Leaf trichome-mediated predator effects on the distribution of herbivorous mites within a kidney bean plant

Some predators prefer to settle on leaf patches with microstructures (e.g., trichomes and domatia), leaving traces on the patches. Herbivorous arthropods, in turn, select leaf patches in response to these traces left by predators. It remains unclear whether traces of predators on leaf patches affect the distribution of herbivorous prey within plants through plant microstructure. Therefore, we examined the distribution of herbivorous mite (Tetranychus urticae) and predatory mite (Phytoseiulus persimilis) by investigating their oviposition pattern. We used a kidney bean plant (Phaseolus vulgaris) with two expanded primary leaves and the first trifoliate leaf, focusing on leaf trichomes as the microstructure. The density of trichomes was higher on the first trifoliate leaf than on the primary leaves and on the abaxial surface of the leaves than on the adaxial surface. Adult female P. persimilis laid more eggs on the first trifoliate leaf to the primary leaves. Although adult female T. urticae preferred to oviposit on the abaxial surface of primary leaves, previous exposure of plants to predators diminished this preference. The altered egg distribution would be a response to the traces of P. persimilis rather than eggs of P. persimilis. Our findings indicate that T. urticae reproduces on leaf patches with traces of predators without altering their oviposition preference. Given that the presence of predator traces is known to reduce the reproduction of T. urticae, it may have a substantial effect on the population of T. urticae in the next generations on kidney bean plants.

The role of the Golden2-like (GLK) transcription factor in regulating terpenoid indole alkaloid biosynthesis in Catharanthus roseus

KEY MESSAGE

A GLK homologue was identified and functionally characterized in Catharanthus roseus. Silencing CrGLK with VIGS or the chloroplast retrograde signaling inducer lincomycin increased terpenoid indole alkaloid biosynthesis. Catharanthus roseus is the sole source of the chemotherapeutic terpenoid indole alkaloids (TIAs) vinblastine and vincristine. TIA pathway genes, particularly genes in the vindoline pathway, are expressed at higher levels in immature versus mature leaves, but the molecular mechanisms responsible for this developmental regulation are unknown. We investigated the role of GOLDEN2-LIKE (GLK) transcription factors in contributing to this ontogenetic regulation since GLKs are active in seedlings upon light exposure and in the leaf's early development, but their activity is repressed as leaves age and senesce. We identified a GLK homologue in C. roseus and functionally characterized its role in regulating TIA biosynthesis, with a focus on the vindoline pathway, by transiently reducing its expression through two separate methods: virus-induced gene silencing (VIGS) and application of chloroplast retrograde signaling inducers, norflurazon and lincomycin. Reducing CrGLK levels with each method reduced chlorophyll accumulation and the expression of the light harvesting complex subunit (LHCB2.2), confirming its functional homology with GLKs in other plant species. In contrast, reducing CrGLK via VIGS or lincomycin increased TIA accumulation and TIA pathway gene expression, suggesting that CrGLK may repress TIA biosynthesis. However, norflurazon had no effect on TIA gene expression, indicating that reducing CrGLK alone is not sufficient to induce TIA biosynthesis. Future work is needed to clarify the specific molecular mechanisms leading to increased TIA biosynthesis with CrGLK silencing. This is the first identification and characterization of GLK in C. roseus and the first investigation of how chloroplast retrograde signaling might regulate TIA biosynthesis.

Age-dependent resistance of a perennial herb, Aristolochia contorta against specialist and generalist leaf-chewing herbivores

