by phenotypic plasticity expressed in heterogeneous environments.

Indirect plant defense against insect herbivores: a review

Plants respond to herbivore attack by launching 2 types of defenses: direct defense and indirect defense. Direct defense includes all plant traits that increase the resistance of host plants to insect herbivores by affecting the physiology and/or behavior of the attackers. Indirect defense includes all traits that by themselves do not have significant direct impact on the attacking herbivores, but can attract natural enemies of the herbivores and thus reduce plant loss. When plants recognize herbivore-associated elicitors, they produce and release a blend of volatiles that can attract predators, parasites, and other natural enemies. Known herbivore-associated elicitors include fatty acid-amino acid conjugates, sulfur-containing fatty acids, fragments of cell walls, peptides, esters, and enzymes. Identified plant volatiles include terpenes, nitrogenous compounds, and indoles. In addition, constitive traits including extrafloral nectars, food bodies, and domatia can be further induced to higher levels and attract natural enemies as well as provide food and shelter to carnivores. A better understanding of indirect plant defense at global and componential levels via advanced high throughput technologies may lead to utilization of indirect defense in suppression of herbivore damage to plants.

Reduced Responsiveness to Volatile Signals Creates a Modular Reward Provisioning in an Obligate Food-for-Protection Mutualism

Plants in more than 100 families secrete extrafloral nectar (EFN) to establish food-for-protection mutualisms with ants. Facultative ant-plants secrete EFN as a jasmonic acid (JA)-dependent response to attract generalist ants. In contrast, obligate ant-plants like the Central American "Swollen-Thorn Acacias" are colonized by specialized ants, although an individual host can carry ant colonies from different species that differ in the degree of protection they provide. We hypothesized that hosts that associate simultaneously with various partners should produce rewards in a modular manner to preferentially reward high quality partners. To test this hypothesis, we applied JA to distinct leaves and quantified cell wall invertase activity (CWIN; a regulator of nectar secretion) and EFN secretion by these "local" (i.e., treated) and the "systemic" (i.e., non-treated) leaves of the same branch. Both CWIN activity and EFN secretion increased in local and systemic leaves of the facultative ant-plant Acacia cochliacantha, but only in the local leaves of the obligate ant-plant, A. cornigera. The systemic EFN secretion in A. cochliacantha was associated with an enhanced emission of volatile organic compounds (VOCs). Such VOCs function as "external signals" that control systemic defense responses in diverse plant species. Indeed, the headspace of JA-treated branches of A. cochliacantha induced EFN secretion in both plant species, whereas the headspace of A. cornigera caused no detectable induction effect. Analyses of the headspace using GC-MS identified six VOCs in the headspace of A. cochliacantha that were not emitted by A. cornigera. Among these VOCs, β-caryophyllene and (cis)-hexenyl isovalerate have already been reported in other plant species to induce defense traits, including EFN secretion. Our observations underline the importance of VOCs as systemic within-plant signals and show that the modular rewarding in A. cornigera is likely to result from a reduced emission of the systemic signal, rather than from a reduced responsiveness to the signal. We suggest that modular rewarding allows hosts to restrict the metabolic investment to specific partners and to efficiently sanction potential exploiters.

Review: Nectar biology: From molecules to ecosystems

Plants attract mutualistic animals by offering a reward of nectar. Specifically, floral nectar (FN) is produced to attract pollinators, whereas extrafloral nectar (EFN) mediates indirect defenses through the attraction of mutualist predatory insects to limit herbivory. Nearly 90% of all plant species, including 75% of domesticated crops, benefit from animal-mediated pollination, which is largely facilitated by FN. Moreover, EFN represents one of the few defense mechanisms for which stable effects on plant health and fitness have been demonstrated in multiple systems, and thus plays a crucial role in the resistance phenotype of plants producing it. In spite of its central role in plant-animal interactions, the molecular events involved in the development of both floral and extrafloral nectaries (the glands that produce nectar), as well as the synthesis and secretion of the nectar itself, have been poorly understood until recently. This review will cover major recent developments in the understanding of (1) nectar chemistry and its role in plant-mutualist interactions, (2) the structure and development of nectaries, (3) nectar production, and (4) its regulation by phytohormones.

