Effect of temperature on glandular trichome exudate composition and pest resistance in geraniums

One inbred geranium line (71-18-6) which exhibits host plant resistance to the twospotted spider mite and foxglove aphid, becomes susceptible to these pests under some temperature conditions, while another resistant line (71-17-7) maintains its resistance. Resistance to these pests is conferred by the presence of glandular trichomes and the exudate they produce. The tall glandular trichome exudate of resistant geraniums has been identified as chiefly C₂₂ and C₂₄ unsaturated anacardic acids. Plants of a resistant line, a susceptible line and the temperature sensitive resistant line were exposed to three different temperature regimes and were analyzed chemically and for resistance using a foxglove aphid bioassay. Results indicate that the loss of resistance in the temperature sensitie line is not due to reduced production of the anacardic acids which comprise the exudate. Similarly, analysis of the exudate present on the leaf exterior indicate the loss of resistance in line 18-6 with increasing temperature is not the result of reduced translocation of the exudate to the trichome tip. Evidence is presented to support the hypothesis that loss of resistance in line 18–6 with increasing temperature is the result of an increased proportion of the shorter chain (C₂₂) anacardic acid in the trichome exudate, which reduces the viscosity of the material and causes it to flow from the trichome tips as the temperature increases. Presentation of the material as an exudate droplet at the trichome tip is apparently required for resistance.     

Differences in distribution and performance of two sap-sucking herbivores on glandular and non-glandular Datura wrightii

1. Datura wrightii Regel (Solanaceae) is polymorphic with regard to trichome type. Some plants are densely covered with short, non-glandular trichomes, whereas other plants in the same populations possess glandular trichomes that excrete a sticky exudate. The hypothesis that glandular trichomes enhance resistance to all small insect herbivores is evaluated.
2. Field censuses in four southern Californian D. wrightii plant populations revealed that glandular plants are indeed resistant to whitefly spp. (Homoptera: Aleyrodidae). Whiteflies are almost exclusively found on non-glandular plants. In contrast, Tupiocoris notatus (Distant) (Heteroptera: Miridae), another sap-sucking herbivore of similar body size, is found predominantly on plants with glandular trichomes.
3. Laboratory experiments showed that whiteflies are unable to colonize glandular D. wrightii phenotypes. After the whitefly adults had landed on the leaves of these plants, they were trapped in the exudate and died.
4. Tupiocoris notatus adults, on the other hand, laid significantly more eggs on glandular plants. The presence of the exudate was shown to be the cue that determined their choice of glandular plants.
5. In no-choice experiments, T. notatus nymphs reared on glandular plants had significantly higher survival rates and had shorter developmental periods than those raised on non-glandular plants. This, combined with the higher oviposition rates, resulted in higher T. notatus population growth rates on glandular plants than on non-glandular plants.
6. Glandular trichomes are not therefore a universal protection against small herbivores. Differences in distribution over the two plant types within the natural herbivore guild on D. wrightii may, among other selection pressures, contribute to the maintenance of the observed trichome polymorphism.     

Isolation and characterization of the CYP71D16 trichome-specific promoter from Nicotiana tabacum L

Trichomes are specialized epidermal cells that produce secretions that are thought to provide a first line of defence against pests and pathogens. Many trichome-secreted compounds are used commercially as flavourings, medicines, etc. Described here is the cloning and characterization of the promoter of a tobacco trichome-specific P450 gene, CYP71D16. This promoter is shown to direct the specific expression of the reporter gene, beta-glucuronidase (GUS), in glandular trichomes of Nicotiana tabacum cv. T.I. 1068 at all developmental stages. With the full promoter, GUS activity was predominantly in the gland cell, with less in the stalk cell adjacent to the gland, and in lower stalk cells. GUS staining was also observed in the most distal trichome stalk cells of non-glandular trichomes found on variety T.I. 1112. Promoter deletion analysis revealed that the region from -223 to +111 bp is sufficient to direct trichome-specific expression, but not strong gland expression. Examination of the literature suggests that this is the first characterized trichome-specific-promoter shown to function at all stages of plant development. This promoter may provide efficient bioengineering to enhance pest and pathogen resistance, and for molecular farming based on the trichome gland system.     