Plants need to balance investments in growth and defense throughout their life to increase their fitness. To optimize fitness, levels of defense against herbivores in perennial plants may vary according to plant age and season. However, secondary plant metabolites often have a detrimental effect on generalist herbivores, while many specialists have developed resistance to them. Therefore, varying levels of defensive secondary metabolites depending on plant age and season may have different effects on the performance of specialist and generalist herbivores colonizing the same host plants. In this study, we analyzed concentrations of defensive secondary metabolites (aristolochic acids) and the nutritional value (C/N ratios) of 1^st-, 2^nd- and 3^rd-year Aristolochia contorta in July (the middle of growing season) and September (the end of growing season). We further assessed their effects on the performances of the specialist herbivore Sericinus montela (Lepidoptera: Papilionidae) and the generalist herbivore Spodoptera exigua (Lepidoptera: Noctuidae). Leaves of 1^st-year A. contorta contained significantly higher concentrations of aristolochic acids than those of older plants, with concentrations tending to decrease over the first-year season. Therefore, when first year leaves were fed in July, all larvae of S. exigua died and S. montela showed the lowest growth rate compared to older leaves fed in July. However, the nutritional quality of A. contorta leaves was lower in September than July irrespective of plant age, which was reflected in lower larval performance of both herbivores in September. These results suggest that A. contorta invests in the chemical defenses of leaves especially at a young age, while the low nutritional value of leaves seems to limit the performance of leaf-chewing herbivores at the end of the season, regardless of plant age.

Evolutionary trade-offs in the chemical defense of floral and fruit tissues across genus Cornus

PREMISE

Defense investment in plant reproductive structures is relatively understudied compared to the defense of vegetative organs. Here the evolution of chemical defenses in reproductive structures is examined in light of the optimal defense, apparency, and resource availability hypotheses within the genus Cornus using a phylogenetic comparative approach in relation to phenology and native habitat environmental data.

METHODS

Individuals representing 25 Cornus species were tracked for reproductive phenology over a full growing season at the Arnold Arboretum of Harvard University. Floral, fruit, and leaf tissue was sampled to quantify defensive chemistry as well as fruit nutritional traits relevant to bird dispersal. Native habitat environmental characteristics were estimated using locality data from digitized herbarium records coupled with global soil and climate data sets.

RESULTS

The evolution of later flowering was correlated with increased floral tannins, and the evolution of later fruiting was correlated with increased total phenolics. Leaves were found to contain the highest tannin activity, while inflorescences contained the highest total flavonoids. Multiple aspects of fruit defensive chemistry were correlated with fruit nutritional traits. Floral and fruit defensive chemistry were evolutionarily correlated with aspects of native habitat temperature, precipitation, and soil characteristics.

CONCLUSIONS

Results provide tentative support for the apparency hypothesis with respect to both flower and fruit phenology, while relative concentrations of secondary metabolites across organs provide mixed support for the optimal defense hypothesis. The evolution of reproductive defense with native habitat provides, at best, mixed support for the resource availability hypothesis.

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.

Leaf metabolic signatures induced by real and simulated herbivory in black mustard (Brassica nigra)

INTRODUCTION

The oxylipin methyl jasmonate (MeJA) is a plant hormone active in response signalling and defence against herbivores. Although MeJA is applied experimentally to mimic herbivory and induce plant defences, its downstream effects on the plant metabolome are largely uncharacterized, especially in the context of primary growth and tissue-specificity of the response.

OBJECTIVES

We investigated the effects of MeJA-simulated and real caterpillar herbivory on the foliar metabolome of the wild plant Brassica nigra and monitored the herbivore-induced responses in relation to leaf ontogeny.

METHODS

As single or multiple herbivory treatments, MeJA- and mock-sprayed plants were consecutively exposed to caterpillars or left untreated. Gas chromatography (GC) and liquid chromatography (LC) time-of-flight mass-spectrometry (TOF-MS) were combined to analyse foliar compounds, including central primary and specialized defensive plant metabolites.

RESULTS

Plant responses were stronger in young leaves, which simultaneously induced higher chlorophyll levels. Both MeJA and caterpillar herbivory induced similar, but not identical, accumulation of tricarboxylic acids (TCAs), glucosinolates (GSLs) and phenylpropanoids (PPs), but only caterpillar feeding led to depletion of amino acids. MeJA followed by caterpillars caused higher induction of defence compounds, including a three-fold increase in the major defence compound allyl-GSL (sinigrin). When feeding on MeJA-treated plants, caterpillars gained less weight indicative of the reduced host-plant quality and enhanced resistance.

