Phenology of glucosinolate concentrations in roots,stems and leaves of Cardamine cordifolia

Total concentrations of isothiocyanate-yielding glucosinolates (IYG) were measured in roots, stems, basal leaves and cauline leaves of the herbaceous perennial Cardamine cordifolia (bittercress, Cruciferae), sampled at three sites in the Colorado Rockies during 1981. Significant variation in quantity was partitioned among plant parts, among sampling dates throughout the growing season, and among the three sites. Roots and basal leaves maintained high and similar concentrations of IYG through the season, while cauline leaves and stems showed seasonal declines, associated partly with flowering. Roots also consistently produced oxazolidinethione-yielding glucosinolates (hydroxylated analogues of IYG), whereas above-ground parts were variable for the presence of these compounds. Seasonal and plant-part variability in glucosinolate content and spatial patchiness of glucosinolate phenotypes contribute to the variation in herbivore occurrence and damage documented in previous studies of this native crucifer.     

Seasonal flux of isothiocyanate-yielding glucosinolates in roots, stems and leaves of Cardamine cordifolia

Concentrations of constituent isothiocyanate-yielding glucosinolates (IYG) declined seasonally in cauline leaves and stems of Cardamine cordifolia, sampled in montane Colorado. All five constituents (isopropyl, 2-butyl, isobutyl, benzyl and 2-phenyl-ethyl glucosinolates) were correlated positively with total concentration over the growing season. In contrast, no seasonal decline was discerned for total and constitutent IYG concentrations in roots and basal leaves, and constituents showed a mixed pattern of positive and negative correlations with total amount. Vegetative organs thus constitute two compartments relative to glucosinolate metabolism. In above-ground stems and associated cauline leaves, IYG concentrations decline individually and in concert over the growing season. In below-ground rhizomes and associated roots and basal leaves, individual compounds fluctuate in concentration, but in the aggregate do not decline seasonally. Metabolic fluxes of individual glucosinolates within and between vegetative organs, as well as changes in total concentrations, may influence feeding by herbivores.     

Herbivory by leaf miners in response to experimental shading of a native crucifer

Summary We tested the hypothesis that light intensity was the direct, proximal mechanism causing significantly higher vulnerability of Bittercress (Cardamine cordifolia A. Gray) clones in the sun to herbivory by a leaf-mining fly (Scaptomyza nigrita Wheeler). Clones in the sun were experimentally shaded. Plant performance and losses to leaf miners were compared to controls in the sun and natural willow shade. Leaf-mining damage was significantly higher on artificially-shaded plants (P<0.01), opposite of our expectation. Shading sun plants shifted their growth pattern toward that of naturally-shaded plants. No significant differences were detected in leaf water status or glucosinolate concentrations, eliminating water stress and variation in defensive posture for mediating the between habitat differences in levels of herbivory. Although soluble sugars varied significantly, they were higher in sun than either shade treatment. Total and free amino nitrogen concentrations were highest in the artificially-shaded plants and lowest in naturally-occurring sun plants. Adult flies were more abundant on sun and on artificially-shaded plants than on naturally-shaded plants. Thus, relative abundance of ovipositing flies in the sun-exposed area, combined with the higher nitrogen availability in artificially-shaded plants, form the most plausible hypothesis for factors mediating the experimentally documented pattern of herbivory.     

Explaining differential herbivory in sun and shade: the case of Aristotelia chilensis saplings

Differential herbivory in contrasting environments is commonly explained by differences in plant traits. When several plant traits are considered, separate correlation analyses between herbivory and candidate traits are typically conducted. This makes it difficult to discern which trait best explain the herbivory patterns, or to avoid spurious inferences due to correlated characters. Aristotelia chilensis saplings sustain greater herbivory in shaded environments than in open habitats. We measured alkaloids, phenolics, trichomes, leaf thickness and water content in the same plants sampled for herbivory. We conducted a multiple regression analysis to estimate the relationship between herbivory and each plant trait accounting for the effect of correlated traits, thus identifying which trait(s) better explain(s) the differential herbivory on A. chilensis. We also estimated insect abundance in both light environments. Palatability bioassays tested whether leaf consumption by the main herbivore on A. chilensis was consistent with field herbivory patterns. Overall insect abundance was similar in open and shaded environments. While saplings in open environments had thicker leaves, lower leaf water content, and higher concentration of alkaloids and phenolics, no difference in trichome density was detected. The multiple regression analysis showed that leaf thickness was the only trait significantly associated with herbivory. Thicker leaves received less damage by herbivores. Sawfly larvae consumed more leaf tissue when fed on shade leaves. This result is consistent with field herbivory and, together with results of insect abundance, renders unlikely that the differential herbivory in A. chilensis was due to greater herbivory pressure in open habitats.     

