Induced Responses to Herbivory and Jasmonate in Three Milkweed Species

Plasticity in resource allocation of the invasive Phytolacca americana: Balancing growth, reproduction, and defense along urban-rural gradients

In response to varying environments along urban and rural gradients, invasive plants may strategically allocate resources to enhance their invasiveness. However, how invasive plants balance their resources for growth, reproduction, and defense as responses to biotic and abiotic factors across these gradients remain unclear. We conducted field surveys on the growth, reproduction, and herbivory of the invasive species Phytolacca americana across diverse urban and rural habitats. Leaf samples were collected to analyze the nutritional content, primary and secondary metabolites. We found that plant growth rates, specific leaf area, leaf nitrogen content, and concentrations of flavonoids and saponins were higher in urban habitats, while reproduction, herbivory, and carbon-to‑nitrogen ratios were lower than those in rural habitats. We also found a trade-off between growth rate and herbivory, as well as trade-offs among defense traits associated with herbivory (e.g., leaf mass per area, the inverse of leaf nitrogen content, and carbon‑nitrogen ratio) and the production of metabolites associated with abiotic stress tolerance (e.g., soluble sugars, flavonoids, and saponins). As earlier studies showed low levels of genetic diversity within and between populations, our findings suggest that the urban-rural gradient patterns of resource allocation are primarily phenotypic plasticity in response to herbivory in rural areas and abiotic factors in urban areas. Our study sheds light on the mechanisms by which urbanization affects plant invasions and offers insights for the implementation of their management strategies.

Drought reduces productivity and anti-herbivore defences, but not mycorrhizal associations, of perennial prairie forbs

During drought, plants allocate resources to aboveground biomass production and belowground carbohydrate reserves, often at the expense of production of defence traits. Additionally, drought has been shown to alter floral resources, with potential implications for plant-pollinator interactions. Although soil symbionts, such as arbuscular mycorrhizal (AM) fungi, can alleviate drought stress in plants, certain levels of drought may negatively impact this relationship, with potential cascading effects. Because of their importance to plant and animal community diversity, we examined effects of drought on biomass production, physical defence properties, nectar production, and associated AM fungal abundance of five common prairie forb species in a greenhouse study. Reduced soil moisture decreased vegetative biomass production. Production of trichomes and latex decreased under drought, relative to well-watered conditions. Ruellia humilis flowers produced less nectar under drought, relative to well-watered conditions. Intra-radical AM fungal colonization was not significantly affected by drought, although extra-radical AM fungal biomass associated with S. azurea decreased following drought. Overall, grassland forb productivity, defence, and nectar production were negatively impacted by moderate drought, with possible negative implications for biotic interactions. Reduced flower and nectar production may lead to fewer pollinator visitors, which may contribute to seed limitation in forb species. Reduced physical defences increase the likelihood of herbivory, further decreasing the ability to store energy for essential functions, such as reproduction. Together, these results suggest drought can potentially impact biotic interactions between plants and herbivores, pollinators, and soil symbionts, and highlights the need for direct assessments of these relationships under climate change scenarios.

A nutrition-defence trade-off drives diet choice in a toxic plant generalist

Plant toxicity shapes the dietary choices of herbivores. Especially when herbivores sequester plant toxins, they may experience a trade-off between gaining protection from natural enemies and avoiding toxicity. The availability of toxins for sequestration may additionally trade off with the nutritional quality of a potential food source for sequestering herbivores. We hypothesized that diet mixing might allow a sequestering herbivore to balance nutrition and defence (via sequestration of plant toxins). Accordingly, here we address diet mixing and sequestration of large milkweed bugs (Oncopeltus fasciatus) when they have differential access to toxins (cardenolides) in their diet. In the absence of toxins from a preferred food (milkweed seeds), large milkweed bugs fed on nutritionally adequate non-toxic seeds, but supplemented their diet by feeding on nutritionally poor, but cardenolide-rich milkweed leaf and stem tissues. This dietary shift corresponded to reduced insect growth but facilitated sequestration of defensive toxins. Plant production of cardenolides was also substantially induced by bug feeding on leaf and stem tissues, perhaps benefitting this cardenolide-resistant herbivore. Thus, sequestration appears to drive diet mixing in this toxic plant generalist, even at the cost of feeding on nutritionally poor plant tissue.

A bioenergetic framework for aboveground terrestrial food webs

Bioenergetic approaches have been greatly influential for understanding community functioning and stability and predicting effects of environmental changes on biodiversity. These approaches use allometric relationships to establish species' trophic interactions and consumption rates and have been successfully applied to aquatic ecosystems. Terrestrial ecosystems, where body mass is less predictive of plant-consumer interactions, present inherent challenges that these models have yet to meet. Here, we discuss the processes governing terrestrial plant-consumer interactions and develop a bioenergetic framework integrating those processes. Our framework integrates bioenergetics specific to terrestrial plants and their consumers within a food web approach while also considering mutualistic interactions. Such a framework is poised to advance our understanding of terrestrial food webs and to predict their responses to environmental changes.

Timing of a plant-herbivore interaction alters plant growth and reproduction

Phenological shifts have the potential to change species interactions, but relatively few studies have used experimental manipulations to examine the effects of variation in timing of an interspecific interaction across a series of life stages of a species. Although previous experimental studies have examined the consequences of phenological timing in plant-herbivore interactions for both plants and their herbivores, less is known about their effects on subsequent plant reproduction. Here, we conducted an experiment to determine how shifts in the phenological timing of monarch (Danaus plexippus) larval herbivory affected milkweed (Asclepias fascicularis) host plant performance, including effects on growth and subsequent effects on flower and seed pod phenology and production. We found that variation in the timing of herbivory affected both plant growth and reproduction, with measurable effects several weeks to several months after herbivory ended. The timing of herbivory had qualitatively different effects on vegetative and reproductive biomass: early-season herbivory had the strongest effects on plant size, whereas late-season herbivory had the strongest effects on the production of viable seeds. These results show that phenological shifts in herbivory can have persistent and qualitatively different effects on different life stages across the season.

Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity. Absence of a correlation between the components of host specificity results in low-performance hosts supporting limited larval development. Most studies of specificity focus on oviposition behavior leaving the importance and basis of palatability and survival under-explored. We conducted a comprehensive review of 127 plant species that have been claimed or tested to be hosts for the monarch butterfly Danaus plexippus to classify them as non-hosts, low performance, or high performance. We performed a meta-analysis to test if performance status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice larval feeding experiment to identify causes of low performance. We identified 34 high performance, 42 low performance, 33 non-hosts, and 18 species with unsubstantiated claims. Mean cardenolide concentration was greater in high- than low-performance hosts and a significant predictor of host status, suggesting possible evolutionary trade-offs in monarch specialization. Other cardenolide properties and trichome density were not significant predictors of host status. In the experiment, we found, of the 62% of larvae that attempted to eat low-performance hosts, only 3.5% survived to adult compared to 85% of those on the high-performance host, demonstrating that multiple factors affect larval host plant specificity. Our study is the first to classify all known host plants for monarchs and has conservation implications for this threatened species.

Growing a glue factory: Open questions in laticifer development

Latex-containing cells called laticifers are present in at least 41 flowering plant families and are thought to have convergently evolved at least 12 times. These cells are known to function in defense, but little is known about the molecular genetic mechanisms of their development. The expansion of laticifers into their distinctive tube shape can occur through two distinct mechanisms, cell fusion and intrusive growth. The mechanism and extent of intrusive laticifer growth are still being investigated. Hormonal regulation by jasmonic acid and ethylene is important for both laticifer differentiation and latex biosynthesis. Current evidence suggests that laticifers can be specified independently of latex production, but extensive latex production requires specified laticifers. Laticifers are an emerging system for studying the intersection of cell identity specification and specialized metabolism.

Between the devil and the deep blue sea: herbivory induces foraging for and uptake of cadmium in a metal hyperaccumulating plant

Plants have been shown to change their foraging behaviour in response to resource heterogeneity. However, an unexplored hypothesis is that foraging could be induced by environmental stressors, such as herbivory, which might increase the demand for particular resources, such as those required for herbivore defence. This study examined the way simulated herbivory affects both root foraging for and uptake of cadmium (Cd), in the metal-hyperaccumulating plant Arabidopsis halleri, which uses this heavy metal as herbivore defence. Simulated herbivory elicited enhanced relative allocation of roots to Cd-rich patches as well as enhanced Cd uptake, and these responses were exhibited particularly by plants from non-metalliferous origin, which have lower metal tolerance. By contrast, plants from a metalliferous origin, which are more tolerant to Cd, did not show any preference in root allocation, yet enhanced Cd sharing between ramets when exposed to herbivory. These results suggest that foraging for heavy metals, as well as their uptake and clonal-sharing, could be stimulated in A. halleri by herbivory impact. Our study provides first support for the idea that herbivory can induce not only defence responses in plants but also affect their foraging, resource uptake and clonal sharing responses.

Historic exposure to herbivores, not constitutive traits, explains plant tolerance to herbivory in the case of two Medicago species (Fabaceae)

Mechanisms that allow plants to survive and reproduce after herbivory are considered to play a key role in plant evolution. In this study, we evaluated how tolerance varies in species with different historic exposure to herbivores considering ontogeny. We exposed the range-restricted species Medicago citrina and its closely related and widespread species M. arborea to one and two herbivory simulations (80 % aerial biomass loss). Physiological and growth parameters related to tolerance capacity were assessed to evaluate constitutive values (without herbivory) and induced tolerance after damage. Constitutive traits were not always related to greater tolerance, and each species compensated for herbivory through different traits. Herbivory damage only led to mortality in M. citrina; adults exhibited root biomass loss and increased oxidative stress after damage, but also compensated aerial biomass. Despite seedlings showed a lower death percentage than adults after herbivory in M. citrina, they showed less capacity to recover control values than adults. Moderate tolerance to M. arborea herbivory and low tolerance to M. citrina is found. Thus, although the constitutive characteristics are maintained in the lineage, the tolerance of plants decreases in M. citrina. That represents how plants respond to the lack of pressure from herbivores in their habitat.

Opposing roles of plant laticifer cells in the resistance to insect herbivores and fungal pathogens

More than 12,000 plant species (ca. 10% of flowering plants) exude latex when their tissues are injured. Latex is produced and stored in specialized cells named "laticifers". Laticifers form a tubing system composed of rows of elongated cells that branch and create an internal network encompassing the entire plant. Laticifers constitute a recent evolutionary achievement in ecophysiological adaptation to specific natural environments; however, their fitness benefit to the plant still remains to be proven. The identification of Euphorbia lathyris mutants (pil mutants) deficient in laticifer cells or latex metabolism, and therefore compromised in latex production, allowed us to test the importance of laticifers in pest resistance. We provided genetic evidence indicating that laticifers represent a cellular adaptation for an essential defense strategy to fend off arthropod herbivores with different feeding habits, such as Spodoptera exigua and Tetranychus urticae. In marked contrast, we also discovered that a lack of laticifer cells causes complete resistance to the fungal pathogen Botrytis cinerea. Thereafter, a latex-derived factor required for conidia germination on the leaf surface was identified. This factor promoted disease susceptibility enhancement even in the non-latex-bearing plant Arabidopsis. We speculate on the role of laticifers in the co-evolutionary arms race between plants and their enemies.

Cardenolides, toxicity, and the costs of sequestration in the coevolutionary interaction between monarchs and milkweeds

For highly specialized insect herbivores, plant chemical defenses are often co-opted as cues for oviposition and sequestration. In such interactions, can plants evolve novel defenses, pushing herbivores to trade off benefits of specialization with costs of coping with toxins? We tested how variation in milkweed toxins (cardenolides) impacted monarch butterfly (Danaus plexippus) growth, sequestration, and oviposition when consuming tropical milkweed (Asclepias curassavica), one of two critical host plants worldwide. The most abundant leaf toxin, highly apolar and thiazolidine ring-containing voruscharin, accounted for 40% of leaf cardenolides, negatively predicted caterpillar growth, and was not sequestered. Using whole plants and purified voruscharin, we show that monarch caterpillars convert voruscharin to calotropin and calactin in vivo, imposing a burden on growth. As shown by in vitro experiments, this conversion is facilitated by temperature and alkaline pH. We next employed toxin-target site experiments with isolated cardenolides and the monarch's neural Na⁺/K⁺-ATPase, revealing that voruscharin is highly inhibitory compared with several standards and sequestered cardenolides. The monarch's typical >50-fold enhanced resistance to cardenolides compared with sensitive animals was absent for voruscharin, suggesting highly specific plant defense. Finally, oviposition was greatest on intermediate cardenolide plants, supporting the notion of a trade-off between benefits and costs of sequestration for this highly specialized herbivore. There is apparently ample opportunity for continued coevolution between monarchs and milkweeds, although the diffuse nature of the interaction, due to migration and interaction with multiple milkweeds, may limit the ability of monarchs to counteradapt.

