Induced indirect defence in a lycaenid-ant association: the regulation of a resource in a mutualism

Indirect defences involve the protection of a host organism by a mutualistic partner. Threat of predation to the host organism may induce the production of rewards and/or signals that attract the mutualistic partner. In laboratory and field experiments we show that threatened lycaenid butterfly larvae (Plebejus acmon) produce more nectar rewards from their gland and were tended by protective ants twice as much as controls. Ant attendance did not affect the leaf consumption or feeding behaviour of larvae in the absence of predators. Inducible nectar production and indirect defence in this system may be a mechanism by which larvae provide rewards for services when they are needed the most. Such a system may stabilize the mutualistic association between lycaenid larvae and ants by preventing exploitation by either partner.     

Herbivores and plant defences affect selection on plant reproductive traits more strongly than pollinators

Pollinators and herbivores can both affect the evolutionary diversification of plant reproductive traits. However, plant defences frequently alter antagonistic and mutualistic interactions, and therefore, variation in plant defences may alter patterns of herbivore‐ and pollinator‐mediated selection on plant traits. We tested this hypothesis by conducting a common garden field experiment using 50 clonal genotypes of white clover (Trifolium repens) that varied in a Mendelian‐inherited chemical antiherbivore defence—the production of hydrogen cyanide (HCN). To evaluate whether plant defences alter herbivore‐ and/or pollinator‐mediated selection, we factorially crossed chemical defence (25 cyanogenic and 25 acyanogenic genotypes), herbivore damage (herbivore suppression) and pollination (hand pollination). We found that herbivores weakened selection for increased inflorescence production, suggesting that large displays are costly in the presence of herbivores. In addition, herbivores weakened selection on flower size but only among acyanogenic plants, suggesting that plant defences reduce the strength of herbivore‐mediated selection. Pollinators did not independently affect selection on any trait, although pollinators weakened selection for later flowering among cyanogenic plants. Overall, cyanogenic plant defences consistently increased the strength of positive directional selection on reproductive traits. Herbivores and pollinators both strengthened and weakened the strength of selection on reproductive traits, although herbivores imposed ~2.7× stronger selection than pollinators across all traits. Contrary to the view that pollinators are the most important agents of selection on reproductive traits, our data show that selection on reproductive traits is driven primarily by variation in herbivory and plant defences in this system.     

Mutualism, hybrid inviability and speciation in a tropical ant-plant

Although biotic interactions are particularly intricate in the tropics, few studies have examined whether divergent adaptations to biotic interactions lead to speciation in tropical organisms. Ant-plant mutualisms are widespread in the tropics. Within Leonardoxa africana, two subspecies present contrasting defences against herbivores. Young leaves of subsp. africana are defended by mutualistic ants, whereas subsp. gracilicaulis satiates herbivores by synchronized leaf production. Subsp. africana possesses hollow internodes and many large foliar nectaries, housing and feeding ants. We detected no genetic introgression between the two subspecies in the contact zone between them. F1 hybrids were present. They were intermediate in phenotype, expressing reduced, nonfunctional but costly myrmecophilic traits. However, they suffered more herbivory than their parents. Hybrids remained small, failing to reach reproductive size, probably due to their maladapted defence phenotype. Hence, there could be a direct link between adaptation to mutualism and reproductive isolation: biotic interactions could be a driver of tropical diversity.     

Increased Host Investment in Extrafloral Nectar (EFN) Improves the Efficiency of a Mutualistic Defensive Service

Extrafloral nectar (EFN) plays an important role as plant indirect defence through the attraction of defending ants. Like all rewards produced in the context of a mutualism, however, EFN is in danger of being exploited by non-ant consumers that do not defend the plant against herbivores. Here we asked whether plants, by investing more in EFN, can improve their indirect defence, or rather increase the risk of losing this investment to EFN thieves. We used the obligate plant-ant Acacia-Pseudomyrmex system and examined experimentally in the field during the dry and the rainy seasons how variations in EFN secretion are related to (i) ant activity, to (ii) the ant-mediated defence against herbivores and (iii) the exploitation of EFN by non-ant consumers. Extrafloral investment enhanced ant recruitment and was positively related to the ant mediated defence against herbivores. The ant-mediated protection from exploiters also increased in proportion to the nectar sugar concentration. Although the daily peak of EFN production coincided with the highest activity of EFN thieves, Pseudomyrmex ferrugineus ants protected this resource effectively from exploiters. Nevertheless, the defensive effects by ants differed among seasons. During the dry season, plants grew slower and secreted more EFN than in the rainy season, and thus, experienced a higher level of ant-mediated indirect defence. Our results show that an increased plant investment in an indirect defence trait can improve the resulting defensive service against both herbivores and exploiters. EFN secretion by obligate ant-plants represents a defensive trait for which the level of investment correlates positively with the beneficial effects obtained.     