Resource availability and the trichome defenses of tomato plants

We conducted two experiments to determine how resource availability influenced allocation by tomato (Lycopersicon esculentum) to trichomes, and how different patterns of trichome allocation by plants grown in different resource environments might then influence the behavior of tobacco hornworm (Manduca sexta) caterpillars. In the first experiment we used high and low levels of light and water, and then, using scanning electron microscopy, determined trichome densities on the leaves and stems. We sampled leaves and stems at several places throughout the plant to determine whether there were within-plant differences in allocation to trichomes. The results of the first experiment showed that resource availability influenced allocation to trichome growth. Patterns in high and low-light supported both the growth-differentiation balance hypothesis (GDBH) and the carbon-nutrient balance hypothesis (CNBH). However, the GDBH was not supported by differences among water treatments. Contrary, to predictions of the GDBH, plants with intermediate growth did not have the highest trichome densities, and plants with similar growth differed in trichome density. Possible biological and artifactual explanations are discussed. The first experiment also showed that there was within-plant variation in allocation to trichomes, and that plant resource availability may influence within-plant variation in allocation to trichomes. In the second experiment, we grew plants in high and low-light, and then monitored the behavior of tobacco hornworms on the stems of these plants in the laboratory. This experiment demonstrated that the light environment that tomato plants were grown in influenced the resting behavior of caterpillars. Furthermore, it demonstrated that both glandular and non-glandular trichomes impeded caterpillars from searching for food. Overall, this study indicated that plant resource availability can influence allocation to trichome defenses, and that these differences may affect insect herbivores.     

Chimeric tomato plants show that aphid resistance and triacylglucose production are epidermal autonomous characters.

Graft chimeras were generated using Lycopersicon pennellii and L. esculentum to determine the contribution of the three meristem layers (L1, L2, and L3) to trichome density, sugar ester production, and aphid resistance. Sugar esters, in the form of triacylglucoses, have been implicated in the aphid resistance of pennellii. One chimera possessed the epidermal layer (L1) of pennellii and the internal tissues (L2 and L3) of the aphid-susceptible esculentum. The second chimera had both the L1 and L2 of pennellii and the L3 of esculentum. Type IV trichome densities did not differ significantly among the chimeras and pennellii. Both chimeras accumulated sugar esters with similar sugar and fatty acid composition as pennellii. The concentration of epicuticular sugar ester on the chimeras was also comparable with that of pennellii. Leaf cage and feeding studies demonstrated that both chimeras are as resistant to aphids as is pennellii. The resistance could be reduced similarly on all three types of plants by removal of the type IV trichome exudate. These results indicate that the presence and density of the type IV trichomes and the amount and type of sugar esters produced are features determined by the genotype of the epidermis. These epidermal features are sufficient to account for the aphid resistance observed in pennellii.     

Suppression of a P450 hydroxylase gene in plant trichome glands enhances natural-product-based aphid resistance

Trichome glands on the surface of many higher plants produce and secrete exudates affecting insects, microbes, and herbivores. Metabolic engineering of gland exudation has potential for improving pest/disease resistance, and for facilitating molecular farming. We identified a cytochrome P450 hydroxylase gene specific to the trichome gland and used both antisense and sense co-suppression strategies to investigate its function. P450-suppressed transgenic tobacco plants showed a > or =41% decrease in the predominant exudate component, cembratriene-diol (CBT-diol), and a > or =19-fold increase in its precursor, cembratriene-ol (CBT-ol). Thus, the level of CBT-ol was raised from 0.2 to > or =4.3% of leaf dry weight. Exudate from antisense-expressing plants had higher aphidicidal activity, and transgenic plants with exudate containing high concentrations of CBT-ol showed greatly diminished aphid colonization responses. Our results demonstrate the feasibility of significantly modifying the natural-product chemical composition and aphid-interactive properties of gland exudates using metabolic engineering. The results also have implications for molecular farming.     

Exploiting physical defence traits for crop protection: leaf trichomes of Rubus idaeus have deterrent effects on spider mites but not aphids