CONCLUSIONS

The metabolomics approach showed that plant responses induced by herbivory extend beyond the regulation of defence metabolism and are tightly modulated throughout leaf development. This leads to a new understanding of the plant metabolic potential that can be exploited for future plant protection strategies.

First description of extrafloral nectaries in Opuntia robusta (Cactaceae): Anatomy and ultrastructure

To our knowledge, there are no studies about the structure and ecological function of extrafloral nectaries (EFNs) in Opuntia robusta. This is the first description of EFNs in O. robusta, where young spines have an interesting structure and a secreting function, which are different from EFNs described in other Cactaceae species. We used light, scanning-electron, and transmission-electron microscopy to examine morphology, anatomy, and ultrastructure of the secretory spines in areoles in female and hermaphrodite individuals of O. robusta. Young cladodes develop areoles with modified and secretory spines as EFNs only active during the early growth phase. EFNs are non-vascularized structures, with no stomata, that consist of a basal meristematic tissue, a middle elongation region, and an apical secretory cone formed by large globular epidermal cells, containing nectar and medullar elongated cells. We observed the presence of Golgi apparatus, vesicles and plastids in the medullar and sup-epidermal cells of the spine. We propose that the nectar is stored in the globular cells at the apex of the spine and secreted by breaking through the globular cells or by pores. We recorded a more frequent presence of ants on younger cladode sprouts producing young secreting spines: this result is parallel with the predictions of Optimal Defense Hypothesis, which states that younger plant organs should be better defended than older ones because their loss produces a higher fitness impairment. Although Diaz-Castelazo's hypothesis states that a more complex structure of EFNs correlates with their lower among-organs dispersion, comparing to less complex EFNs, non-vascularized structure of EFNs in O. robusta is not associated with their higher among-organs dispersion likened to O. stricta, which produces vascularized EFNs. We provide evidence that this characteristic is not a good taxonomic feature of Opuntia genus. Moreover, the comparison of EFNs of O. robusta and O. stricta suggests that the hypothesis of Diaz-Castelazo should be revised: it is rather a rule but not a law.

Testing the optimal defense hypothesis in nature: Variation for glucosinolate profiles within plants

Plants employ highly variable chemical defenses against a broad community of herbivores, which vary in their susceptibilities to specific compounds. Variation in chemical defenses within the plant has been found in many species; the ecological and evolutionary influences on this variation, however, are less well-understood. One central theory describing the allocation of defenses in the plant is the Optimal Defense Hypothesis (ODH), which predicts that defenses will be concentrated in tissues that are of high fitness value to the plant. Although the ODH has been repeatedly supported within vegetative tissues, few studies have compared vegetative and reproductive tissues, and the results have not been conclusive. We quantified variation in glucosinolate profile and tissue value between vegetative and reproductive tissues in Boechera stricta, a close relative of Arabidopsis. B. stricta manufactures glucosinolates, a set of defensive compounds that vary genetically and are straightforward to quantify. Genetic diversity in glucosinolate profile has been previously demonstrated to be important to both herbivory and fitness in B. stricta; however, the importance of glucosinolate variation among tissues has not. Here, we investigate whether allocation of glucosinolates within the plant is consistent with the ODH. We used both clipping experiments on endogenous plants and ambient herbivory in a large-scale transplant experiment at three sites to quantify fitness effects of loss of rosette leaves, cauline leaves, and flowers and fruits. We measured glucosinolate concentration in leaves and fruits in the transplant experiment, and asked whether more valuable tissues were more defended. We also investigated within-plant variation in other aspects of the glucosinolate profile. Our results indicated that damage to fruits had a significantly larger effect on overall fitness than damage to leaves, and that fruits had much higher concentrations of glucosinolates, supporting the ODH. This is, to the best of our knowledge, the first study to explicitly compare both tissue value and chemical defense concentrations between vegetative and reproductive tissues under natural conditions.

