Constitutive Defense Research Articles

The Role of Herbivore-Induced Plant Volatiles in Tri-trophic Interactions and Pest Management

In agricultural ecosystems, the tri trophic interactions between plants, herbivores and their natural enemies are crucial for maintaining ecological balance and crop health. Natural enemies of pests predominantly regulate herbivore populations, emphasizing the importance of these interactions. Plants exhibit both induced and constitutive defences, Plants use herbivore-induced plant volatiles (HIPVs) as a key mechanism to attract natural enemies that defend them against the herbivores. HIPVs are chemical signals emitted in response to herbivore damage and can attract predators and parasitoids, indirectly they protect the plants. This review explores the role of HIPVs in plant defense, focusing on their evolutionary, chemical and practical aspects. with HIPVs acting as indirect defenses by recruiting natural enemies. The interaction between physical traits, chemical signals and semiochemicals, including volatile organic compounds (VOCs), significantly influences these interactions. Olfactometers, such as the Y-tube and four-arm designs, have been proven to be pivotal in studying insect responses to these volatile compounds, providing insights into their behavior and effectiveness in pest management. Despite advances in understanding HIPVs, research indicates variability in volatile emissions and their effectiveness across different herbivore species and developmental stages. Further studies are needed to improve the specificity of HIPV research, incorporating field-based analyses to capture natural environmental complexities. Future research should focus on optimizing HIPV applications in pest management, understanding spatial and temporal scales of HIPV interactions, and addressing the influence of environmental factors on HIPV efficacy.

Mathematical modeling predicts that endemics by generalist insects are eradicated if nearly all plants produce constitutive defense

Plants with constitutive defense chemicals exist widely in nature. The phenomenon is backed by abundant data from plant chemical ecology. Sufficient data are also available to conclude that plant defenses act as deterrent and repellent to attacking herbivores, particularly deleterious generalist insects. In the wild, generalist species are usually not endemic, meaning they are not restricted to certain plant species in a region. Therefore, our objective is to inspect theoretically whether evolution of chemical defenses in all plant species eradicate an endemic by any generalist species. The objective is addressed by developing deterministic ordinary differential equations under the following conditions: Plants without constitutive defenses are susceptible to oviposition by generalist insects, while they become defended against generalists by storing chemical defenses. From the models, we explicitly obtain that a generalist-free stable state is only possible if the vast majority of all plant individuals have chemical defenses. The model also allows one to predict the highest possible percentage of undefended plant individuals, which may be considered as free-riders.

First comprehensive evaluation of genetic variability for bacterial blight resistance in wild Punica granatum L. populations from the North-Western Himalayas

Identifying and selecting disease-resistant sources from wild germplasm pools is pivotal due to their ecological benefits and cost-effectiveness. This study encompassed the survey and collection of wild pomegranate genotypes alongside commercial cultivars, followed by screening against native pathogens. Additionally, biochemical analyses were conducted to elucidate defense mechanisms in both resistant and susceptible genotypes. Cuttings from selected germplasm across four districts were obtained, grown for one year, and assessed for pathogenicity and biochemical responses. Microscopic and molecular analyses confirmed the pathogen as Xanthomonas axonopodis pv. punicae (Xap), identified by its distinctive morphology and a 1500 bp amplicon along with phylogeny analysis. Germplasm screening via detached and attached leaf assays identified genotypes with minimal disease severity, highlighting SH-14 and SH-16 as highly resistant in the attached leaf assay. Principal component analysis categorized genotypes based on disease parameters, with PC1 explaining approximately 75 % of the variability. Agglomerative hierarchical clustering further grouped genotypes into ten clusters, emphasizing clusters IX and X for their significant resistance traits. Biochemical studies revealed that the resistant genotype SH-16 exhibited higher levels of constitutive defense components like chlorophyll and vitamin C, as well as induced defense components such as phenols, flavonoids, and antioxidants, compared to the susceptible cultivar Bhagwa. These findings underscore the critical roles of both constitutive and induced defense mechanisms in conferring pomegranate resistance to Xap. The study's comprehensive approach in evaluating genetic and biochemical resistance provides valuable insights for breeding disease-resistant pomegranate varieties, marking a significant advancement in exploring pomegranate wild relatives from the Himalayan region for their resistance to bacterial blight.