Do Leaf Cutting Ants Cut Undetected? Testing the Effect of Ant-Induced Plant Defences on Foraging Decisions in Atta colombica

Leaf-cutting ants (LCAs) are polyphagous, yet highly selective herbivores. The factors that govern their selection of food plants, however, remain poorly understood. We hypothesized that the induction of anti-herbivore defences by attacked food plants, which are toxic to either ants or their mutualistic fungus, should significantly affect the ants' foraging behaviour. To test this "induced defence hypothesis," we used lima bean (Phaseolus lunatus), a plant that emits many volatile organic compounds (VOCs) upon herbivore attack with known anti-fungal or ant-repellent effects. Our results provide three important insights into the foraging ecology of LCAs. First, leaf-cutting by Atta ants can induce plant defences: Lima bean plants that were repeatedly exposed to foraging workers of Atta colombica over a period of three days emitted significantly more VOCs than undamaged control plants. Second, the level to which a plant has induced its anti-herbivore defences can affect the LCAs' foraging behaviour: In dual choice bioassays, foragers discriminated control plants from plants that have been damaged mechanically or by LCAs 24 h ago. In contrast, strong induction levels of plants after treatment with the plant hormone jasmonic acid or three days of LCA feeding strongly repelled LCA foragers relative to undamaged control plants. Third, the LCA-specific mode of damaging leaves allows them to remove larger quantities of leaf material before being recognized by the plant: While leaf loss of approximately 15% due to a chewing herbivore (coccinelid beetle) was sufficient to significantly increase VOC emission levels after 24 h, the removal of even 20% of a plant's leaf area within 20 min by LCAs did not affect its VOC emission rate after 24 h. Taken together, our results support the "induced defence hypothesis" and provide first empirical evidence that the foraging behaviour of LCAs is affected by the induction of plant defence responses.     

Patterns in the phytochemistry of arctic plants

Ninety-one species of arctic tundra plants were screened for possible chemical defences against herbivory. Tannins were found in one-third of the species, whereas two-thirds of the plants contained alkaloids. Anthraquinones, cyanogenic glycosides and saponins accounted for only 6% of the positive reactions. The results were used to test the apparency hypothesis for the distribution of chemical plant defences which states that rare or unpredictable species should contain qualitative defences (toxins, such as alkaloids) while common or predictable species should contain quantitative defences (digestibility-reducing compounds, such as tannins). Abundance of plant species showed no relationship to chemical content, except that the relatively abundant shrubs more often contained tannins than the relatively rare forbs. Common graminoids (grasses and related taxa) did not contain tannins and data for the other classes of compounds did not support the apparency hypothesis. Graminoids appeared to rely on tolerance to grazing rather than chemical defences and common ericaceous plants produced both digestibility-reducing and toxic compounds.     

Chemical convergence between a guild of facultative myrmecophilous caterpillars and host plants

1. Ants exert strong selective pressure on herbivorous insects, although some caterpillars can live in symbiosis with them using chemical defensive strategies.
2. We investigated the adaptive resemblance of cuticular hydrocarbons (CHCs) in multitrophic systems involving a guild of facultative myrmecophilous caterpillar species (Lepidoptera: Lycaenidae), tending ants (Hymenoptera: Formicidae), and host plants from three families. We hypothesised that the CHCs of the caterpillars would resemble those of their host plants (chemical camouflage).
3. We analysed CHCs using gas chromatography/mass spectrometry. Morisita's similarity index (SI) was used to compare CHC profiles of caterpillar species with different types of ant associations (commensal or mutualistic), ants, and host plants.
4. We found strong convergence between caterpillars' CHCs and plants, especially for commensal species that do not provide secretion rewards for ants. Moreover, we found unexpected chemical convergence among mutualistic (trophobiotic) caterpillar species that offer caloric reward secretions to ants.
5. These results show that the studied caterpillars acquire CHCs through their diet and that they vary according to host plant species and type of ant association (commensalism or mutualism). This ‘chemical camouflage’ of myrmecophilous caterpillars may have arisen as a defensive strategy allowing coexistence with ants on plants, whereas ‘chemical conspicuousness’ may have evolved in the context of honest signalling between mutualistic partners.
6. We suggest the existence of chemical mimicry among myrmecophilous species, especially between mutualistic caterpillars. Cuticular chemical mixtures can play a key adaptive role in decreasing ant attacks and increasing caterpillar survival in multimodal sensory systems.