Leaf trichomes and foliar chemistry mediate defence against glasshouse thrips; Heliothrips haemorrhoidalis (Bouché) in Rhododendron simsii

Herbivore defence mechanisms are a costly diversion of resources away from growth and reproduction. Thus time-limited and tissue specific expression in critical plant parts is more efficient as defined by optimal defence theory. Surprisingly little is known about Rhododendron herbivore defence but it may be mediated by combined chemical and physical mechanisms. Rhododendron simsii Planch. survives cyclic infestations of a leaf-feeding thrips, Heliothrips haemorrhoidalis (Bouché), which severely damage mature leaves but avoid terminal young leaves suggesting specific, localised defence expression. We examined correlations between the distribution of thrips and feeding damage with density of trichomes and the concentration of the diterpenoid, grayanotoxin I, a compound implicated in but not previously reported to mediate invertebrate defence in Rhododendron. Our data show that as leaves matured the number of thrips and area of feeding damage increased as trichome density and grayanotoxin I concentration decreased, this inverse correlation suggesting trichomes and grayanotoxin I mediate defence in younger leaf tissue. Grayanotoxin I was tested against H. haemorrhoidalis and was toxic to immature life stages and repellent to the adult thrips, reducing numbers of first instars emerging on leaves when applied at ecologically relevant concentrations. This work demonstrates that the pattern of defensive traits in foliage of a species of Rhododendron is key to its ability to tolerate cyclic infestations of a generalist herbivore, effectively conserving vital tissues required for growth and reproduction.

Herbivory and relative growth rates of Pieris rapae are correlated with host constitutive salicylic acid and flowering time

Treatment of plants with exogenous salicylic acid (SA) improves resistance to many bacterial pathogens, but can suppress resistance to insect herbivores. While plants vary naturally in constitutive SA, whether such differences are predictive of resistance to insect herbivores has not been studied previously. We examined the possible role of this endogenous SA in structuring the interactions between the cabbage white butterfly, Pieris rapae, and ten hosts in the mustard family (Brassicaceae). Because P. rapae has multiple generations that utilize different hosts across the year, we included five spring-flowering mustards and five summer-flowering mustards that co-occur in ruderal habitats in upstate New York. Under common garden conditions, the spring flowering mustards (Capsella bursa-pastoris, Draba verna, Cardamine impatiens, Barbarea vulgaris, and Arabidopsis thaliana) were significantly more resistant to P. rapae, supporting 42 % less herbivory (P = 0.015) and 64 % lower relative growth rates (P = 0.007), relative to the summer flowering mustards (Sisymbrium altissimum, Brassica nigra, Sinapis arvense, Lepidium campestre, and Arabis canadensis). Leaf total constitutive SA explained significant variation in larval herbivory (R² = 75.3 %, P = 0.007) and relative growth rates (R² = 59.4 %, P = 0.043). The three species with the lowest levels of constitutive SA (Capsella bursa-pastoris, Draba verna, and Cardamine impatiens) were the most resistant to larvae. Barbarea vulgaris and Arabis canadensis were notable exceptions, exhibiting high SA concentrations and intermediate resistance to P. rapae. These results suggest a curvilinear relationship between leaf constitutive SA and the herbivory by P. rapae, and they provide some insight into the ecology and possible management of this economically important crop pest.

A Multi-Layered Screening Method to Identify Plant Regulatory Genes

We used a seven-step process to identify genes involved in glucosinolate biosynthesis and metabolism in the Chinese cabbage (Brassica rapa). We constructed an annotated data set with 34,570 unigenes from B. rapa and predicted 11,526 glucosinolate-related candidate genes using expression profiles generated across nine stages of development on a 47k-gene microarray. Using our multi-layered screening method, we screened 392 transcription factors, 843 pathway genes, and 4,162 ortholog genes associated with glucosinolate-related biosynthesis. Finally, we identified five genes by comparison of the pathway-network genes including the transcription-factor genes and the ortholog-ontology genes. The five genes were anchored to the chromosomes of B. rapa to characterize their genetic-map positions, and phylogenetic reconstruction with homologous genes was performed. These anchored genes were verified by reverse-transcription polymerase chain reaction. While the five genes identified by our multi-layered screen require further characterization and validation, our study demonstrates the power of multi-layered screening after initial identification of genes on microarrays.