Plants possess anti‐herbivore defences that could be exploited for crop protection. The potential for deploying physical defence traits for more sustainable pest management (i.e. reduced pesticide application) has not been fully realised. Using a perennial crop (red raspberry, Rubus idaeus), we take the novel approach of quantifying within‐ and between‐genotype variation in a resistance trait, leaf trichome density, to determine precisely the effect of trichomes on host plant preference and suitability for two shoot‐feeding arthropods, the European large raspberry aphid (Amphorophora idaei) and two‐spotted spider mite (Tetranychus urticae). Additionally, we tested whether this trait influenced searching behaviour of a generalist herbivore predator (lacewing larvae, Chrysoperla carnea). Although there was no consistent genotypic variation in R. idaeus suitability for T. urticae, our hypothesis that T. urticae would avoid high leaf trichome density was supported on certain genotypes. The deterrent effect was mainly on egg deposition rather than leaf selection by adults, with up to sixfold differences in leaf preference depending on the genotypes offered. By contrast, there was significant genotypic variation in R. idaeus suitability for A. idaei (10‐fold variation in aphid abundance), but, contrary to our prediction, aphid preference and infestation levels were unrelated to leaf trichome density. Instead, A. idaei performed best on vigorous genotypes, indicating that plant tolerance traits contributed to R. idaeus suitability for aphids. Leaf trichomes had little effect on the behaviour of the beneficial control agent C. carnea larvae. We conclude that physical anti‐herbivore defences, specifically leaf trichomes, could be deployed to deter particular arthropod pests. However, the mechanistic approach adopted here is necessary to avoid antagonistic effects on other pests or on natural enemies.     

Effects of trichomes on the behavior and distribution of Platyprepia virginalis caterpillars

Trichomes are an important physical resistance mechanism of plants, as they reduce insect herbivore movement, feeding, and digestion. However, we know little about how trichomes influence herbivore distributions and populations. We conducted laboratory and field experiments to evaluate the preferences of Platyprepia virginalis (Boisduval) (Lepidoptera: Arctiidae) caterpillars to natural and manipulated densities of trichomes on their primary food, Lupinus arboreus Sims (Fabaceae). We then conducted field surveys to determine whether variation in trichome density among lupine bushes affected caterpillar spatial distribution on the landscape. Platyprepia virginalis caterpillars preferred lupine leaves with fewer trichomes in choice and no‐choice experiments. In the field, caterpillar feeding damage was found more often on leaves with fewer trichomes. These preferences scaled up to the level of bushes in the landscape such that more caterpillars were found on bushes with lower trichome densities than on bushes with higher trichome densities. This is one of few studies to show the potential for trichome density to influence herbivore population size and distribution in a natural system at a landscape level. The results are consistent with trichomes functioning as a resistance mechanism with consequences for herbivore choice, performance, and distribution.     

Tobacco NtLTP1, a glandular-specific lipid transfer protein, is required for lipid secretion from glandular trichomes

Glandular trichomes are the phytochemical factories of plants, and they secrete a wide range of commercially important natural products such as lipids, terpenes and flavonoids. Herein, we report that the Nicotiana tabacum LTP1 (NtLTP1) gene, which is specifically expressed in long glandular trichomes, plays a role in lipid secretion from trichome heads. NtLTP1 mRNA is abundantly transcribed in trichomes, but NtLTP3, NtLTP4 and NtLTP5 are not. In situ hybridization revealed that NtLTP1 mRNAs accumulate specifically in long trichomes and not in short trichomes or epidermal cells. X-gluc staining of leaves from a transgenic plant expressing the NtLTP1 promoter fused to a GUS gene revealed that NtLTP1 protein accumulated preferentially on the tops of long glandular trichomes. GFP fluorescence from transgenic tobacco plants expressing an NtLTP1-GFP fusion protein was localized at the periphery of cells and in the excreted liquid droplets from the glandular trichome heads. In vitro assays using a fluorescent 2-p-toluidinonaphthalene-6-sulfonate probe indicated that recombinant NtLTP1 had lipid-binding activity. The overexpression of NtLTP1 in transgenic tobacco plants resulted in the increased secretion of trichome exudates, including epicuticular wax. In transgenic NtLTP1-RNAi lines, liquid secretion from trichomes was strongly reduced, but epicuticular wax secretion was not altered. Moreover, transgenic tobacco plants overexpressing NtLTP1 showed increased protection against aphids. Taken together, these data suggest that NtLTP1 is abundantly expressed in long glandular trichomes, and may play a role in lipid secretion from long glandular trichomes.     