Exploiting Induced Plant Resistance for Sustainable Pest Management: Mechanisms, Elicitors, and Applications: A Review

Plants and phytophagous (plant-eating) arthropods have coevolved over millions of years, leading to the development of both constitutive and inducible defense mechanisms in plants. Despite this long history of coexistence, it remains unclear how to precisely regulate each host-arthropod interaction to achieve an equilibrium that maximizes crop yield. The defensive chemicals produced by plants can significantly affect herbivores’ feeding behaviour, growth, and survival. These chemicals may be generated constitutively (continuously) or induced in response to herbivore damage. Induced resistance, a strategy where plants enhance their defences after being attacked, holds potential for reducing the number of insecticides needed in pest management. Chemical elicitors, which trigger the production of secondary metabolites that confer resistance to insects, can be used to manipulate host plant resistance, specifically by inducing resistance. By understanding the mechanisms underlying induced resistance, it is possible to predict which herbivores will be impacted by these responses. Applying induced response elicitors to crop plants can strengthen their natural defences against herbivore damage. Additionally, it is possible to genetically modify plants so that they constitutively produce defense chemicals, providing continuous protection in areas where herbivory is a constant threat. As part of integrated pest management, induced resistance can be used to develop crop cultivars that readily trigger an inducible response in the event of a mild infestation, contributing to sustainable agricultural practices and reducing reliance on chemical pesticides.

Transcriptome, hormonal, and secondary metabolite changes in leaves of DEFENSE NO DEATH 1 (DND1) silenced potato plants

DEFENSE NO DEATH 1 (DND1) is a cyclic nucleotide-gated ion channel protein. Earlier, it was shown that the silencing of DND1 in the potato (Solanum tuberosum L.) leads to resistance to late blight, powdery mildew, and gray mold diseases. At the same time, however, it can reduce plant growth and cause leaf necrosis. To obtain knowledge of the molecular events behind the pleiotropic effect of DND1 downregulation in the potato, metabolite and transcriptome analyses were performed on three DND1 silenced lines of the cultivar ‘Désirée.’ A massive increase in the salicylic acid content of leaves was detected. Concentrations of jasmonic acid and chlorogenic acid and their derivatives were also elevated. Expression of 1866 genes was altered in the same way in all three DND1 silenced lines, including those related to the synthesis of secondary metabolites. The activation of several alleles of leaf rust, late blight, and other disease resistance genes, as well as the induction of pathogenesis-related genes, was detected. WRKY and NAC transcription factor families were upregulated, whereas bHLHs were downregulated, indicating their central role in transcriptome changes. These results suggest that the maintenance of the constitutive defense state leads to the reduced growth of DND1 silenced potato plants.

CmMYC2-CmMYBML1 module orchestrates the resistance to herbivory by synchronously regulating the trichome development and constitutive terpene biosynthesis in Chrysanthemum.

Trichomes are specialized epidermal outgrowths covering the aerial parts of most terrestrial plants. There is a large species variability in occurrence of different types of trichomes such that the molecular regulatory mechanism underlying the formation and the biological function of trichomes in most plant species remain unexplored. Here, we used Chrysanthemum morifolium as a model plant to explore the regulatory network in trichome formation and terpenoid synthesis and unravel the physical and chemical roles of trichomes in constitutive defense against herbivore feeding. By analyzing the trichome-related genes from transcriptome database of the trichomes-removed leaves and intact leaves, we identified CmMYC2 to positively regulate both development of T-shaped and glandular trichomes as well as the content of terpenoids stored in glandular trichomes. Furthermore, we found that the role of CmMYC2 in trichome formation and terpene synthesis was mediated by interaction with CmMYBML1. Our results reveal a sophisticated molecular mechanism wherein the CmMYC2-CmMYBML1 feedback inhibition loop regulates the formation of trichomes (non-glandular and glandular) and terpene biosynthesis, collectively contributing to the enhanced resistance to Spodoptera litura larvae feeding. Our findings provide new insights into the novel regulatory network by which the plant synchronously regulates trichome density for the physical and chemical defense against herbivory.

Constitutive Level of Specialized Secondary Metabolites Affects Plant Phytohormone Response to Above- and Belowground Herbivores