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

BACKGROUND

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

SCOPE

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

Defence on demand: mechanisms behind optimal defence patterns

BACKGROUND

The optimal defence hypothesis (ODH) predicts that tissues that contribute most to a plant's fitness and have the highest probability of being attacked will be the parts best defended against biotic threats, including herbivores. In general, young sink tissues and reproductive structures show stronger induced defence responses after attack from pathogens and herbivores and contain higher basal levels of specialized defensive metabolites than other plant parts. However, the underlying physiological mechanisms responsible for these developmentally regulated defence patterns remain unknown.

SCOPE

This review summarizes current knowledge about optimal defence patterns in above- and below-ground plant tissues, including information on basal and induced defence metabolite accumulation, defensive structures and their regulation by jasmonic acid (JA). Physiological regulations underlying developmental differences of tissues with contrasting defence patterns are highlighted, with a special focus on the role of classical plant growth hormones, including auxins, cytokinins, gibberellins and brassinosteroids, and their interactions with the JA pathway. By synthesizing recent findings about the dual roles of these growth hormones in plant development and defence responses, this review aims to provide a framework for new discoveries on the molecular basis of patterns predicted by the ODH.

CONCLUSIONS

Almost four decades after its formulation, we are just beginning to understand the underlying molecular mechanisms responsible for the patterns of defence allocation predicted by the ODH. A requirement for future advances will be to understand how developmental and defence processes are integrated.

Relative importance of genetic, ontogenetic, induction, and seasonal variation in producing a multivariate defense phenotype in a foundation tree species

Plant adaptations for defense against herbivory vary both among species and among genotypes. Moreover, numerous forms of within-plant variation in defense, including ontogeny, induction, and seasonal gradients, allow plants to avoid expending resources on defense when herbivores are absent. We used an 18-year-old cottonwood common garden composed of Populus fremontii, Populus angustifolia, and their naturally occurring F(1) hybrids (collectively referred to as "cross types") to quantify and compare the relative influences of three hierarchical levels of variation (between cross types, among genotypes, and within individual genotypes) on univariate and multivariate phytochemical defense traits. Within genotypes, we evaluated ontogeny, induction (following cottonwood leaf beetle herbivory), and seasonal variation. We compared the effect sizes of each of these sources of variation on the plant defense phenotype. Three major patterns emerged. First, we observed significant differences in concentrations of defense phytochemicals among cross types, and/or among genotypes within cross types. Second, we found significant genetic variation for within-plant differences in phytochemical defenses: (a) based on ontogeny, levels of constitutive phenolic glycosides were nearly three times greater in the mature zone than in the juvenile zone within one cottonwood cross type, but did not significantly differ within another cross type; (b) induced levels of condensed tannins increased up to 65 % following herbivore damage within one cottonwood cross type, but were not significantly altered in another cross type; and (c) concentrations of condensed tannins tended to increase across the season, but did not do so across all cross types. Third, our estimates of effect size demonstrate that the magnitude of within-plant variation in a phytochemical defense can rival the magnitude of differences in defense among genotypes and/or cross types. We conclude that, in cottonwood and likely other plant species, multiple forms of within-individual variation have the potential to substantially influence ecological and evolutionary processes.