Role of leaf glandular trichomes of melon plants in deterrence of Aphis gossypii Glover

External characteristics of the leaf epidermis and their effects on behaviour of Aphis gossypii Glover were evaluated in two Cucumis melo L. genotypes, ‘Bola de Oro’ (aphid susceptible) and TGR‐1551 (aphid resistant) in order to explore their role in the early rejection of TGR‐1551 by this aphid. No differential effects of epicuticular waxes on aphid behaviour were observed. The type, distribution and number of trichomes on melon leaves were also studied. Pubescence in melon, measured as the number of non‐glandular trichomes per cm², was not sufficient to prevent aphid settling. However, there was a high density of type I glandular trichomes on leaves of the aphid‐resistant genotype. According to microscopic observations and stain testing, these trichomes store and secrete phenols and flavonoids. Free‐choice tests were conducted to determine the effect of these glandular trichomes on A. gossypii preference, revealing that aphids reject leaf disks of TGR‐1551 from the onset of the experiment. Additional experiments after removal of leaf type I glandular trichome exudates showed that A. gossypii preferred washed TGR‐1551 leaf disks over unwashed disks, while this effect was not observed in experiments using washed and unwashed ‘Bola de Oro’ leaf disks. These results suggest that a high density of glandular trichomes and chemicals secreted by them deter A. gossypii and disturb aphid settling on TGR‐1551.     

Aphid repellent sesquiterpenes in glandular trichomes of Solanum berthaultii and S. tuberosum

Exudate from glandular trichomes of the wild potato species, Solanum berthaultii Hawkes, and the cultivated potato, S. tuberosum L., contained volatile substances including sesquiterpenes when examined with gas chromatography/mass spectrometry procedures. Sesquiterpenes were the major volatile constituents of Type A trichome exudate but were a minor constituent of Type B exudate. Sesquiterpene mixtures of the two potato species differed qualitatively and quantitatively. -Caryophyllene and E--Farnesene were major components identified in Type A trichome exudate of the cultivated potato. Sesquiterpene mixtures of both potato species were repellent to the green peach aphid, Myzus persicae (Sulzer). The role of trichome sesquiterpenes as a determinant of aphid behavior on the two potato species and in the expression of host resistance is discussed.

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Résumé L'examen en chromatographie gazeuse et en spectométrie de masse a révélé la présence de substances volatiles contenant des sesquiterpènes dans l'exsudat des trichômes glandulaires des pommes de terre sauvage (Solanum berthaultii) et cultivée (S. tuberosum). Les sesquiterpènes étaient le principal composé volatile de l'exsudat de type A, tandis qu'ils n'étaient que faiblement représentés dans l'exsudat de type B. Les mélanges de sesquiterpènes des deux espèces différaient qualitativement et quantitativement. Le -Caryophyllène et le E--Farnesène constituaient les principaux composés identifiés dans l'exsudat de trichôme de type A de S. tuberosum. Les mélanges sesquiterpéniques des deux espèces ont repousée Myzus persicae Sulzer. La discussion a porté sur l'influence des sesquiterpènes des trichômes dans la définition du comportement des pucerons sur ces deux plantes et sur la manifestation de la résistance de l'hôte.

     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use. 2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used. 3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated. 4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera. 5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies. 6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.     

Glandular trichomes of Solanum berthaultii and its hybrids with Solanum tuberosum affect nymphal emergence, development, and survival of Empoasca fabae (Homoptera: Cicadellidae)

The influence of foliar glandular trichomes on developmental biology of the potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), was examined by selective removal of trichomes on resistant and susceptible potato germplasm comprising two glandular trichome-bearing genotypes (PI 473331 and PI 473334) of wild potato, Solanum berthaultii (Hawkes), two interspecific hybrids of S. berthaultii with Solanum tuberosum L. (Q174-2 and NY123), and two commercial S. tuberosum cultivars ('Elba' and 'Allegany'). Adult mortality was affected, whereas nymphal emergence was unaffected by removal of glandular trichome exudates. No nymphs were observed emerging from leaves of PI 473331 or PI 473334. Regardless of the trichome removal treatment, no nymphs completed development on PI 473331, and only a small percentage of nymphs survived to adulthood on NY123 and PI 473334. PI 473331 had the greatest frequency of adults and nymphs with trichome exudate deposition on their bodies, followed by PI 473334. Nymphs, but not adults, had exudate accumulation when caged on foliage of Q174-2. Removal of type A trichome heads and type B droplets eliminated exudate deposition. The results of these studies provide evidence that phenolic oxidation chemistry and the physical barrier imposed by trichomes of S. berthaultii are responsible for much of the observed resistance but that other factors possibly linked to the presence of trichomes aid in the expression on resistance.     