Ants at Plant Wounds: A Little-Known Trophic Interaction with Evolutionary Implications for Ant-Plant Interactions

Extrafloral nectaries (EFNs) allow plants to engage in mutualisms with ants, preventing herbivory in exchange for food. EFNs occur scattered throughout the plant phylogeny and likely evolved independent from herbivore-created wounds subsequently visited by ants collecting leaked sap. Records of wound-feeding ants are, however, anecdotal. By surveying 38,000 trees from 40 species, we conducted the first quantitative ecological study of this overlooked behavior. Ant-wound interactions were widespread (0.5% of tree individuals) and occurred on 23 tree species. Interaction networks were opportunistic, closely resembling ant-EFN networks. Fagaceae, a family lacking EFNs, was strongly overrepresented. For Fagaceae, ant occurrence at wounds correlated with species-level leaf damage, potentially indicating that wounds may attract mutualistic ants, which supports the hypothesis of ant-tended wounds as precursors of ant-EFN mutualisms. Given that herbivore wounds are common, wound sap as a steadily available food source might further help to explain the overwhelming abundance of ants in (sub)tropical forest canopies.

Plant spines deter herbivory by restricting caterpillar movement

The spines of flowering plants are thought to function primarily in defence against mammalian herbivores; however, we previously reported that feeding by Manduca sexta caterpillars on the leaves of horsenettle plants (Solanum carolinense) induces increased development of internode spines on new growth. To determine whether and how spines impact caterpillar feeding, we conducted assays with three Solanaceous plant species that vary in spine numbers (S. carolinense, S. atropurpureum and S. aethiopicum) and also manipulated spine numbers within each species. We found that M. sexta caterpillars located experimentally isolated target leaves much more quickly on plants with experimentally removed spines compared with plants with intact spines. Moreover, it took caterpillars longer to defoliate species with relatively high spine numbers (S. carolinense and particularly Satropurpureum) compared with S. aethiopicum, which has fewer spines. These findings suggest that spines may play a significant role in defence against insect herbivores by restricting herbivore movement and increasing the time taken to access feeding sites, with possible consequences including longer developmental periods and increased vulnerability or apparency to predators.

Impact of an Invasive Insect and Plant Defense on a Native Forest Defoliator

Eastern hemlock (Tsuga canadensis [L.] Carriére) in the United States is threatened by the invasive hemlock woolly adelgid (Adelges tsugae Annand). The native hemlock looper (Lambdina fiscellaria Guenée) also appears to have played a role in previous population declines of this conifer. Although these two insects co-occur in much of the adelgid's invaded range, their interactions remain unstudied. We assessed looper performance and preference on both uninfested and adelgid-infested foliage from adelgid-susceptible hemlocks, as well as on uninfested foliage from an eastern hemlock that is naturally adelgid-resistant. Larvae reared on uninfested foliage from adelgid-susceptible hemlocks experienced 60% mortality within the first two weeks of the experiment, and pupated at a lower weight than larvae fed adelgid-infested foliage. Despite differences in foliage source, this first look and strong pattern suggests that the hemlock looper performs better (pupates earlier, weighs more) on adelgid-infested foliage. In addition, trends suggested that larvae reared on foliage from the adelgid-resistant tree survived better, pupated earlier, and weighed more than in the other treatments. Larvae preferred adelgid-resistant over adelgid-susceptible foliage. Our results suggest that looper perform slightly better on adelgid-infested foliage and that plant resistance to xylem-feeding adelgid may increase susceptibility to foliar-feeding looper larvae.