Differential allocation of constitutive and induced chemical defenses in pine tree juveniles: a test of the optimal defense theory

Optimal defense theory (ODT) predicts that the within-plant quantitative allocation of defenses is not random, but driven by the potential relative contribution of particular plant tissues to overall fitness. These predictions have been poorly tested on long-lived woody plants. We explored the allocation of constitutive and methyl-jasmonate (MJ) inducible chemical defenses in six half-sib families of Pinus radiata juveniles. Specifically, we studied the quantitative allocation of resin and polyphenolics (the two major secondary chemicals in pine trees) to tissues with contrasting fitness value (stem phloem, stem xylem and needles) across three parts of the plants (basal, middle and apical upper part), using nitrogen concentration as a proxy of tissue value. Concentration of nitrogen in the phloem, xylem and needles was found to be greater higher up the plant. As predicted by the ODT, the same pattern was found for the concentration of non-volatile resin in the stem. However, in leaf tissues the concentrations of both resin and total phenolics were greater towards the base of the plant. Two weeks after MJ application, the concentrations of nitrogen in the phloem, resin in the stem and total phenolics in the needles increased by roughly 25% compared with the control plants, inducibility was similar across all plant parts, and families differed in the inducibility of resin compounds in the stem. In contrast, no significant changes were observed either for phenolics in the stems, or for resin in the needles after MJ application. Concentration of resin in the phloem was double that in the xylem and MJ-inducible, with inducibility being greater towards the base of the stem. In contrast, resin in the xylem was not MJ-inducible and increased in concentration higher up the plant. The pattern of inducibility by MJ-signaling in juvenile P. radiata is tissue, chemical-defense and plant-part specific, and is genetically variable.

Mechanisms of optimal defense patterns in Nicotiana attenuata: flowering attenuates herbivory-elicited ethylene and jasmonate signaling

To defend themselves against herbivore attack, plants produce secondary metabolites, which are variously inducible and constitutively deployed, presumably to optimize their fitness benefits in light of their fitness costs. Three phytohormones, jasmonates (JA) and their active forms, the JA-isoleucine (JA-Ile) and ethylene (ET), are known to play central roles in the elicitation of induced defenses, but little is known about how this mediation changes over ontogeny. The Optimal Defense Theory (ODT) predicts changes in the costs and benefits of the different types of defenses and has been usefully extrapolated to their modes of deployment. Here we studied whether the herbivore-induced accumulation of JA, JA-Ile and ET changed over ontogeny in Nicotiana attenuata, a native tobacco in which inducible defenses are particularly well studied. Herbivore-elicited ET production changed dramatically during six developmental stages, from rosette through flowering, decreasing with the elongation of the first corollas during flower development. This decrease was largely recovered within a day after flower removal by decapitation. A similar pattern was found for the herbivore-induced accumulation of JA and JA-Ile. These results are consistent with ODT predictions and suggest that the last steps in floral development control the inducibility of at least three plant hormones, optimizing defense-growth tradeoffs.

The genetics of phenotypic plasticity in plant defense: trichome production in Mimulus guttatus

Insect herbivory is a major driving force of plant evolution. Phenotypic plasticity and developmental variation provide a means for plants to cope with variable herbivory. We characterized the genetics of developmental variation and phenotypic plasticity in trichome density, a putative defensive trait of Mimulus guttatus (yellow monkeyflower). Our results are evaluated in relation to the optimal defense theory, which provides testable predictions for plastic and developmental patterns in defense traits. We found that both developmental stage and simulated insect damage affected trichome production, but in different ways. Plants were more likely to produce at least some trichomes on later leaves than on earlier leaves, regardless of damage. Damage did not affect the average probability of producing trichomes, but it did increase the density of hairs on trichome-positive plants. We mapped trichome quantitative trait loci (QTL) by selectively genotyping a large panel of recombinant inbred lines derived from two highly divergent populations. Several highly pleiotropic QTL influenced multiple aspects of the trichome phenotype (constitutive, developmental, and/or plastic responses). Only one of the QTL influenced trichome induction following damage. In a result that is consistent with a central prediction of optimal defense theory, the high allele at this location was from the ancestral population with low constitutive trichome production.