Plants defend themselves chemically against herbivory through secondary metabolites and phytohormones. Few studies have investigated how constitutive variation in secondary metabolites contributes to systemic herbivory response. We hypothesized that plants with lower constitutive defenses would induce a stronger phytohormone response to spatially separated herbivory than plants with high constitutive defense. We used growth chamber bioassays to investigate how aboveground herbivory by Colorado potato beetle (Leptinotarsa decemlineata, CPB) and belowground herbivory by northern root-knot nematode (Meloidogyne hapla, RKN) altered phytohormones and glycoalkaloids in roots and shoots of two lines of wild potato (Solanum chacoense). These lines had different constitutive levels of chemical defense, particularly leptine glycoalkaloids, which are only present in aboveground tissues. We also determined how these differences influenced the preference and performance of CPB. The susceptible wild potato line responded to aboveground damage by CPB through induction of jasmonic acid (JA) and OPDA. However, when challenged by both RKN and CPB, the susceptible line retained high levels of JA, but not OPDA. Beetles gained more mass after feeding on the susceptible line compared to the resistant line, but were not affected by nematode presence. Belowground, JA, JA-Isoleucine, and OPDA were higher in the resistant line compared to the susceptible line, and some compounds demonstrated response to local herbivory. In contrast, the susceptible line did not induce phytohormone defenses belowground. These findings allow us to predict that constitutive level of defense may influence the threshold of herbivory that may lead to plant-mediated effects on spatially separated herbivores.

The role of uncertainty and negative feedback loops in the evolution of induced immune defenses.

Organisms use constitutive or induced defenses against pathogens and other external threats. Constitutive defenses are constantly on, whereas induced defenses are activated when needed. Each of these strategies has costs and benefits, which can affect the type of defense that evolves in response to pathogens. In addition, induced defenses are usually regulated by multiple negative feedback mechanisms that prevent overactivation of the immune response. However, it is unclear how negative feedback affects the costs, benefits, and evolution of induced responses. To address this gap, we developed a mechanistic model of the well-characterized Drosophila melanogaster immune signaling network that includes 3 separate mechanisms of negative feedback as a representative of the widespread phenomenon of multilevel regulation of induced responses. We show that, under stochastic fly-bacteria encounters, an induced defense is favored when bacterial encounters are rare or uncertain, but in ways that depend on the bacterial proliferation rate. Our model also predicts that the specific negative regulators that optimize the induced response depend on the bacterial proliferation rate, linking negative feedback mechanisms to the factors that favor induction.

Synergistic and antagonistic interactions between plant defences and biological pest control

Plants have evolved various defence mechanisms against herbivorous pests, ranging from physical barriers (constitutive defences) to the release of toxins (induced defences) in response to an attack. Plant defences are not always target-specific and can also act against natural enemies of these pests. This could potentially become problematic when considering both plant defences and predators as part of an Integrated Pest Management (IPM) scheme. Here we used a predator-prey model to capture the population dynamics of a biological control system to investigate the conditions where both plant defences and biological control act synergistically, leading to better pest control, and under what condition antagonistic outcomes could be expected. Our results demonstrate that both antagonistic and synergistic interactions are observable under small changes in key parameters, such as predation and growth rates. We then compared the qualitative model predictions against a set of population dynamic experiments using the Orius laevigatus (Hemiptera: Anthocoridae) - Frankliniella occidentalis (Thysanoptera: Thripidae) predator-pest system on tomato plants. In particular, we investigated the effect of plant defence-induced changes in pest growth rates and predation rates. For this we compared wild-type tomato with a mutant unable to synthesise jasmonic acid, an induced defence mechanism against pest attack, whereas changes to predation rates were realised by using O. laevigatus predator strains that are, due to differences in size, differentially affected by the plant's trichome-based constitutive defences. In agreement with model predictions, the qualitative dynamics confirmed the existence of both antagonistic and synergistic outcomes. Our results highlight the importance of considering multi-trophic interactions for the development of effective IPMs.

Patterns of constitutive and induced herbivore defence are complex, but share a common genetic basis in annual and perennial monkeyflower