Interaction of glucosinolate content of Arabidopsis thaliana mutant lines and feeding and oviposition by generalist and specialist lepidopterans

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is an insect specialized on glucosinolate-containing Brassicaceae that uses glucosinolates in host-plant recognition. We used wild-type and mutants of Arabidopsis thaliana (L.) Heynh. (Brassicaceae) to investigate the interaction between plant glucosinolate and myrosinase content and herbivory by larvae of the generalist Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) and the specialist P. xylostella. We also measured glucosinolate changes as a result of herbivory by these larvae to investigate whether herbivory and glucosinolate induction had an effect on oviposition preference by P. xylostella. Feeding by H. armigera and P. xylostella larvae was 2.1 and 2.5 times less, respectively, on apk1 apk2 plants (with almost no aliphatic glucosinolates) than on wild-type plants. However, there were no differences in feeding by H. armigera and P. xylostella larvae on wild-type, gsm1 (different concentrations of aliphatic glucosinolates compared to wild-type plants), and tgg1 tgg2 plants (lacking major myrosinases). Glucosinolate induction (up to twofold) as a result of herbivory occurred in some cases, depending on both the plant line and the herbivore. For H. armigera, induction, when observed, was noted mostly for indolic glucosinolates, while for P. xylostella, induction was observed in both aliphatic and indolic glucosinolates, but not in all plant lines. For H. armigera, glucosinolate induction, when observed, resulted in an increase of glucosinolate content, while for P. xylostella, induction resulted in both a decrease and an increase in glucosinolate content. Two-choice tests with wild-type and mutant plants were conducted with larvae and ovipositing moths. There were no significant differences in preference of larvae and ovipositing moths between wild-type and gsm1 mutants and between wild-type and tgg1 tgg2 mutants. However, both larvae and ovipositing moths preferred wild-type over apk1 apk2 mutants. Two-choice oviposition tests were also conducted with P. xylostella moths comparing undamaged plants to plants being attacked by larvae of either P. xylostella or H. armigera. Oviposition preference by P. xylostella was unaffected as a result of larval plant damage, even in the cases where herbivory resulted in glucosinolate induction.     

Intraspecific variation in defense against a generalist lepidopteran herbivore in populations of Eruca sativa (Mill.)

Populations of Eruca sativa (Brassicaceae) from desert and Mediterranean (Med) habitats in Israel differ in their defense against larvae of the generalist Spodoptera littoralis but not the specialist Pieris brassicae. Larvae of the generalist insect feeding on plants of the Med population gained significantly less weight than those feeding on the desert plants, and exogenous application of methyl jasmonate (MJ) on leaves of the Med plants significantly reduced the level of damage created by the generalist larvae. However, MJ treatment significantly induced resistance in plants of the desert population, whereas the generalist larvae caused similar damage to MJ‐induced and noninduced plants. Analyses of glucosinolates and expression of genes in their synthesis pathway indicated that defense in plants of the Med population against the generalist insect is governed by the accumulation of glucosinolates. In plants of the desert population, trypsin proteinase inhibitor activity was highly induced in response to herbivory by S. littoralis. Analysis of genes in the defense‐regulating signaling pathways suggested that in response to herbivory, differences between populations in the induced levels of jasmonic acid, ethylene, and salicylic acid mediate the differential defenses against the insect. In addition, expression analysis of myrosinase‐associated protein NSP2 suggested that in plants of the desert population, glucosinolates breakdown products were primarily directed to nitrile production. We suggest that proteinase inhibitors provide an effective defense in the desert plants, in which glucosinolate production is directed to the less toxic nitriles. The ecological role of nitrile production in preventing infestation by specialists is discussed.     