     

Plant age and the inducibility of extrafloral nectaries in Vicia faba

Insect herbivory imposes a strong selection pressure on plants. As a result, plants have evolved a wide array of defences, including resistance traits that help them reduce the negative impact of herbivores. Along one axis of variation, these traits can be divided into direct resistance (physical and chemical defences) and indirect resistance (the recruitment of natural enemies of the herbivore via extrafloral nectar and other incentives). Along a second axis of variation, resistance can be split into constitutive resistance, which is always present, and induced resistance, which is expressed more strongly following damage to plant tissues. Interestingly, the strength and efficacy of all of constitutive-direct, constitutive-indirect, induced-direct, and induced-indirect resistance can vary with plant age and ontological stage. Here, we examine the effect of plant age on an induced-indirect resistance trait, the deployment of extrafloral nectaries (EFNs) to attract pugnacious ants, in a short-lived annual, broad bean (Vicia faba L.). We demonstrate that in severely damaged plants, the induction of EFNs is greater in older plants (5?C6?weeks) than in younger plants (2?C4?weeks); however, in more moderately damaged plants, the induction of EFNs is unaffected by plant age. This suggests the hypothesis that a plant??s ability to induce extrafloral nectar, and therefore recruit more ant ??bodyguards,?? may be related to the interaction of plant age and severity of damage.     

Low nutritive quality as a plant defence: Effects of herbivore-mediated interactions

Summary A plant may lower its nutritive quality, for herbivores, by using secondary compounds, morphological characters and/or having a lowered nutrient content. If such traits decrease the amount of resources lost through herbivory, then they act as antiherbivore defences. However, if herbivores compensate for the lowered nutrient availability, by increasing their intake rates or by prolonging their feeding periods, then this may render the defence useless. I analyse the conditions for evolution of this type of plant defences in a game theoretical model. The predictions of the model depend on the amount of compensatory feeding performed by the herbivores and on the herbivores' mobility in relation to the spatial structure of the plant population. When herbivores cannot compensate for a lowered nutritive quality, the defence can evolve irrespective of the type of herbivore. When herbivores can compensate for such defences, the outcome depends on how the herbivores compensate. In situations where herbivores compensate only on defended plants, which could correspond to immobile herbivores, this type of defence can evolve only if the level of compensation is lower than a certain critical value. When herbivores compensate more on defended than on undefended plants, e.g. because of low mobility, the outcome depends on the level of compensation performed on defended plants. If this level of compensation is high, then the model predicts a stable coexistence of defended and undefended plants and, if it is low, then the populations can consist of only defended plants. When herbivores compensate more on undefended plants than on defended ones, e.g. highly mobile herbivores, the result is populations consisting of either only defended plants, or only undefended plants. Consequently, the fact that herbivores may compensate for lowered nutrient quality does not, as such, nullify the notion of low nutrient quality as a plant defence. However, compensatory feeding may restrict the conditions for the evolution of such defences.     

Physical and chemical properties of primary defences in Tenebrio molitor

Prevention and reaction are the foundation for any defence system. In insects, the primary defences against pathogens and parasites limit invasion; the secondary ones (e.g. immune system) act when the cuticle and other primary defences fail. Because investment in both aspects of defence may be costly, they should be regulated in a plastic or variable way in accordance with the risk of infection. The mealworm beetle Tenebrio molitor L. changes cuticle colour and its resistance to fungal infection when subject to high population density, although such resistance is a result of the primary (cuticle) defences rather than the secondary (immunological) ones. The present study tests the hypothesis that the physical and chemical properties of the primary defences in T. molitor change with cuticular darkness. Beetles expressing black phenotypes (or with darker cuticle) have a thicker cuticle, with four well organized layers (epi‐, exo‐, endocuticle and formation zone) and more melanin than tan beetles. The cuticle properties investigated in the present study are likely to be the underlying mechanisms of pathogen resistance in black beetles, including the content of carbonylated proteins, which in black beetles was almost half that of tan beetles after exposure to ultraviolet radiation. It is proposed that, in polyphenic insects (such as mealworm beetles), primary and secondary defences are regulated pleiotropically, with the genes responsible for the expression of one defence having a positive effect on others, whereas, in polymorphic insects, there is no such link and so investment in one defence may impair others.     