Defense of Milkweed Bugs (Heteroptera: Lygaeinae) against Predatory Lacewing Larvae Depends on Structural Differences of Sequestered Cardenolides

Predators and parasitoids regulate insect populations and select defense mechanisms such as the sequestration of plant toxins. Sequestration is common among herbivorous insects, yet how the structural variation of plant toxins affects defenses against predators remains largely unknown. The palearctic milkweed bug Lygaeus equestris (Heteroptera: Lygaeinae) was recently shown to sequester cardenolides from Adonis vernalis (Ranunculaceae), while its relative Horvathiolus superbus also obtains cardenolides but from Digitalis purpurea (Plantaginaceae). Remarkably, toxin sequestration protects both species against insectivorous birds, but only H. superbus gains protection against predatory lacewing larvae. Here, we used a full factorial design to test whether this difference was mediated by the differences in plant chemistry or by the insect species. We raised both species of milkweed bugs on seeds from both species of host plants and carried out predation assays using the larvae of the lacewing Chrysoperla carnea. In addition, we analyzed the toxins sequestered by the bugs via liquid chromatography (HPLC). We found that both insect species gained protection by sequestering cardenolides from D. purpurea but not from A. vernalis. Since the total amount of toxins stored was not different between the plant species in H. superbus and even lower in L. equestris from D. purpurea compared to A. vernalis, the effect is most likely mediated by structural differences of the sequestered toxins. Our findings indicate that predator-prey interactions are highly context-specific and that the host plant choice can affect the levels of protection to various predator types based on structural differences within the same class of chemical compounds.

The role of toxic nectar secondary compounds in driving differential bumble bee preferences for milkweed flowers

While morphological differences such as tongue length are often featured as drivers of pollinator floral preferences, differences in chemical detection and tolerance to secondary compounds may also play a role. We sought to better understand the role of secondary compounds in floral preference by examining visitation of milkweed flowers, which can contain toxic cardenolides in their nectar, by bumble bees (Bombus spp.), some of their most abundant and important pollinators. We examine bumble bee species visitation of common milkweed (Asclepias syriaca) compared to other flowers in the field and test whether observed preferences may be influenced by avoidance and tolerance of cardenolides, as measured by the cardenolide ouabain, in the lab. We reveal that common milkweed is visited predominantly by one bumble bee species, Bombus griseocollis, in a ratio much higher than the abundance of this species in the community. We confirmed the presence and toxicity of cardenolides in A. syriaca nectar. Lab experiments revealed that B. griseocollis, compared to the common bumble bees B. impatiens and B. bimaculatus, exhibit greater avoidance of cardenolides, but only at levels that start to induce illness, whereas the other species exhibit either no or reduced avoidance of cardenolides, resulting in illness and mortality in these bees. Toxicity experiments reveal that B. griseocollis also has a substantially higher tolerance for cardenolides than B. impatiens. Together, these results support a potential evolutionary association between B. griseocollis and milkweed that may involve increased ability to both detect and tolerate milkweed cardenolides.

Ecology of the Western Queen Butterfly Danaus gilippus thersippus (Lepidoptera: Nymphalidae) in the Mojave and Sonoran Deserts

The purpose of this study was to assess the ecological knowledge surrounding the western queen butterfly, Danaus gilippus thersippus (H. Bates). Specifically, our objectives were to synthesize existing data and knowledge on the ecology of the queen and use results of this assessment to inform the direction of future research on this understudied species. We identified six core areas for assessment: distribution, the biodiversity of plant resources, western queen and their host plant phenology, chemical ecology, and four key life history traits. We mapped the distribution of D. g. thersippus from museum specimen records, citizen science (e.g., iNaturalist) and image sharing app-based observations, along with other observational data enumerating all current known plant resources and long-range movements. We assembled 14 larval food plants, six pyrrolizidine alkaloids plants and six nectar plants distributed in the western Mojave and Sonoran Desert regions of the United States and Baja California. We report on its phenology and its long-range movement. Butterfly species have declined across the western US, and western monarch populations have declined by 97%. Danaus g. thersippus has received little research attention compared with its famous congener D. plexippus L. Danaus g. thersippus' desert distribution may be at its temperature limits for the species distribution and for its rare host plant Asclepias nyctaginifolia.

Independent evolution of ancestral and novel defenses in a genus of toxic plants (Erysimum, Brassicaceae)

Phytochemical diversity is thought to result from coevolutionary cycles as specialization in herbivores imposes diversifying selection on plant chemical defenses. Plants in the speciose genus Erysimum (Brassicaceae) produce both ancestral glucosinolates and evolutionarily novel cardenolides as defenses. Here we test macroevolutionary hypotheses on co-expression, co-regulation, and diversification of these potentially redundant defenses across this genus. We sequenced and assembled the genome of E. cheiranthoides and foliar transcriptomes of 47 additional Erysimum species to construct a phylogeny from 9868 orthologous genes, revealing several geographic clades but also high levels of gene discordance. Concentrations, inducibility, and diversity of the two defenses varied independently among species, with no evidence for trade-offs. Closely related, geographically co-occurring species shared similar cardenolide traits, but not glucosinolate traits, likely as a result of specific selective pressures acting on each defense. Ancestral and novel chemical defenses in Erysimum thus appear to provide complementary rather than redundant functions.