Abstract Despite multiple ecological and evolutionary hypotheses that predict patterns of phenotypic relationships between plant growth, reproduction and constitutive and/or induced resistance to herbivores, these hypotheses do not make any predictions about the underlying molecular genetic mechanisms that mediate these relationships. We investigated how divergent plant life‐history strategies in the yellow monkeyflower and a life‐history altering locus, DIV1, influence plasticity of phytochemical herbivory resistance traits in response to attack by two herbivore species with different diet breadth. Life‐history strategy (annual vs. perennial) and the DIV1 locus significantly influenced levels of constitutive herbivory resistance, as well as resistance induction following both generalist and specialist herbivory. Perennial plants had higher total levels of univariate constitutive and induced defence than annuals, regardless of herbivore type. Annuals induced less in response to generalist herbivory than did perennials, while induction response was equivalent across the ecotypes for specialist herbivory. The effects of the DIV1 locus on levels of constitutive and induced defence were dependent on genetic background, the annual versus perennial haplotype of DIV1 and herbivore identity. The patterns of univariate induction due to DIV1 were non‐additive and did not always match expectations based on patterns of divergence for annual/perennial parents. For example, perennial plants had higher levels of constitutive and induced defence than did annuals, but when the annual DIV1 was present in the perennial genetic background induction response to herbivory was higher than for the perennial parent lines. Patterns for multivariate defence arsenals generally echoed those of univariate, with annual and perennial monkeyflowers and those with alternative versions of DIV1 differing significantly in constitutive and induced resistance. Like univariate resistance, induced multivariate defence arsenals were affected by herbivore identity. Our results highlight the complexity of the genetic mechanisms underlying plastic response to herbivory. While a genetic locus underlying substantial phenotypic variation in life‐history strategy and constitutive defence also influences defence plasticity, the induction response also depends on genetic background. This result demonstrates the potential for some degree of evolutionary independence between constitutive and induced defence, or induced defence and life‐history strategy, in monkeyflowers. Read the free Plain Language Summary for this article on the Journal blog.

Over-accumulation of chloroplast-nucleus located WHIRLY1 in barley leads to a decrease in growth and an enhanced stress resistance.

WHIRLY1 is a chloroplast-nucleus located DNA/RNA-binding protein with functions in development and stress tolerance. By overexpression of HvWHIRLY1 in barley, one line with a 10-fold and two lines with a 50-fold accumulation of the protein were obtained. In these lines, the relative abundance of the nuclear form exceeded that of the chloroplast form. Growth of the plants was shown to be compromised in a WHIRLY1 abundance-dependent manner. Over-accumulation of WHIRLY1 in chloroplasts had neither an evident impact on nucleoid morphology nor on the composition of the photosynthetic apparatus. Nevertheless, oeW1 plants were found to be compromised in the light reactions of photosynthesis as well as in carbon fixation. The reduction in growth and photosynthesis was shown to be accompanied by a decrease in the levels of cytokinins and an increase in the level of jasmonic acid. Gene expression analyses revealed that in nonstress conditions the oeW1 plants had enhanced levels of pathogen response (PR) gene expression indicating activation of constitutive defense. During growth in continuous light of high irradiance PR gene expression increased indicating that under stress conditions oeW1 are capable to further enhance defense.

Sleeping but not defenceless: seed dormancy and protection.

To ensure their vital role in disseminating the species, dormant seeds have developed adaptive strategies to protect themselves against pathogens and predators. This is orchestrated through the synthesis of an array of constitutive defences that are put in place in a developmentally regulated manner, which are the focus of this review. We summarize the defence activity and the nature of the molecules coming from the exudate of imbibing seeds that leak into their vicinity, also referred to as the spermosphere. As a second layer of protection, the dual role of the seed coat will be discussed; as a physical barrier and a multi-layered reservoir of defence compounds that are synthesized during seed development. Since imbibed dormant seeds can persist in the soil for extensive periods, we address the question of whether during this time a constitutively regulated defence programme is switched on to provide further protection, via the well-defined pathogenesis-related (PR) protein family. In addition, we review the hormonal and signalling pathways that might be involved in the interplay between dormancy and defence and point out questions that need further attention.

Конституционное право и политический реализм: негативная теория свободы слова

Classic positive theories of freedom of speech explain enhanced constitutional defense of speech in terms of fundamental values served by expression (pursuit of truth, democratic governance or personal autonomy). Negative approach, which is the subject of this article, focuses not on the high social value of speech, but on the unique risks of its suppression. The dangers of repressive censorship mainly stem from personal interests of office-holders (legislators as well as law-enforcement officers) or from intolerance. Psychological reasons and the architecture of censorial decision-making turn speech into an especially fragile object of regulation. The idea underlying the constitutional status of freedom of speech is the distrust of government decision-making in speech regulation. Judicial review should pay close attention to the design of abridging laws and justifications provided for them. These laws should be worded clearly and should not allow for selective viewpoint censorship. The simplicity of negative approach and its political realism make it the most plausible theory for interpreting constitutional article on freedom of speech (expression).

Simulated Herbivory Affects the Volatile Emissions of Oak Saplings, while Neighbourhood Affects Flavan-3-ols Content of Their Leaves.