Genetic control of natural variation in Arabidopsis glucosinolate accumulation

Glucosinolates are biologically active secondary metabolites of the Brassicaceae and related plant families that influence plant/insect interactions. Specific glucosinolates can act as feeding deterrents or stimulants, depending upon the insect species. Hence, natural selection might favor the presence of diverse glucosinolate profiles within a given species. We determined quantitative and qualitative variation in glucosinolates in the leaves and seeds of 39 Arabidopsis ecotypes. We identified 34 different glucosinolates, of which the majority are chain-elongated compounds derived from methionine. Polymorphism at only five loci was sufficient to generate 14 qualitatitvely different leaf glucosinolate profiles. Thus, there appears to be a modular genetic system regulating glucosinolate profiles in Arabidopsis. This system allows the rapid generation of new glucosinolate combinations in response to changing herbivory or other selective pressures. In addition to the qualitative variation in glucosinolate profiles, we found a nearly 20-fold difference in the quantity of total aliphatic glucosinolates and were able to identify a single locus that controls nearly three-quarters of this variation.     

Genotype–environment interactions affect flower and fruit herbivory and plant chemistry of Arabidopsis thaliana in a transplant experiment

Large differences exist in flower and fruit herbivory between dune and inland populations of plants of Arabidopsis thaliana (Brassicaceae). Two specialist weevils Ceutorhynchus atomus and C. contractus (Curculionidae) and their larvae are responsible for this pattern in herbivory. We test, by means of a reciprocal transplant experiment, whether these differences reflect environmental influences or genetic variation in plant defense level. All plants suffered more damage after being transplanted to the dune site than after being transplanted to the inland site. Plants of inland origin suffered more flower and fruit herbivory than plants of dune origin when grown at the dune transplant site, but differences were much smaller at the inland site. Both flower damage by adult weevils and fruit damage by their larvae were subject to significant genotype × environment interactions. The observed pattern in herbivory is a strong indication for local adaption of plant defense to the level of herbivory by Ceutorhynchus. In order to identify the mechanism of defense, a quantitative analysis of glucosinolates was performed on the seeds with HPLC. Highly significant differences were found in glucosinolate types and total concentration. These patterns were mainly determined by the origin of the plants (dune or inland) and by a genotype × environment interaction. Herbivory was not significantly correlated to the concentration of glucosinolates in seeds. We therefore analyzed the total metabolic composition of seeds, using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in the water–methanol fractions: more glucosinolate and sucrose in the dune and more fatty acids, lipids and sinapoylmalate in the inland populations. We discuss which of these chemical factors could explain the marked differences in damage between populations.     

Comparative analysis of quantitative trait loci controlling glucosinolates,myrosinase and insect resistance in Arabidopsis thaliana

Evolutionary interactions among insect herbivores and plant chemical defenses have generated systems where plant compounds have opposing fitness consequences for host plants, depending on attack by various insect herbivores. This interplay complicates understanding of fitness costs and benefits of plant chemical defenses. We are studying the role of the glucosinolate-myrosinase chemical defense system in protecting Arabidopsis thaliana from specialist and generalist insect herbivory. We used two Arabidopsis recombinant inbred populations in which we had previously mapped QTL controlling variation in the glucosinolate-myrosinase system. In this study we mapped QTL controlling resistance to specialist (Plutella xylostella) and generalist (Trichoplusia ni) herbivores. We identified a number of QTL that are specific to one herbivore or the other, as well as a single QTL that controls resistance to both insects. Comparison of QTL for herbivory, glucosinolates, and myrosinase showed that T. ni herbivory is strongly deterred by higher glucosinolate levels, faster breakdown rates, and specific chemical structures. In contrast, P. xylostella herbivory is uncorrelated with variation in the glucosinolate-myrosinase system. This agrees with evolutionary theory stating that specialist insects may overcome host plant chemical defenses, whereas generalists will be sensitive to these same defenses.     