Species-specific, age-varying plant traits affect herbivore growth and survival

Seasonal windows of opportunity represent intervals of time within a year during which organisms have improved prospects of achieving life history aims such as growth or reproduction, and may be commonly structured by temporal variation in abiotic factors, bottom-up factors, and top-down factors. Although seasonal windows of opportunity are likely to be common, few studies have examined the factors that structure seasonal windows of opportunity in time. Here, we experimentally manipulated host-plant age in two milkweed species (Asclepias fascicularis and Asclepias speciosa) in order to investigate the role of plant-species-specific and plant-age-varying traits on the survival and growth of monarch caterpillars (Danaus plexippus). We show that the two plant species showed diverging trajectories of defense traits with increasing age. These species-specific and age-varying host-plant traits significantly affected the growth and survival of monarch caterpillars through both resource quality- and quantity-based constraints. The effects of plant age on monarch developmental success were comparable to and sometimes larger than those of plant-species identity. We conclude that species-specific and age-varying plant traits are likely to be important factors with the potential to structure seasonal windows of opportunity for monarch development, and examine the implications of these findings for both broader patterns in the ontogeny of plant defense traits and the specific ecology of milkweed-monarch interactions in a changing world.

Plant Defense by Latex: Ecological Genetics of Inducibility in the Milkweeds and a General Review of Mechanisms, Evolution, and Implications for Agriculture

Latex occurs in 10% of plant families, has evolved independently many times, and is the most effective defense of milkweeds against its chewing herbivores. Here we report on new experiments on the heritability and inducibility of latex in several milkweed species. In addition, we review what is known about the genetic and environmental determinants of latex exudation, hormonal regulation, evolution within and among species, and the role and frequency of latex in agricultural crops. We first evaluated genotype-by-environment interactions using ~20 full-sibling genetic families in each of seven Asclepias species treated as controls or attacked by monarch butterfly caterpillars. All species showed substantial genetic variation for latex exudation and six of seven species responded to monarch herbivory (two species increased latex, two species decreased, and two showed variation among genetic families). Exogenous application of jasmonic acid (JA) to three species induced a consistent increase in latex (including species which showed a decline following caterpillar herbivory). We next evaluated three hypotheses for what determines genetic variation for induced latex in A. syriaca: 1) a trade-off with constitutive investment, 2) differential endogenous JA induction, or 3) variation in responsiveness to JA. We only found support for the second hypothesis: genetic families with a stronger JA-burst showed the greatest latex exudation following herbivory. We conclude that most species exhibit a genetic and inducible basis for latex, although genetic variation in inducibility is not pervasive. Finally, we summarized studies across 22 species of Asclepias and found that neither a species' latitude nor its phylogenetic position predicted latex inducibility. Nonetheless, a negative association between constitutive and induced latex across species indicates a macroevolutionary trade-off in allocation to this defense. Our review indicates that jasmonic acid is a key regulator of latex exudation, laticifer morphology, and defensive metabolites within latex. Biotic and abiotic factors strongly modulate latex expression. A survey of latex in food crops revealed that latex and analogous exudates (gums, resins, mucilage) are more common than expected based on their distribution across all plants. In conclusion, despite its widespread occurrence, the literature on latex is currently dominated by rubber trees and milkweeds, and we look forward to the broadening of ecological, agricultural, and mechanistic research into other systems.

Leaf traits mediate changes in invertebrate herbivory along broad environmental gradients on Mt. Kilimanjaro, Tanzania

Temperature, primary productivity, plant functional traits, and herbivore abundances are considered key predictors of leaf herbivory but their direct and indirect contributions to community-level herbivory are not well understood along broad climatic gradients. Here, we determined elevational herbivory patterns and used a path analytical approach to disentangle the direct and indirect effects of climate, land use, net primary productivity (NPP), herbivore abundance, and plant functional traits on community-level invertebrate herbivory along the extensive elevational and land use gradients at Mt. Kilimanjaro, Tanzania. We recorded standing leaf herbivory caused by leaf chewers, leaf miners and leaf gallers on 55 study sites distributed in natural and anthropogenic habitats along a 3,060 m elevation gradient. We related the total community-level herbivory to climate (temperature and precipitation), NPP, plant functional traits (specific leaf area, leaf carbon-to-nitrogen [CN] ratio and leaf nitrogen-to-phosphorus [NP] ratio) and herbivore abundances. Leaf herbivory ranged from 5% to 11% along the elevation gradient. Total leaf herbivory showed unimodal pattern in natural habitats but a strongly contrasting bimodal pattern in anthropogenic habitats. We also detected some variation in the patterns of leaf herbivory along environmental gradients across feeding guilds with leaf chewers being responsible for a disproportionally large part of herbivory. Path analyses indicated that the variation in leaf herbivory was mainly driven by changes in leaf CN and NP ratios which were closely linked to changes in NPP in natural habitats. Similarly, patterns of leaf herbivory in anthropogenic habitats were best explained by variation in leaf CN ratios and a negative effect of land use. Our study elucidates the strong role of leaf nutrient stoichiometry and its linkages to climate and NPP for explaining the variation in leaf herbivory along broad climatic gradients. Furthermore, the study suggests that climatic changes and nutrient inputs in the course of land use change may alter leaf herbivory and consequently energy and nutrient fluxes in terrestrial habitats.