Reduced chemical defence in ant‐plants? A critical re‐evaluation of a widely accepted hypothesis

Since its original formulation by Janzen in 1966, the hypothesis that obligate ant‐plants (myrmecophytes) defended effectively against herbivores by resident mutualistic ants have reduced their direct, chemical defence has been widely adopted. We tested this hypothesis by quantifying three classes of phenolic compounds (hydrolysable tannins, flavonoids, and condensed tannins) spectrophotometrically in the foliage of 20 ant‐plant and non‐ant‐plant species of the three unrelated genera Leonardoxa,Macaranga and Acacia (and three other closely related Mimosoideae from the genera Leucaena, Mimosa and Prosopis). We further determined biological activities of leaf extracts of the mimosoid species against fungal spore germination (as measure of pathogen resistance), seed germination (as measure of allelopathic activity), and caterpillar growth (as measure of anti‐herbivore defence).
Condensed tannin content in three of four populations of the non‐myrmecophytic Leonardoxa was significantly higher than in populations of the myrmecophyte. In contrast, we observed no consistent differences between ant‐plants and non‐ant‐plants in the Mimosoideae and in the genus Macaranga, though contents of phenolic compounds varied strongly among different species in each of these two plant groups. Similarly, among the investigated Mimosoideae, biological activity against spore or seed germination and caterpillar growth varied considerably but showed no clear relation with the existence of an obligate mutualism with ants. Our results did not support the hypothesis of ‘trade‐offs’ between indirect, biotic and direct, chemical defence in ant‐plants.
A critical re‐evaluation of the published data suggests that support for this hypothesis is more tenuous than is usually believed. The general and well‐established phenomenon that myrmecophytes are subject to severe attack by herbivores when deprived of their ants still lacks an explanation. It remains to be studied whether the trade‐off hypothesis holds true only for specific compounds (such as chitinases and amides whose cost may be the direct negative effects on plants’ ant mutualists), or whether the pattern of dramatically reduced direct defence of ant‐plants is caused by classes of defensive compounds not yet studied.     

Production of food bodies on the reproductive organs of myrmecophytic Macaranga species (Euphorbiaceae): effects on interactions with herbivores and pollinators

In protective ant–plant mutualisms, plants offer ants food (such as extrafloral nectar and/or food bodies) and ants protect plants from herbivores. However, ants often negatively affect plant reproduction by deterring pollinators. The aggressive protection that mutualistic ants provide to some myrmecophytes may enhance this negative effect in comparison to plant species that are facultatively protected by ants. Because little is known about the processes by which myrmecophytes are pollinated in the presence of ant guards, we examined ant interactions with herbivores and pollinators on plant reproductive organs. We examined eight myrmecophytic and three nonmyrmecophytic Macaranga species in Borneo. Most of the species studied are pollinated by thrips breeding in the inflorescences. Seven of eight myrmecophytic species produced food bodies on young inflorescences and/or immature fruits. Food body production was associated with increased ant abundance on inflorescences of the three species observed. The exclusion of ants from inflorescences of one species without food rewards resulted in increased herbivory damage. In contrast, ant exclusion had no effect on the number of pollinator thrips. The absence of thrips pollinator deterrence by ants may be due to the presence of protective bracteoles that limit ants, but not pollinators, from accessing flowers. This unique mechanism may account for simultaneous thrips pollination and ant defense of inflorescences.     