Host plant resistance among tomato accessions to the spider mite <Emphasis Type="Italic">Tetranychus evansi</Emphasis> in Kenya

The spider mite Tetranychus evansi has a broad range of host plants. Control of T. evansi has been a big challenge to tomato farmers due to its fast rate of reproduction, development of resistance to chemical pesticides and its ability to use weeds as alternative hosts when the tomato plants are not available. The aim of the current study was to determine the host plant acceptance and the relative contributions of trichomes in the control of the red spider mite by comparing the survival, development and oviposition rates of the red spider mite on eight tomato accessions. Leaflets from eight tomato varieties were assayed with the spider mites to determine the egg laying capacity and developmental time of the spider mites on the tomato accessions as well as the trichome densities. Densities of trichome types I, IV, V and VI varied among the tomato accessions. Variation in types I, IV and VI accounted for most of the variation in mite responses. The varieties with high densities of types IV and VI had the highest fecundity and mite development did not go beyond the larval stage. The developmental time varied significantly among the tomato accessions. The results indicated that the higher the density of trichome type I the lower the adult survival. The findings indicated possible resistance of some of the tested tomato accessions against T. evansi which is partially associated with trichomes types and density.     

Inheritance of potato aphid resistance in hybrids between Lycopersicon esculentum and L. pennellii

Summary The potato aphid, Macrosiphum euphorbiae Thomas, is an important pest of tomato, Lycopersicon esculentum Mill., because it transmits tomato viruses and directly reduces crop yields by its feeding. This study was conducted to determine whether the wild tomato species, Lycopersicon pennellii (Corr.) D'Arcy, would be useful as a source of potato aphid resistance for tomato. Type IV trichome density and aphid resistance were assessed in six generations (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) from crosses between L. pennellii (LA 716) and two tomato cultivars, New Yorker and VF Vendor. Weighted leastsquares were used in joint scaling tests to estimate the relative importance of gene effects on type IV trichome density and potato aphid resistance of the hybrids. A simple additive-dominance model adequately explained the variation in type IV trichome density. Models which included digenic epistatic effects were required to explain the variation in aphid resistance. Standard unit heritability estimates of aphid resistance in the backcross to L. esculentum were obtained by regression of BC₁F₂ off-spring families on BC₁F₁ parents. Regression coefficients and heritability estimates varied between years with the level and uniformity of the aphid infestation. In the 1985–1986 growing seasons, when aphid infestations were uniform, aphid resistance exhibited a moderate level of heritability (29.8% ± 14.1% and 47.1% ± 11.5% in New Yorker and VF Vendor backcross populations, respectively). The non-uniform aphid infestation of 1984 resulted in lower heritability estimates in the 1984–1985 growing seasons (16.1% ± 15.7% and 21.9% ± 14.8% in the New Yorker and VF Vendor backcross populations, respectively). Selection for potato aphid resistance would probably be most efficient if it were delayed until gene combinations are fixed in later generations, because of the large epistatic effects and the low heritability of this trait in seasons with variable aphid infestations.     

Indirect cost of a defensive trait: variation in trichome type affects the natural enemies of herbivorous insects on <Emphasis Type="Italic">Datura wrightii</Emphasis>

The costs and benefits of defensive traits in plants can have an ecological component that arises from the effect of defenses on the natural enemies of herbivores. We tested if glandular trichomes in Datura wrightii, a trait that confers resistance to several species of herbivorous insects, impose an ecological cost by decreasing rates of predation by the natural enemies of herbivores. For two common herbivores of D. wrightii, Lema daturaphila and Tupiocoris notatus, several generalized species of natural enemies exhibited lower rates of predation on glandular compared to non-glandular plants. Lower rates of predation were associated with reductions in the residence time and foraging efficiency of natural enemies on plants with glandular trichomes, but not with direct toxic effects of glandular exudate. Our results suggest that the benefit of resistance to herbivores conferred by glandular trichomes might be offset by the detrimental effect of this trait on the natural enemies of herbivores, and that the fitness consequences of this trichome defense might depend on the composition and abundance of the natural-enemy community.     