Cardenolide Intake, Sequestration, and Excretion by the Monarch Butterfly along Gradients of Plant Toxicity and Larval Ontogeny

Monarch butterflies, Danaus plexippus, migrate long distances over which they encounter host plants that vary broadly in toxic cardenolides. Remarkably little is understood about the mechanisms of sequestration in Lepidoptera that lay eggs on host plants ranging in such toxins. Using closely-related milkweed host plants that differ more than ten-fold in cardenolide concentrations, we mechanistically address the intake, sequestration, and excretion of cardenolides by monarchs. We show that on high cardenolide plant species, adult butterflies saturate in cardenolides, resulting in lower concentrations than in leaves, while on low cardenolide plants, butterflies concentrate toxins. Butterflies appear to focus their sequestration on particular compounds, as the diversity of cardenolides is highest in plant leaves, lower in frass, and least in adult butterflies. Among the variety of cardenolides produced by the plant, sequestered compounds may be less toxic to the butterflies themselves, as they are more polar on average than those in leaves. In accordance with this, results from an in vitro assay based on inhibition of Na⁺/K⁺ ATPase (the physiological target of cardenolides) showed that on two milkweed species, including the high cardenolide A. perennis, extracts from butterflies have lower inhibitory effects than leaves when standardized by cardenolide concentration, indicating selective sequestration of less toxic compounds from these host plants. To understand how ontogeny shapes sequestration, we examined cardenolide concentrations in caterpillar body tissues and hemolymph over the course of development. Caterpillars sequestered higher concentrations of cardenolides as early instars than as late instars, but within the fifth instar, concentration increased with body mass. Although it appears that large amounts of sequestration occurs in early instars, a host switching experiment revealed that caterpillars can compensate for feeding on low cardenolide host plants with substantial sequestration in the fifth instar. We highlight commonalities and striking differences in the mechanisms of sequestration depending on host plant chemistry and developmental stage, which have important implications for monarch defense.

Toxicity of Milkweed Leaves and Latex: Chromatographic Quantification Versus Biological Activity of Cardenolides in 16 Asclepias Species

Cardenolides are classically studied steroidal defenses in chemical ecology and plant-herbivore coevolution. Although milkweed plants (Asclepias spp.) produce up to 200 structurally different cardenolides, all compounds seemingly share the same well-characterized mode of action, inhibition of the ubiquitous Na⁺/K⁺ ATPase in animal cells. Over their evolutionary radiation, milkweeds show a quantitative decline of cardenolide production and diversity. This reduction is contrary to coevolutionary predictions and could represent a cost-saving strategy, i.e. production of fewer but more toxic cardenolides. Here we test this hypothesis by tandem cardenolide quantification using HPLC (UV absorption of the unsaturated lactone) and a pharmacological assay (in vitro inhibition of a sensitive Na⁺/K⁺ ATPase) in a comparative study of 16 species of Asclepias. We contrast cardenolide concentrations in leaf tissue to the subset of cardenolides present in exuding latex. Results from the two quantification methods were strongly correlated, but the enzymatic assay revealed that milkweed cardenolide mixtures often cause stronger inhibition than equal amounts of a non-milkweed reference cardenolide, ouabain. Cardenolide concentrations in latex and leaves were positively correlated across species, yet latex caused 27% stronger enzyme inhibition than equimolar amounts of leaf cardenolides. Using a novel multiple regression approach, we found three highly potent cardenolides (identified as calactin, calotropin, and voruscharin) to be primarily responsible for the increased pharmacological activity of milkweed cardenolide mixtures. However, contrary to an expected trade-off between concentration and toxicity, later-diverging milkweeds had the lowest amounts of these potent cardenolides, perhaps indicating an evolutionary response to milkweed's diverse community of specialist cardenolide-sequestering insect herbivores.

Population Variation, Environmental Gradients, and the Evolutionary Ecology of Plant Defense against Herbivory

A central tenet of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a trade-off between plant growth and defense. Yet this trade-off, commonly found among species occupying divergent resource environments, may not occur across populations of single species. We hypothesized that more favorable climates and higher levels of herbivory would lead to increases in growth and defense across plant populations. We evaluated whether plant growth and defense traits covaried across 18 populations of showy milkweed (Asclepias speciosa) inhabiting an east-west climate gradient spanning 25° of longitude. A suite of traits impacting defense (e.g., latex, cardenolides), growth (e.g., size), or both (e.g., specific leaf area [SLA], trichomes) were measured in natural populations and in a common garden, allowing us to evaluate plastic and genetically based variation in these traits. In natural populations, herbivore pressure increased toward warmer sites with longer growing seasons. Growth and defense traits showed strong clinal patterns and were positively correlated. In a common garden, clines with climatic origin were recapitulated only for defense traits. Correlations between growth and defense traits were also weaker and more negative in the common garden than in the natural populations. Thus, our data suggest that climatically favorable sites likely facilitate the evolution of greater defense at minimal costs to growth, likely because of increased resource acquisition.

Relative Selectivity of Plant Cardenolides for Na+/K+-ATPases From the Monarch Butterfly and Non-resistant Insects

A major prediction of coevolutionary theory is that plants may target particular herbivores with secondary compounds that are selectively defensive. The highly specialized monarch butterfly (Danaus plexippus) copes well with cardiac glycosides (inhibitors of animal Na+/K+-ATPases) from its milkweed host plants, but selective inhibition of its Na+/K+-ATPase by different compounds has not been previously tested. We applied 17 cardiac glycosides to the D. plexippus-Na+/K+-ATPase and to the more susceptible Na+/K+-ATPases of two non-adapted insects (Euploea core and Schistocerca gregaria). Structural features (e.g., sugar residues) predicted in vitro inhibitory activity and comparison of insect Na+/K+-ATPases revealed that the monarch has evolved a highly resistant enzyme overall. Nonetheless, we found evidence for relative selectivity of individual cardiac glycosides reaching from 4- to 94-fold differences of inhibition between non-adapted Na+/K+-ATPase and D. plexippus-Na+/K+-ATPase. This toxin receptor specificity suggests a mechanism how plants could target herbivores selectively and thus provides a strong basis for pairwise coevolutionary interactions between plants and herbivorous insects.

The Effects of Milkweed Induced Defense on Parasite Resistance in Monarch Butterflies, Danaus plexippus

Many plants express induced defenses against herbivores through increasing the production of toxic secondary chemicals following damage. Phytochemical induction can directly or indirectly affect other organisms within the community. In tri-trophic systems, increased concentrations of plant toxins could be detrimental to plants if herbivores can sequester these toxins as protective chemicals for themselves. Thus, through trophic interactions, induction can lead to either positive or negative effects on plant fitness. We examined the effects of milkweed (Asclepias spp.) induced defenses on the resistance of monarch caterpillars (Danaus plexippus) to a protozoan parasite (Ophryocystis elektroscirrha). Milkweeds contain toxic secondary chemicals called cardenolides, higher concentrations of which are associated with reduced parasite growth. Previous work showed that declines in foliar cardenolides caused by aphid attack render monarch caterpillars more susceptible to infection. Here, we ask whether cardenolide induction by monarchs increases monarch resistance to disease. We subjected the high-cardenolide milkweed A. curassavica and the low-cardenolide A. syriaca to caterpillar grazing, and reared infected and uninfected caterpillars on these plants. As expected, monarchs suffered less parasite growth and disease when reared on A. curassavica than on A. syriaca. We also found that herbivory increased cardenolide concentrations in A. curassavica, but not A. syriaca. However, cardenolide induction in A. curassavica was insufficient to influence monarch resistance to the parasite. Our results suggest that interspecific variation in cardenolide concentration is a more important driver of parasite defense than plasticity via induced defenses in this tri-trophic system.