Conspicuous extra-floral nectaries are inducible in Vicia faba

Mutualistic interactions are dynamic associations that vary depending on the costs and benefits to each of the interacting parties. Phenotypic plasticity in mutualistic interactions allows organisms to produce rewards to attract mutualists when the benefits of their presence outweigh the costs of producing the rewards. In ant–plant defensive mutualisms, defences are indirect as plants produce extra‐floral nectaries (EFN) to attract predatory ants to deter herbivores. Here we demonstrate that in broad bean, Vicia faba, the overall number of EFNs on a plant increases dramatically following leaf damage. In two damage treatments, removal of: (1) one‐third of one leaf in a single leaf pair or (2) one‐third of both halves of a single leaf pair, resulted in a 59 and 106% increase in the number of EFNs on the plants, respectively, over 1 week. We suggest that the increased production of visually conspicuous EFNs is an adaptive inducible response, to attract predatory arthropods when risk of herbivory increases.     

An investigation into induced plant defences

This laboratory exercise involves techniques that can be used to investigate induced plant defences. Runner beans are damaged, and the leaves that subsequently grow are tested for the presence of additional defence mechanisms. A control (undamaged) group is kept for comparison. Induced physical defences are indicated by an increase in the density of hairs (trichomes) on the leaves, and induced chemical defences by a decrease in the palatability of the leaves. The latter is tested by a bioassay using garden snails. These basic methods can be used to investigate a variety of defensive responses in plants.     

Plant structural traits and their role in anti-herbivore defence

We consider the role that key structural traits, such as spinescence, pubescence, sclerophylly and raphides, play in protecting plants from herbivore attack. Despite the likelihood that many of these morphological characteristics may have evolved as responses to other environmental stimuli, we show that each provides an important defence against herbivore attack in both terrestrial and aquatic ecosystems. We conclude that leaf-mass–area is a robust index of sclerophylly as a surrogate for more rigorous mechanical properties used in herbivory studies. We also examine herbivore counter-adaptations to plant structural defence and illustrate how herbivore attack can induce the deployment of intensified defensive measures. Although there have been few studies detailing how plant defences vary with age, we show that allocation to structural defences is related to plant ontogeny. Age-related changes in the deployment of structural defences plus a paucity of appropriate studies are two reasons why relationships with other plant fitness characteristics may be obscured, although we describe studies where trade-offs between structural defence and plant growth, reproduction, and chemical defences have been demonstrated. We also show how resource availability influences the expression of structural defences and demonstrate how poorly our understanding of plant structural defence fits into contemporary plant defence theory. Finally, we suggest how a better understanding of plant structural defence, particularly within the context of plant defence syndromes, would not only improve our understanding of plant defence theory, but enable us to predict how plant morphological responses to climate change might influence interactions at the individual (plant growth trade-offs), species (competition), and ecosystem (pollination and herbivory) levels.     

Silicon-augmented resistance of plants to herbivorous insects: a review

Silicon (Si) is one of the most abundant elements in the earth's crust, although its essentiality in plant growth is not clearly established. However, the importance of Si as an element that is particularly beneficial for plants under a range of abiotic and biotic stresses is now beyond doubt. This paper reviews progress in exploring the benefits at two‐ and three‐trophic levels and the underlying mechanism of Si in enhancing the resistance of host plants to herbivorous insects. Numerous studies have shown an enhanced resistance of plants to insect herbivores including folivores, borers, and phloem and xylem feeders. Silicon may act directly on insect herbivores leading to a reduction in insect performance and plant damage. Various indirect effects may also be caused, for example, by delaying herbivore establishment and thus an increased chance of exposure to natural enemies, adverse weather events or control measures that target exposed insects. A further indirect effect of Si may be to increase tolerance of plants to abiotic stresses, notably water stress, which can in turn lead to a reduction in insect numbers and plant damage. There are two mechanisms by which Si is likely to increase resistance to herbivore feeding. Increased physical resistance (constitutive), based on solid amorphous silica, has long been considered the major mechanism of Si‐mediated defences of plants, although there is recent evidence for induced physical defence. Physical resistance involves reduced digestibility and/or increased hardness and abrasiveness of plant tissues because of silica deposition, mainly as opaline phytoliths, in various tissues, including epidermal silica cells. Further, there is now evidence that soluble Si is involved in induced chemical defences to insect herbivore attack through the enhanced production of defensive enzymes or possibly the enhanced release of plant volatiles. However, only two studies have tested for the effect of Si on an insect herbivore and third trophic level effects on the herbivore's predators and parasitoids. One study showed no effect of Si on natural enemies, but the methods used were not favourable for the detection of semiochemical‐mediated effects. Work recently commenced in Australia is methodologically and conceptually more advanced and an effect of Si on the plants' ability to generate an induced response by acting at the third trophic level was observed. This paper provides the first overview of Si in insect herbivore resistance studies, and highlights novel, recent hypotheses and findings in this area of research. Finally, we make suggestions for future research efforts in the use of Si to enhance plant resistance to insect herbivores.     