To what extent particular plant defences against herbivorous insects are constitutive or inducible will depend on the costs and benefits in their neighbourhood. Some defensive chemicals in leaves are thought to be costly and hard to produce rapidly, while others, including volatile organic compounds that attract natural enemies, might be cheaper and can be released rapidly. When surrounding tree species are more closely related, trees can face an increased abundance of both specialist herbivores and their parasitoids, potentially increasing the benefits of constitutive and inducible defences. To test if oaks (Quercus robur) respond more to herbivore attacks with volatile emission than with changes in leaf phenolic chemistry and carbon to nitrogen ratio (C: N), and whether oaks respond to the neighbouring tree species, we performed an experiment in a forest in Poland. Oak saplings were placed in neighbourhoods dominated by oak, beech, or pine trees, and half of them were treated with the phytohormone methyl jasmonate (elicitor of anti-herbivore responses). Oaks responded to the treatment by emitting a different volatile blend within 24h, while leaf phenolic chemistry and C: N remained largely unaffected after 16 days and multiple treatments. Leaf phenolics were subtly affected by the neighbouring trees with elevated flavan-3-ols concentrations in pine-dominated plots. Our results suggest that these oaks rely on phenols as a constitutive defence and when attacked emit volatiles to attract natural enemies. Further studies might determine if the small effect of the neighbourhood on leaf phenolics is a response to different levels of shading, or if oaks use volatile cues to assess the composition of their neighbourhood.

Phenotyping, genetics, and "-omics" approaches to unravel and introgress enhanced resistance against apple scab (Venturia inaequalis) in apple cultivars (Malus × domestica).

Apple scab disease, caused by the fungus Venturia inaequalis, endangers commercial apple production globally. It is predominantly managed by frequent fungicide sprays that can harm the environment and promote the development of fungicide-resistant strains. Cultivation of scab-resistant cultivars harboring diverse qualitative Rvi resistance loci and quantitative trait loci associated with scab resistance could reduce the chemical footprint. A comprehensive understanding of the host-pathogen interaction is, however, needed to efficiently breed cultivars with enhanced resistance against a variety of pathogenic strains. Breeding efforts should not only encompass pyramiding of Rvi loci and their corresponding resistance alleles that directly or indirectly recognize pathogen effectors, but should also integrate genes that contribute to effective downstream defense mechanisms. This review provides an overview of the phenotypic and genetic aspects of apple scab resistance, and currently known corresponding defense mechanisms. Implementation of recent "-omics" approaches has provided insights into the complex network of physiological, molecular, and signaling processes that occur before and upon scab infection, thereby revealing the importance of both constitutive and induced defense mechanisms. Based on the current knowledge, we outline advances toward more efficient introgression of enhanced scab resistance into novel apple cultivars by conventional breeding or genetic modification techniques. However, additional studies integrating different "-omics" approaches combined with functional studies will be necessary to unravel effective defense mechanisms as well as key regulatory genes underpinning scab resistance in apple. This crucial information will set the stage for successful knowledge-based breeding for enhanced scab resistance.

A hidden herbivory effect on Sphagnum reproduction.

Defence theories provide predictions about trade-offs in the allocation of resources to defence and growth. However, very little is known about how pressure from herbivores influences the allocation of resources during reproduction. Two common peatland bryophyte species, Sphagnum angustifolium and S. capillifolium, were chosen as study species. Vegetative and reproductive shoots of both Sphagnum species were subjected to treatments with and without herbivores in a lab experiment. After 4 weeks of exposure to herbivores in a growth chamber, we measured biomass production, net photosynthesis rate, defence traits (phenolics in leachate and phenolics in extract), nonstructural carbohydrates (soluble sugar and starch), and reproductive traits (capsule number, weight and diameter, and spore germination) of both Sphagnum species. Reproductive shoots had higher constitutive defence than vegetative shoots in S. angustifolium, and a similar pattern was observed in S. capillifolium. With herbivory, reproductive shoots showed stronger induced defence (released more phenolics) than vegetative shoots in S. capillifolium, but not in S. angustifolium. Herbivory had no effect on capsule number, weight, or diameter, but reduced spore germination percentage by more than half in both species. Our study highlights the hidden effects of herbivory on reproduction of Sphagnum and indicates the presence of maternal effects in bryophytes. Ecologists will benefit from examining both quality- and quantity-based traits when attempting to estimate the herbivory effect on plant fitness.