Multidrug transporters and organic anion transporting polypeptides protect insects against the toxic effects of cardenolides

In the struggle against dietary toxins, insects are known to employ target site insensitivity, metabolic detoxification, and transporters that shunt away toxins. Specialized insects across six taxonomic orders feeding on cardenolide-containing plants have convergently evolved target site insensitivity via specific amino acid substitutions in the Na/K-ATPase. Nonetheless, in vitro pharmacological experiments have suggested a role for multidrug transporters (Mdrs) and organic anion transporting polypeptides (Oatps), which may provide a basal level of protection in both specialized and non-adapted insects. Because the genes coding for these proteins are evolutionarily conserved and in vivo genetic evidence in support of this hypothesis is lacking, here we used wildtype and mutant Drosophila melanogaster (Drosophila) in capillary feeder (CAFE) assays to quantify toxicity of three chemically diverse, medically relevant cardenolides. We examined multiple components of fitness, including mortality, longevity, and LD50, and found that, while the three cardenolides each stimulated feeding (i.e., no deterrence to the toxin), all decreased lifespan, with the most apolar cardenolide having the lowest LD50 value. Flies showed a clear non-monotonic dose response and experienced high levels of toxicity at the cardenolide concentration found in plants. At this concentration, both Mdr and Oatp knockout mutant flies died more rapidly than wildtype flies, and the mutants also experienced more adverse neurological effects on high-cardenolide-level diets. Our study further establishes Drosophila as a model for the study of cardenolide pharmacology and solidifies support for the hypothesis that multidrug and organic anion transporters are key players in insect protection against dietary cardenolides.

Behavioral Sabotage of Plant Defenses by Insect Folivores

Plant susceptibility to herbivore attack is determined not just by the suite of defenses present in different tissues of the plant, but also by the capabilities of the herbivore for tolerating, circumventing, or disarming the defenses. This article reviews the elaborate behaviors exhibited by leaf-chewing insects that appear to function specifically to deactivate hostplant defenses. Shortcomings in our understanding and promising areas for future research are highlighted. Behaviors covered include vein cutting, trenching, girdling, leaf clipping, and application of fluids from exocrine glands. Many of these behaviors have a widespread distribution, having evolved independently in multiple insect lineages. Insects utilizing the behaviors include significant agricultural, horticultural, and forestry pests, as well as numerous species important in natural ecosystems. Behavioral, ecological, and phylogenetic studies have documented the importance of the behaviors and their ancient history, but the molecular analysis of how the behaviors affect plant physiology has scarcely begun.

Milkweed butterfly resistance to plant toxins is linked to sequestration, not coping with a toxic diet

Insect resistance to plant toxins is widely assumed to have evolved in response to using defended plants as a dietary resource. We tested this hypothesis in the milkweed butterflies (Danaini) which have progressively evolved higher levels of resistance to cardenolide toxins based on amino acid substitutions of their cellular sodium-potassium pump (Na(+)/K(+)-ATPase). Using chemical, physiological and caterpillar growth assays on diverse milkweeds (Asclepias spp.) and isolated cardenolides, we show that resistant Na(+)/K(+)-ATPases are not necessary to cope with dietary cardenolides. By contrast, sequestration of cardenolides in the body (as a defence against predators) is associated with the three levels of Na(+)/K(+)-ATPase resistance. To estimate the potential physiological burden of cardenolide sequestration without Na(+)/K(+)-ATPase adaptations, we applied haemolymph of sequestering species on isolated Na(+)/K(+)-ATPase of sequestering and non-sequestering species. Haemolymph cardenolides dramatically impair non-adapted Na(+)/K(+)-ATPase, but had systematically reduced effects on Na(+)/K(+)-ATPase of sequestering species. Our data indicate that major adaptations to plant toxins may be evolutionarily linked to sequestration, and may not necessarily be a means to eat toxic plants. Na(+)/K(+)-ATPase adaptations thus were a potential mechanism through which predators spurred the coevolutionary arms race between plants and insects.

Low tolerance to simulated herbivory in Hawaiian seedlings despite induced changes in photosynthesis and biomass allocation

BACKGROUND AND AIMS

Seedling herbivory is an important factor underlying plant community diversity and structure. While considerable research has characterized seedling defence in terms of resistance, very little is known about seedling tolerance of herbivory. Moreover, few studies have attempted to identify mechanisms of tolerance across a range of plant species.

METHODS

Seedling tolerance of simulated herbivory was tested in a diverse pool of ten Hawaiian plant species, including several lobeliad species (family Campanulaceae), a grass, a herb and common woody trees and shrubs. Tolerance was measured as the relative survival and growth of damaged plants receiving 50 % defoliation with simultaneous jasmonic acid application compared with undamaged control plants, assessed 1·5 and 5 weeks after damage. Putative mechanisms of tolerance were measured, including photosynthetic parameters, light use efficiency, and biomass allocation reflecting growth priorities, and analysed using species-level regression analyses on tolerance indices.

KEY RESULTS

No species fully tolerated 50 % defoliation at either harvest date, and simulated herbivory significantly reduced shoot as well as root biomass. Lobeliad species had particularly low tolerance. Species varied considerably in size, biomass allocation parameters and their constitutive (pre-damage) and induced (post-damage) photosynthetic parameters. However, only constitutive levels of non-photochemical quenching were significantly related to tolerance, indicating that species with more efficient light use (and less heat dissipation) are better at tolerating damage than species with high levels of heat dissipation.