Bottom-up and top-down herbivore regulation mediated by glucosinolates in Brassica oleracea var. acephala

Quantitative differences in plant defence metabolites, such as glucosinolates, may directly affect herbivore preference and performance, and indirectly affect natural enemy pressure. By assessing insect abundance and leaf damage rate, we studied the responses of insect herbivores to six genotypes of Brassica oleracea var. acephala, selected from the same cultivar for having high or low foliar content of sinigrin, glucoiberin and glucobrassicin. We also investigated whether the natural parasitism rate was affected by glucosinolates. Finally, we assessed the relative importance of plant chemistry (bottom-up control) and natural enemy performance (top-down control) in shaping insect abundance, the ratio of generalist/specialist herbivores and levels of leaf damage. We found that high sinigrin content decreased the abundance of the generalist Mamestra brassicae (Lepidoptera, Noctuidae) and the specialist Plutella xylostella (Lepidoptera, Yponomeutidae), but increased the load of the specialist Eurydema ornatum (Hemiptera, Pentatomidae). Plants with high sinigrin content suffered less leaf injury. The specialist Brevicoryne brassicae (Hemiptera, Aphididae) increased in plants with low glucobrassicin content, whereas the specialists Pieris rapae (Lepidoptera, Pieridae), Aleyrodes brassicae (Hemiptera, Aleyrodidae) and Phyllotreta cruciferae (Coleoptera, Chrysomelidae) were not affected by the plant genotype. Parasitism rates of M. brassicae larvae and E. ornatum eggs were affected by plant genotype. The ratio of generalist/specialist herbivores was positively correlated with parasitism rate. Although both top-down and bottom-up forces were seen to be contributing, the key factor in shaping both herbivore performance and parasitism rate was the glucosinolate concentration, which highlights the impact of bottom-up forces on the trophic cascades in crop habitats.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

不同生境下空心莲子草响应模拟昆虫采食的生长和化学防御策略

外来植物往往可以入侵多种生境并受到多种昆虫的采食,而不同生境条件将可能会影响这些入侵植物对昆虫采食的防御策略。以入侵我国的克隆植物——空心莲子草为研究对象,分别选择生长在水生生境、水陆两栖生境和陆生生境中的无性个体(分株),通过50%去叶处理模拟昆虫采食,分析不同生境下空心莲子草对模拟昆虫采食处理的生长及化学防御响应的差异。模拟昆虫采食处理显著抑制了陆生生境、水陆两栖生境以及水生生境下空心莲子草的根、茎、叶和总生物量,但对3种生境下空心莲子草的生物量分配(根冠比、根生物量分配、茎生物量分配和叶生物量分配)均无显著影响。陆生生境下空心莲子草根、茎和总生物量显著高于水陆两栖生境和水生生境,根冠比显著低于水陆两栖生境和水生生境。模拟昆虫采食处理显著降低了空心莲子草的木质素含量,而对单宁和总酚含量影响不显著。生境对木质素含量无显著影响,但陆生生境下空心莲子草单宁含量显著高于水陆两栖生境和水生生境,且总酚含量显著高于水陆两栖生境,表明陆生生境中空心莲子草具有更强的防御能力。空心莲子草木质素含量与总生物量无显著相关性,但在模拟采食情况下,其总酚含量与总生物量呈显著负相关,而无论模拟昆虫采食处理存在与否,空心莲子草单宁含量与总生物量均呈显著正相关。因此,空心莲子草存在昆虫介导的生长和化学防御之间的权衡,在昆虫采食的情况下可通过减少生长来增加对化学防御物质的投入,但生境对空心莲子草这种生长-防御权衡的影响十分有限。     

Constitutive and herbivore-inducible glucosinolate concentrations in oilseed rape (Brassica napus) leaves are not affected by Bt Cry1Ac insertion but change under elevated atmospheric CO2 and O3

Glucosinolates are plant secondary compounds involved in direct chemical defence by cruciferous plants against herbivores. The glucosinolate profile can be affected by abiotic and biotic environmental stimuli. We studied changes in glucosinolate patterns in leaves of non-transgenic oilseed rape (Brassica napus ssp. oleifera) under elevated atmospheric CO₂ or ozone (O₃) concentrations and compared them with those from transgenic for herbivore-resistance (Bacillus thuringiensis Cry1Ac endotoxin), to assess herbivory dynamics. Both elevated CO₂ and O₃ levels decreased indolic glucosinolate concentrations in transgenic and non-transgenic lines, whereas O₃ specifically increased the concentration of an aromatic glucosinolate, 2-phenylethylglucosinolate. The herbivore-inducible indolic glucosinolate response was reduced in elevated O₃ whereas elevated CO₂ altered the induction dynamics of indolic and aliphatic glucosinolates. Herbivore-resistant Bt plants experienced minimal leaf damage after target herbivore Plutella xylostella feeding, but exhibited comparatively similar increase in glucosinolate concentrations after herbivory as non-transgenic plants, indicating that the endogenous glucosinolate defence was not severely compromised by transgenic modifications. The observed differences in constitutive and inducible glucosinolate concentrations of oilseed rape under elevated atmospheric CO₂ and O₃ might have implications for plant–herbivore interactions in Brassica crop-ecosystems in future climate scenarios.     