The ontogeny of plant defense and herbivory: characterizing general patterns using meta-analysis

Defense against herbivores often changes dramatically as plants develop. Hypotheses based on allocation theory and herbivore selection patterns predict that defense should increase or decrease, respectively, across ontogeny, and previous research partly supports both predictions. Thus, it remains unclear which pattern is more common and what factors contribute to variability among studies. We conducted a meta-analysis of 116 published studies reporting ontogenetic patterns in plant defense traits and herbivory. Patterns varied depending on plant life form (woody, herbaceous, grass), type of herbivore (insect, mollusk, mammal), and type of defense trait (secondary chemistry, physical defense, tolerance). In woody plants, chemical defense increased during the seedling stage, followed by an increase in physical defenses during the vegetative juvenile stage. Mammalian herbivores showed a strong preference for mature compared to juvenile tissues in woody plants. Herbs experienced a significant increase in secondary chemistry across the entire ontogenetic trajectory, although the magnitude of increase was greatest during the seedling stage. Correspondingly, mollusks preferred young compared to older herbs. Future research investigating growth/defense trade-offs, allometry, herbivore selection patterns, and ecological costs would shed light on the mechanisms driving the ontogenetic patterns observed.

Within-plant distribution of phenolic glycosides and extrafloral nectaries in trembling aspen (Populus tremuloides; Salicaceae)

Expression of foliar secondary compounds and extrafloral nectaries (EFNs) within the same leaves may be incompatible if secondary compounds repel beneficial insects that might otherwise be attracted to EFNs. This study examined the within-plant distributions of phenolic glycosides and EFNs in trembling aspen, Populus tremuloides, and their relationships to herbivore damage. Populus tremuloides expresses extrafloral nectaries (EFNs) on a subset of its leaves. We studied short and tall naturally occurring aspen ramets across multiple sites in interior Alaska. Contrary to our expectations, foliar phenolic glycoside concentrations were approximately 30% greater on leaves bearing EFNs than on leaves without EFNs. The mean concentration of foliar phenolic glycosides in short ramets was nine times that in tall ramets. Phenolic glycoside concentration was negatively related to leaf mining damage by Phyllocnistis populiella (Lepidoptera; Gracilliadae) at concentrations greater than 27 mg/g, whereas the presence of EFNs was unrelated to mining damage. The positive association of chemical defensive compounds and EFNs in leaves suggests that, for species with variation in EFN expression, negative correlations between herbivory and EFN expression may arise indirectly from associated effects of other, correlated types of defense.

Nicotiana tabacum TTG1 contributes to ParA1-induced signalling and cell death in leaf trichomes

Leaf trichomes serve as a physical barrier and can also secrete antimicrobial compounds to protect plants from attacks by insects and pathogens. Besides the use of the physical and chemical mechanisms, leaf trichomes might also support plant responses by communicating the extrinsic cues to plant intrinsic signalling pathways. Here we report a role of leaf trichomes in tobacco (Nicotiana tabacum) hypersensitive cell death (HCD) induced by ParA1, an elicitin protein from a plant-pathogenic oomycete. After localized treatment with ParA1, reactive oxygen species were produced first in the leaf trichomes and then in mesophylls. Reactive oxygen species are a group of intracellular signals that are crucial for HCD to develop and for cells to undergo cell death subsequent to chromatin condensation, a hallmark of HCD. These events were impaired when the production of hydrogen peroxide (H(2)O(2)) was inhibited by catalase or a NADPH-oxidase inhibitor applied to trichomes, suggesting the importance of H(2)O(2) in the pathway of HCD signal transduction from the trichomes to mesophylls. This pathway was no longer activated when leaf trichomes were treated with C51S, a ParA1 mutant protein defective in its interaction with N. tabacum TTG1 (NtTTG1), which is a trichome protein that binds ParA1, rather than C51S, in vitro and in trichome cells. The ParA1-NtTTG1 interaction and the HCD pathway were also abrogated when NtTTG1 was silenced in the trichomes. These observations suggest that NtTTG1 plays an essential role in HCD signal transduction from leaf trichomes to mesophylls.