CONCLUSIONS

Native Hawaiian plants expressed low tolerance to a conservative level of simulated herbivory. Root growth decreased in response to damage, but this was not associated with greater tolerance, suggesting this response may be due to allocation constraints following defoliation and not due to adaptive plasticity. Conservation of native island plants threatened by invasive herbivores should prioritize protection for seedlings for improved regeneration and the persistence of native plants in disturbed habitats.

Characterization of a Chitosanase from Jelly Fig (Ficus awkeotsang Makino) Latex and Its Application in the Production of Water-Soluble Low Molecular Weight Chitosans

A chitosanase was purified from jelly fig latex by ammonium sulfate fractionation (50–80% saturation) and three successive column chromatography steps. The purified enzyme was almost homogeneous, as determined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and gel activity staining. The molecular mass of the enzyme was 20.5 kDa. The isoelectric point (pI) was <3.5, as estimated by isoelectric focusing electrophoresis on PhastGel IEF 3-9. Using chitosan as the substrate, the optimal pH for the enzyme reaction was 4.5; the kinetic parameters K_m and V_max were 0.089 mg mL^-1 and 0.69 μmol min^-1 mg^-1, respectively. The enzyme showed activity toward chitosan polymers which exhibited various degrees of deacetylation (21–94%). The enzyme hydrolyzed 70–84% deacetylated chitosan polymers most effectively. Substrate specificity analysis indicated that the enzyme catalyzed the hydrolysis of chitin and chitosan polymers and their derivatives. The products of the hydrolysis of chitosan polymer derivatives, ethylene glycol (EG) chitosan, carboxymethyl (CM) chitosan and aminoethyl (AE) chitosan, were low molecular weight chitosans (LMWCs); these products were referred to as EG-LMWC, CM-LMWC and AE-LMWC, respectively. The average molecular weights of EG-LMWC, CM-LMWC and AE-LMWC were 11.2, 11.2 and 8.89 kDa, respectively. All of the LMWC products exhibited free radical scavenging activities toward ABTS^•+, superoxide and peroxyl radicals.

Quantity over quality: light intensity, but not red/far-red ratio, affects extrafloral nectar production in Senna mexicana var. chapmanii

Extrafloral nectar (EFN) mediates food‐for‐protection mutualisms between plants and insects and provides plants with a form of indirect defense against herbivory. Understanding sources of variation in EFN production is important because such variations affect the number and identity of insect visitors and the effectiveness of plant defense. Light represents a potentially crucial tool for regulating resource allocation to defense, as it not only contributes energy but may help plants to anticipate future conditions. Low red/far‐red (R/FR) light ratios can act as a signal of the proximity of competing plants. Exposure to such light ratios has been shown to promote competitive behaviors that coincide with reduced resource allocation to direct chemical defenses. Little is known, however, about how such informational light signals might affect indirect defenses such as EFN, and the interactions that they mediate. Through controlled glasshouse experiments, we investigated the effects of light intensity, and R/FR light ratios, on EFN production in Senna mexicana var. chapmanii. Plants in light‐limited conditions produced significantly less EFN, and leaf damage elicited increased EFN production regardless of light conditions. Ratios of R/FR light, however, did not appear to affect EFN production in either damaged or undamaged plants. Understanding the effects of light on indirect defenses is of particular importance for plants in the threatened pine rockland habitats of south Florida, where light conditions are changing in predictable ways following extensive fragmentation and subsequent mismanagement. Around 27% of species in these habitats produce EFN and may rely on insect communities for defense.

Specificity of herbivore-induced hormonal signaling and defensive traits in five closely related milkweeds (Asclepias spp.)

Despite the recognition that phytohormonal signaling mediates induced responses to herbivory, we still have little understanding of how such signaling varies among closely related species and may generate herbivore-specific induced responses. We studied closely related milkweeds (Asclepias) to link: 1) plant damage by two specialist chewing herbivores (milkweed leaf beetles Labidomera clivicolis and monarch caterpillars Danaus plexippus); 2) production of the phytohormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA); 3) induction of defensive cardenolides and latex; and 4) impacts on Danaus caterpillars. We first show that A. syriaca exhibits induced resistance following monarch herbivory (i.e., reduced monarch growth on previously damaged plants), while the defensively dissimilar A. tuberosa does not. We next worked with a broader group of five Asclepias, including these two species, that are highly divergent in defensive traits yet from the same clade. Three of the five species showed herbivore-induced changes in cardenolides, while induced latex was found in four species. Among the phytohormones, JA and ABA showed specific responses (although they generally increased) to insect species and among the plant species. In contrast, SA responses were consistent among plant and herbivore species, showing a decline following herbivore attack. Jasmonic acid showed a positive quantitative relationship only with latex, and this was strongest in plants damaged by D. plexippus. Although phytohormones showed qualitative tradeoffs (i.e., treatments that enhanced JA reduced SA), the few significant individual plant-level correlations among hormones were positive, and these were strongest between JA and ABA in monarch damaged plants. We conclude that: 1) latex exudation is positively associated with endogenous JA levels, even among low-latex species; 2) correlations among milkweed hormones are generally positive, although herbivore damage induces a divergence (tradeoff) between JA and SA; 3) induction of cardenolides and latex are not necessarily physiologically linked; and 4) even very closely related species show highly divergent induction, with some species showing strong defenses, hormonally-mediated induction, and impacts on herbivores, while other milkweed species apparently use alternative strategies to cope with insect attack.