Histological analysis of <i>Xylella fastidiosa</i> infection in <i>Quercus pyrenaica</i> in Northern Portugal

<abstract> <p><italic>Quercus pyrenaica</italic> Willd thrives in the intermediate zone between the Mediterranean sclerophyllous and the temperate deciduous forest. In December 2022, the presence of the bacteria <italic>Xylella fastidiosa (Xf)</italic> was confirmed in samples collected from a <italic>Quercus pyrenaica</italic> located in Sabrosa, Vila Real, Portugal. Following <italic>Xf</italic> infection, the transport of water and nutrients is hindered due to the occlusion of xylem vessels. This loss of hydraulic conductivity may lead to vessel blockage and subsequent embolism formation. The objective of this study was to investigate the interaction between <italic>Xf</italic> and <italic>Quercus pyrenaica</italic> tissues, as well as the mechanism by which the bacteria can spread through the plant's xylem vessels, ultimately resulting in the formation of vascular plugs. At the time of the sample collection (10 months post-detection), symptoms of Bacterial Leaf Scorch (BLS) began to appear. Examination of xylem vessels using both light and scanning electron microscopy (SEM) revealed the presence of various types of occlusions, predominantly tyloses. Additionally, fibrillar networks, gums, starch grains, and crystals were observed. The stem vessels exhibited significantly more occlusions compared to the leaves. Furthermore, individual bacterial cells were observed to be attached to the vessel wall. This implies that occlusions were primarily induced by tyloses and gums as a defensive response to the invasion of vascular pathogens, in addition to the pathogen itself. This study highlights the presence of starch grains in stems, which may function as a refilling mechanism, thereby preventing the loss of hydraulic conductivity in plants and potentially acting as a means to entrap the bacteria. These mechanisms exemplify the constitutive defense systems of the plant against <italic>Xf</italic>. Understanding the interaction between <italic>Xylella fastidiosa</italic> and <italic>Quercus pyrenaica</italic> is crucial, given that the latter species occupies nearly 95% of the natural distribution area of Portugal.</p> </abstract>

Selection pressure by specialist and generalist insect herbivores leads to optimal constitutive plant defense. A mathematical model.

Brassicaceae plants have the glucosinolate-myrosinase defense system, jointly active against herbivory. However, constitutive glucosinolate (GLS) defense is observed to occur at levels that do not deter all insects from feeding. That prompts the question of why Brassicaceae plants have not evolved a higher constitutive defense. The answer may lie in the contrasting relationship between plant defense and host plant preference of specialist and generalist herbivores. GLS content increases a plant's susceptibility to specialist insects. In contrast, generalists are deterred by the plant GLSs. Although GLSs can attract the natural enemies (predators and parasitoids) of these herbivores, enemies can reduce herbivore pressure to some extent only. So, plants can be overrun by specialists if GLS content is too high, whereas generalists can invade the plants if it is too low. Therefore, an optimal constitutive plant defense can minimize the overall herbivore pressure. To explain the optimal defense theoretically, we model the contrasting host selection behavior of insect herbivores and the emergence of their natural enemies by non-autonomous ordinary differential equations, where the independent variable is the plant GLS concentration. From the model, we quantify the optimal amount of GLSs, which minimizes total herbivore (specialists and generalists) pressure. That quite successfully explains the evolution of constitutive defense in plants from the perspective of optimality theory.

Trade-offs among leaf toughness, constitutive chemical defense, and growth rates in oaks are influenced by the level of leaf mass per area

Key messageAmong the variety of leaf defensive strategies to counteract herbivory attacks, the oak species analyzed in this study maximize investment in no more than one, with high-LMA oaks developing very tough leaves and low-LMA oaks favoring between high concentrations of condensed tannins or high growth rates.ContextPlants develop a variety of defense strategies to counteract herbivory attacks, from physical and chemical defenses to tolerance strategies. Tradeoffs between strategies have been widely assessed from a resource allocation perspective, but there is a need to consider eventual interactions among them.AimWe evaluate the among-species tradeoff between three main constitutive plant defense traits, while considering the leaf mass per area ratio (LMA) as a proxy of leaf construction investment on area basis.MethodsLeaf toughness measured as work of fracture, condensed tannins, and relative growth rate (RGR) were analyzed in a set of 19 Quercus L. species with contrasting LMA.ResultsMost species had low values either in two or in the three traits analyzed. Moreover, the highest values of work of fracture appeared in the species with high LMA; the highest values of condensed tannins were found in the species with the lowest LMA; and high values of RGR were measured in species with intermediate or lower values of LMA.ConclusionOaks showed a trade-off among leaf defensive strategies influenced by LMA. Oaks with high LMA developed very tough leaves while oaks with low LMA presented lower values of toughness but favored between one of the two other strategies.

Silicon-based anti-herbivore defense in tropical tree seedlings.

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding anti-herbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiple-choice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species.