Characterization of the Arabidopsis TU8 Glucosinolate Mutation,an Allele of TERMINAL FLOWER2

Glucosinolates are a group of defense-related secondary metabolites found in Arabidopsis and other cruciferous plants. Levels of leaf glucosinolates are regulated during plant development and increase in response to mechanical damage or insect feeding. The Arabidopsis TU8 mutant has a developmentally altered leaf glucosinolate profile: aliphatic glucosinolate levels drop off more rapidly, consistent with the early senescence of the mutant, and the levels of two indole glucosinolates are uniformly low. In TU8 seeds, four long-chain aliphatic glucosinolates have significantly increased levels, whereas the indolyl-3-methyl glucosinolate level is significantly reduced relative to wild type. Genetic mapping and DNA sequencing identified the TU8 mutation as tfl2-6, a new allele of TERMINAL FLOWER2 (TFL2), the only Arabidopsis homolog of animal HETEROCHROMATIN PROTEIN1 (HP1). TU8 (tfl2-6) has other previously identified tfl2 phenotypes, including an early transition to flowering, altered meristem structure, and stunted leaves. Analysis of two additional alleles, tfl2-1 and tfl2-2, showed glucosinolate profiles similar to those of line TU8 (tfl2-6).     

Glucosinolates in Sesamoides canescens and S. pygmaea: Identification of 2-(α-l-arabinopyranosyloxy)-2-phenylethylglucosinolate

A new natural product, 2-(α-l-arabinopyranosyloxy)-2-phenylethylglucosinolate, has been isolated from Sesamoides canescens. This glucosinolate together with 2-hydroxy-2-phenylethylglucosinolate and 2-phenetliylglucosinolate in a 1:1:1 ratio constitutes about 90 % of the total glucosinolate pool in green parts of the plant. Phenethylglucosinolate constitutes about 70 % of the total glucosinolate pool in green parts of S. pygmaea together with minor amounts of the two other glucosinolates. In addition, both plants contain at least seven other glucosinolates. The structure of the new natural product has been confirmed by transformations into d-glucose, l-arabinose, N-(2-(α-l-arabinopyranosyloxy)-2-phenylethyl)thiourea, 3-(α-l-arabinopyranosyloxy)-3-phenylpropionitrile and 3-hydroxy-3-phenylpropionic acid, respectively. The significance of this investigation is briefly discussed in relation to the methods used in glucosinolate analysis, chemotaxonomy and possible catabolic transformation of glucosinolates into amines.     

Dynamics of Ni-based defence and organic defences in the Ni hyperaccumulator, Streptanthus polygaloides (Brassicaceae)

Plants use chemical defences to reduce damage from herbivores and the effectiveness of these defences can be altered by biotic and abiotic factors, such as herbivory and soil resource availability. Streptanthus polygaloides , a nickel (Ni) hyperaccumulator, possesses both Ni-based defences and organic defences (glucosinolates), but the extent to which these defences interact and respond to environmental conditions is unknown. S. polygaloides plants were grown on high-Ni and low-Ni soil and concentrations of Ni and glucosinolates were compared with those of the congeneric non-hyperaccumulator, S. insignus spp. insignus , grown under the same conditions. Ni contents were highest (4000 μg g⁻¹ dry tissue) in S. polygaloides plants grown on high-Ni soil. Glucosinolate content was significantly higher in S. insignus than in S. polygaloides suggesting that plants defended by Ni produce a lower concentration of organic defences. In a separate experiment, high-Ni S. polygaloides plants were exposed to simulated herbivory or live folivores to determine the inducibility of Ni-based and organic defences. Contents of Ni were not affected by either herbivory treatment, whereas glucosinolate concentrations were >30% higher in damaged plants. We concluded that the Ni-based defence of S. polygaloides is not induced by herbivory.