Prickly poppies can get pricklier: ontogenetic patterns in the induction of physical defense traits

Plant ontogeny is a common source of variation in defense and herbivory. Yet, few studies have investigated how the induction of physical defense traits changes across plant ontogeny. Physical defense traits are costly to produce, and thus, it was predicted that induction as a cost-saving strategy would be particularly favorable for seedlings, leading to ontogenetic declines in the inducibility of these traits. We tested for induction of three different physical defense traits (prickles, latex and leaf toughness) in response to mechanical defoliation and jasmonic acid application using prickly poppies (Argemone glauca and A. mexicana, Papaveraceae) as a model system. Genetic variation in the induction of physical defenses was tested using maternal sib-ships sampled from multiple populations. Both species induced higher densities of laminar prickles, although the magnitude of induction was much higher in the endemic Hawaiian prickly poppy, A. glauca, than in the cosmopolitan A. mexicana. The magnitude of prickle induction was also higher in young compared to older juvenile plant stages in A. glauca, demonstrating a strong role of ontogeny. Neither latex exudation nor leaf toughness was induced in either species. Although significant genetic variation was detected within and among populations for constitutive expression of physical defense traits in Argemone, there was no evidence for genetic variation in the induction of these traits. This study provides the first evidence for the induction of physical defenses in prickly poppies, emphasizing how an ontogenetically explicit framework can reveal new insights into plant defense. Moreover, this study illustrates how sister species comparisons between island vs. continental plants can provide new insights into plant functional and evolutionary ecology, highlighting a fruitful area for future research on more species pairs.

Light regulation of plant defense

Precise allocation of limited resources between growth and defense is critical for plant survival. In shade-intolerant species, perception of competition signals by informational photoreceptors activates shade-avoidance responses and reduces the expression of defenses against pathogens and insects. The main mechanism underlying defense suppression is the simultaneous downregulation of jasmonate and salicylic acid signaling by low ratios of red:far-red radiation. Inactivation of phytochrome B by low red:far-red ratios appears to suppress jasmonate responses by altering the balance between DELLA and JASMONATE ZIM DOMAIN (JAZ) proteins in favor of the latter. Solar UVB radiation is a positive modulator of plant defense, signaling through jasmonate-dependent and jasmonate-independent pathways. Light, perceived by phytochrome B and presumably other photoreceptors, helps plants concentrate their defensive arsenals in photosynthetically valuable leaves. The discovery of connections between photoreceptors and defense signaling is revealing novel mechanisms that control key resource allocation decisions in plant canopies.

An emerging understanding of mechanisms governing insect herbivory under elevated CO2

By changing the chemical composition of foliage, the increase in atmospheric CO(2) is fundamentally altering insect herbivory. The responses of folivorous insects to these changes is, however, highly variable. In this review we highlight emerging mechanisms by which increasing CO(2) alters the defense chemistry and signaling of plants. The response of allelochemicals affecting insect performance varies under elevated CO(2), and results suggest this is driven by changes in plant hormones. Increasing CO(2) suppresses the production of jasmonates and ethylene and increases the production of salicylic acid, and these differential responses of plant hormones affect specific secondary chemical pathways. In addition to changes in secondary chemistry, elevated CO(2) decreases rates of water loss from leaves, increases temperature and feeding rates, and alters nutritional content. New insights into the mechanistic responses of secondary chemistry to elevated CO(2) increase our ability to predict the ecological and evolutionary responses of plants attacked by insects.

Attenuation of the jasmonate burst, plant defensive traits, and resistance to specialist monarch caterpillars on shaded common milkweed (Asclepias syriaca)

Plant responses to herbivory and light competition are often in opposing directions, posing a potential conflict for plants experiencing both stresses. For sun-adapted species, growing in shade typically makes plants more constitutively susceptible to herbivores via reduced structural and chemical resistance traits. Nonetheless, the impact of light environment on induced resistance has been less well-studied, especially in field experiments that link physiological mechanisms to ecological outcomes. Accordingly, we studied induced resistance of common milkweed (Asclepias syriaca, a sun-adapted plant), and linked hormonal responses, resistance traits, and performance of specialist monarch caterpillars (Danaus plexippus) in varying light environments. In natural populations, plants growing under forest-edge shade showed reduced levels of resistance traits (lower leaf toughness, cardenolides, and trichomes) and enhanced light-capture traits (higher specific leaf area, larger leaves, and lower carbon-to-nitrogen ratio) compared to paired plants in full sun. In a field experiment repeated over two years, only milkweeds growing in full sun exhibited induced resistance to monarchs, whereas plants growing in shade were constitutively more susceptible and did not induce resistance. In a more controlled field experiment, plant hormones were higher in the sun (jasmonic acid, salicylic acid, abscisic acid, indole acidic acid) and were induced by herbivory (jasmonic acid and abscisic acid). In particular, the jasmonate burst following herbivory was halved in plants raised in shaded habitats, and this correspondingly reduced latex induction (but not cardenolide induction). Thus, we provide a mechanistic basis for the attenuation of induced plant resistance in low resource environments. Additionally, there appears to be specificity in these interactions, with light-mediated impacts on jasmonate-induction being stronger for latex exudation than cardenolides.

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na⁺/K⁺-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.

Plant latex and other exudates as plant defense systems: roles of various defense chemicals and proteins contained therein

Plant latex and other exudates are saps that are exuded from the points of plant damage caused either mechanically or by insect herbivory. Although many (ca. 10%) of plant species exude latex or exudates, and although the defensive roles of plant latex against herbivorous insects have long been suggested by several studies, the detailed roles and functions of various latex ingredients, proteins and chemicals, in anti-herbivore plant defenses have not been well documented despite the wide occurrence of latex in the plant kingdom. Recently, however, substantial progress has been made. Several latex proteins, including cysteine proteases and chitin-related proteins, have been shown to play important defensive roles against insect herbivory. In the mulberry (Morus spp.)-silkworm (Bombyx mori) interaction, an old and well-known model system of plant-insect interaction, plant latex and its ingredients--sugar-mimic alkaloids and defense protein MLX56--are found to play key roles. Complicated molecular interactions between Apocynaceae species and its specialist herbivores, in which cardenolides and defense proteins in latex play key roles, are becoming more and more evident. Emerging observations suggested that plant latex, analogous to animal venom, is a treasury of useful defense proteins and chemicals that has evolved through interspecific interactions. On the other hand, specialist herbivores developed sophisticated adaptations, either molecular, physiological, or behavioral, against latex-borne defenses. The existence of various adaptations in specialist herbivores itself is evidence that latex and its ingredients function as defenses at least against generalists. Here, we review molecular and structural mechanisms, ecological roles, and evolutionary aspects of plant latex as a general defense against insect herbivory and we discuss, from recent studies, the unique characteristics of latex-borne defense systems as transport systems of defense substances are discussed based on recent studies.