Caterpillar mimicry by plant galls as a visual defense against herbivores

Plant galls, induced by arthropods and various other organisms have an intimate relationship with host plants, and gall-inducers have limited mobility. In addition to their own photosynthesis, galls are resource sinks rich with nutrients, with neighboring plant organs commonly serving as external photosynthate sources. Galls, if not well defended, may therefore be attractive food sources for herbivores. Galls produced by some aphids, jumping plant lice, thrips, and gall midges in Japan, Palearctic region and in the Middle East visually resemble lepidopteran caterpillars. I propose that such visual resemblance may reduce herbivory of galls and surrounding plant tissues, resulting in an increase in galler survival due to reduced gall damage and in enhanced galler growth due to improved nutrient inflow to the galls, when herbivores avoid colonizing or consuming plant parts that look as if they have been occupied by other herbivores. Potential predators and parasitoids of caterpillars may be attracted to the caterpillar-like galls and then attack real caterpillars and other invertebrate herbivores, which would also be beneficial for both gallers and their hosts.

Benefits of jasmonate-dependent defenses against vertebrate herbivores in nature

Endogenous jasmonates are important regulators of plant defenses. If and how they enable plants to maintain their reproductive output when facing community-level herbivory under natural conditions, however, remains unknown. We demonstrate that jasmonate-deficient Nicotiana attenuata plants suffer more damage by arthropod and vertebrate herbivores than jasmonate-producing plants in nature. However, only damage by vertebrate herbivores translates into a significant reduction in flower production. Vertebrate stem peeling has the strongest negative impact on plant flower production. Stems are defended by jasmonate-dependent nicotine, and the native cottontail rabbit Sylvilagus nuttallii avoids jasmonate-producing N. attenuata shoots because of their high levels of nicotine. Thus, endogenous jasmonates enable plants to resist different types of herbivores in nature, and jasmonate-dependent defenses are important for plants to maintain their reproductive potential when facing vertebrate herbivory. Ecological and evolutionary models on plant defense signaling should aim at integrating arthropod and vertebrate herbivory at the community level.

Defense pattern of Chinese cork oak across latitudinal gradients: influences of ontogeny, herbivory, climate and soil nutrients

Knowledge of latitudinal patterns in plant defense and herbivory is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. Using a widely distributed species in East Asia, Quercus variabilis, we aim to reveal defense patterns of trees with respect to ontogeny along latitudinal gradients. Six leaf chemical (total phenolics and total condensed tannin concentrations) and physical (cellulose, hemicellulose, lignin and dry mass concentration) defensive traits as well as leaf herbivory (% leaf area loss) were investigated in natural Chinese cork oak (Q. variabilis) forests across two ontogenetic stages (juvenile and mature trees) along a ~14°-latitudinal gradient. Our results showed that juveniles had higher herbivory values and a higher concentration of leaf chemical defense substances compared with mature trees across the latitudinal gradient. In addition, chemical defense and herbivory in both ontogenetic stages decreased with increasing latitude, which supports the latitudinal herbivory-defense hypothesis and optimal defense theory. The identified trade-offs between chemical and physical defense were primarily determined by environmental variation associated with the latitudinal gradient, with the climatic factors (annual precipitation, minimum temperature of the coldest month) largely contributing to the latitudinal defense pattern in both juvenile and mature oak trees.

Three's a Crowd: Trade-Offs between Attracting Pollinators and Ant Bodyguards with Nectar Rewards in Turnera

Many plants attract insect pollinators with floral nectar (FN) and ant "bodyguards" with extrafloral nectar (EFN). If nectar production is costly or physiologically linked across glands, investment in one mutualism may trade off with investment in the other. We confirmed that changes in FN and EFN availability alter pollination and ant defense mutualisms in a field population of Turnera ulmifolia. Plants with additional FN tended to produce more seeds, while plants with reduced EFN production experienced less florivory. We then mimicked the consumptive effects of mutualists by removing FN or EFN daily for 50 days in a full factorial design using three Turnera species (T. joelii, T. subulata, and T. ulmifolia) in a glasshouse experiment. For T. ulmifolia and T. subulata, but not T. joelii, removing either nectar reduced production of the other, showing for the first time that EFN and FN production can trade off. In T. subulata, increased investment in FN decreased seed set, suggesting that nectar production can have direct fitness costs. Through the linked expression of EFN and FN, floral visitors may negatively affect biotic defense, and extrafloral nectary visitors may negatively affect pollination.