Water Stress Strengthens Mutualism Among Ants,Trees, and Scale Insects

Abiotic environmental variables strongly affect the outcomes of species interactions. For example, mutualistic interactions between species are often stronger when resources are limited. The effect might be indirect: water stress on plants can lead to carbon stress, which could alter carbon-mediated plant mutualisms. In mutualistic ant–plant symbioses, plants host ant colonies that defend them against herbivores. Here we show that the partners'' investments in a widespread ant–plant symbiosis increase with water stress across 26 sites along a Mesoamerican precipitation gradient. At lower precipitation levels, Cordia alliodora trees invest more carbon in Azteca ants via phloem-feeding scale insects that provide the ants with sugars, and the ants provide better defense of the carbon-producing leaves. Under water stress, the trees have smaller carbon pools. A model of the carbon trade-offs for the mutualistic partners shows that the observed strategies can arise from the carbon costs of rare but extreme events of herbivory in the rainy season. Thus, water limitation, together with the risk of herbivory, increases the strength of a carbon-based mutualism.     

保护性的蚂蚁-植物相互作用及其调节机制研究综述

蚂蚁-植物互利关系是生态和进化研究中的模式系统之一。该文分析总结了近年来有关蚂蚁对植物的保护作用及其调节机制的研究进展。植物通过给蚂蚁提供食物体、蚁菌穴和蜜露吸引蚂蚁, 通过自身的物理、化学方式调节与蚂蚁的互利关系, 使蚂蚁能有效地保护自已, 防止欺骗和寄生的发生。反过来, 蚂蚁可以减少植食性动物对植物的伤害和取食, 减少叶片损伤, 提高种子产量和质量, 提高植物的竞争优势等。虽然蚂蚁对植物的保护作用的强度受到多种生物与非生物因素的影响, 变异性较大, 但在大多数情况下, 蚂蚁-植物之间仍呈显著的正相互作用。同时, 蚂蚁-植物的相互作用还具有广泛的生态影响, 尤其会大大降低林冠上节肢动物群落的物种多样性和多度。未来的研究需要加强蚂蚁-植物互利关系的起源与维持机制、对蚂蚁自身的影响、与生物入侵的关系, 以及进化生态学等方面的研究。     

Trade-off among antiherbivore defences in a south american blackberry (Rubus bogotensis)

Summary The idea of trade-offs among antiherbivore defences in plants is examined using data from a South American blackberry (Rubus bogotensis). Two distinct morphs of R. bogotensis, one with glandular trichomes and one without, were compared with respect to leaf toughness, number of prickles and prickle length. The two morphs were sympatric and grew under similar environmental conditions. The morph lacking trichomes had significantly tougher leaves and also tended to have more and longer prickles. Bioassay showed that Ithomiid larvae fed to a lesser extent on tough leaves than on more tender ones. Correlations between antiherbivore defences within each phenotype revealed three significant or almost significant negative relationships. The comparisons support the hypothesis that trade-offs exist among antiherbivore defences.     

Plasticity in the expression of direct and indirect defence traits of young plants of <Emphasis Type="Italic">Mallotus japonicus</Emphasis> in relation to soil nutritional conditions

Although soil nutrients can influence the defence strategy of plants that have multiple defence traits, to date, there have been few studies to examine this. To evaluate the effect of soil nutrients on multiple plant defences, we cultivated Mallotus japonicus under three soil nutritional conditions in the field, and experimentally examined the expression of a physical defence trait (trichomes), chemical traits (pellucid dots), and biotic traits (extrafloral nectaries (EFNs) and pearl bodies) of the plants, and the number of ants visiting them. Under the low soil nutritional condition, plants strongly expressed the physical defence by trichomes and chemical defence by pellucid dots, meaning that the young plants adopted direct defences under the poor soil nutritional condition. Under the high soil nutritional condition, in contrast, the plants strongly expressed the indirect defence traits. They produced abundant EFNs and pearl bodies, and attracted many ants. These results suggest that young plants of M. japonicus use different defence modes in response to different soil nutritional conditions.