The mechanism of trichome resistance in Anaphalis margaritacea to the meadow spittlebug Philaenus spumarius

This study demonstrated how simple, lanate trichomes on the stem of Anaphalis margaritacea (D.C.) inhibit feeding by young nymphs of Philaenus spumarius (L.). Nymphs were caged on stems which were subsequently sectioned and examined for feeding marks. The lack of stylet punctures at sites where nymphs were unable to feed indicated that trichomes prevented the start of stylet penetration rather than interfering with the progress of the stylets after penetration had begun. Trichome resistance declined progressively with spittlebug instar from complete resistance to first instar nymphs through complete absence of resistance to fifth instar nymphs. Discriminant analysis of the effect of number of trichomes per mm², vertical density, and height on trichome resistance to third instar nymphs revealed that trichome height was the critical factor in resistance. Variation in the percent of nymphs in each instar able to feed when caged on the stem was (1) positively correlated with degree to which the average rostrum length of the instar exceeded the trichome height, (2) independent of trichome density, which was uniformly high on all stems. Trichomes prevented stylet penetration where trichome height exceeded the length of the nymph's rostrum, probably because the necessary stimuli were not received when the sensilla on the tip of the rostrum could not touch the surface of the stem. A similar mechanism may explain resistance of simple trichomes to crop pests in the Cicadellidae.

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Résumé Ce travail montre comment des trichomes simples et laineux sur les tiges d'Anaphalis margaritacea (D.C.) inhibent la prise d'aliments par les jeunes larves de Philaenus spumarius L. Les larves ont été encagées sur des tiges qui ont été ultérieurement coupées et les piqûres alimentaires dénombrées. L'absence de piqûres par les stylets aux lieux où les larves ont été incapables de s'alimenter a montré que les trichomes ont empêché le début de la pénétration des stylets plus qu'ils n'ont entravé leur progression après le début de la pénétration. La résistance des trichomes diminue progressivement: totale pour le premier stade larvaire, elle est devenue nulle au cinquième stade. Une analyse discriminante des effets de la densité des trichome par mm², de la densité verticale, de leur hauteur sur la résistance des trichomes aux larves de 3ème stade, a révélé que la hauteur du trichome est le facteur de résistance. La variation dans le pourcentage de larves de chaque stade encagées sur la tige, capables de consommer, présentait une relation positive avec le degré par lequel la longueur du rostre dépasse la hauteur du trichome, quelle que soit la densité des trichomes qui est toujours élevée sur toutes les tiges. Les trichomes ont empêché la pénétration des stylets quand leur hauteur a dépassé la longueur du rostre, probablement parce que les stimuli nécessaires n'ont pas été reçus quant l'extrémité du rostre ne pouvait pas atteindre la surface de la tige. Un mécanisme semblable peut expliquer la résistance de trichomes simples aux Cicadellidae.

     

Costs of glandular trichomes in Datura wrightii: a three-year study

Models accounting for genetic variation for resistance to herbivores within plant populations often postulate a balance between the costs of that resistance and its benefits. The production of glandular trichomes by Datura wrightii was shown to be costly in a previous one-year study because plants producing glandular trichomes (sticky plants), a factor conferring resistance to some insect herbivores, also produced 45% fewer seeds than plants producing nonglandular trichomes (velvety plants) when grown in a common garden. Because sticky plants tended to be larger than velvety plants but produced fewer seed capsules, we postulated an allocation trade-off in which velvety plants are more reproduction-dominated whereas sticky plants are more growth-dominated. If a greater commitment to vegetative growth eventually allows sticky plants to compensate for reduced seed production, we would expect a reduction or elimination of the cost of resistance over time in this perennial plant. We monitored growth, survival, and seed production of plants from defined crosses of local populations for three years in a common garden when exposed to and protected from herbivores, and with and without supplemental water. The majority of plants exposed to herbivores had died by the end of the study. We used standard life-table methods to determine the net reproductive rate (R0) and the finite rate of increase (lambda) of plants of each trichome type. After three years, when plants were protected from herbivores, sticky plants were 187-245% larger than velvety plants, depending upon irrigation treatment, but sticky plants continued to be less efficient in producing seeds per unit of canopy volume. Even though the total seed production of sticky plants eventually equaled that of velvety plants, the advantage of earlier reproduction by velvety plants increased lambda by 55-230% over that of sticky plants, depending upon herbivore and irrigation treatment. Exposure to herbivores reduced lambda by 69-83%, depending upon plant type and irrigation treatment, whereas supplemental irrigation increased lambda by 29-175%, depending upon plant type and exposure to herbivores. Although there was a large allocation trade-off between growth and reproduction, the benefits of such a trade-off did not emerge before most plants were killed by herbivores. The cost of producing glandular trichomes strictly for herbivore resistance continued to exceed its benefits, and in the absence of other, unmeasured benefits from the suite of life-history characters associated with glandular trichome production, natural selection is expected to eliminate this costly resistance trait from D. wrightii populations.