The Venus Flytrap Dionaea muscipula Counts Prey-Induced Action Potentials to Induce Sodium Uptake

Carnivorous plants, such as the Venus flytrap (Dionaea muscipula), depend on an animal diet when grown in nutrient-poor soils. When an insect visits the trap and tilts the mechanosensors on the inner surface, action potentials (APs) are fired. After a moving object elicits two APs, the trap snaps shut, encaging the victim. Panicking preys repeatedly touch the trigger hairs over the subsequent hours, leading to a hermetically closed trap, which via the gland-based endocrine system is flooded by a prey-decomposing acidic enzyme cocktail. Here, we asked the question as to how many times trigger hairs have to be stimulated (e.g., now many APs are required) for the flytrap to recognize an encaged object as potential food, thus making it worthwhile activating the glands. By applying a series of trigger-hair stimulations, we found that the touch hormone jasmonic acid (JA) signaling pathway is activated after the second stimulus, while more than three APs are required to trigger an expression of genes encoding prey-degrading hydrolases, and that this expression is proportional to the number of mechanical stimulations. A decomposing animal contains a sodium load, and we have found that these sodium ions enter the capture organ via glands. We identified a flytrap sodium channel DmHKT1 as responsible for this sodium acquisition, with the number of transcripts expressed being dependent on the number of mechano-electric stimulations. Hence, the number of APs a victim triggers while trying to break out of the trap identifies the moving prey as a struggling Na⁺-rich animal and nutrition for the plant.

Video Abstract

•

Carnivorous Dionaea muscipula captures and processes nutrient- and sodium-rich prey

•

Via mechano-sensor stimulation, an animal meal is recognized, captured, and processed

•

Mechano-electrical waves induce JA signaling pathways that trigger prey digestion

•

Number of stimulations controls the production of digesting enzymes and uptake modules

Shared weapons of blood- and plant-feeding insects: Surprising commonalities for manipulating hosts

Insects that reprogram host plants during colonization remind us that the insect side of plant-insect story is just as interesting as the plant side. Insect effectors secreted by the salivary glands play an important role in plant reprogramming. Recent discoveries point to large numbers of salivary effectors being produced by a single herbivore species. Since genetic and functional characterization of effectors is an arduous task, narrowing the field of candidates is useful. We present ideas about types and functions of effectors from research on blood-feeding parasites and their mammalian hosts. Because of their importance for human health, blood-feeding parasites have more tools from genomics and other - omics than plant-feeding parasites. Four themes have emerged: (1) mechanical damage resulting from attack by blood-feeding parasites triggers "early danger signals" in mammalian hosts, which are mediated by eATP, calcium, and hydrogen peroxide, (2) mammalian hosts need to modulate their immune responses to the three "early danger signals" and use apyrases, calreticulins, and peroxiredoxins, respectively, to achieve this, (3) blood-feeding parasites, like their mammalian hosts, rely on some of the same "early danger signals" and modulate their immune responses using the same proteins, and (4) blood-feeding parasites deploy apyrases, calreticulins, and peroxiredoxins in their saliva to manipulate the "danger signals" of their mammalian hosts. We review emerging evidence that plant-feeding insects also interfere with "early danger signals" of their hosts by deploying apyrases, calreticulins and peroxiredoxins in saliva. Given emerging links between these molecules, and plant growth and defense, we propose that these effectors interfere with phytohormone signaling, and therefore have a special importance for gall-inducing and leaf-mining insects, which manipulate host-plants to create better food and shelter.