Herbivore-Mediated Effects of Glucosinolates on Different Natural Enemies of a Specialist Aphid

The cabbage aphid Brevicoryne brassicae is a specialist herbivore that sequesters glucosinolates from its host plant as a defense against its predators. It is unknown to what extent parasitoids are affected by this sequestration. We investigated herbivore-mediated effects of glucosinolates on the parasitoid wasp Diaeretiella rapae and the predator Episyrphus balteatus. We reared B. brassicae on three ecotypes of Arabidopsis thaliana that differ in glucosinolate content and on one genetically transformed line with modified concentrations of aliphatic glucosinolates. We tested aphid performance and the performance and behavior of both natural enemies. We correlated this with phloem and aphid glucosinolate concentrations and emission of volatiles. Brevicoryne brassicae performance correlated positively with concentrations of both aliphatic and indole glucosinolates in the phloem. Aphids selectively sequestered glucosinolates. Glucosinolate concentration in B. brassicae correlated negatively with performance of the predator, but positively with performance of the parasitoid, possibly because the aphids with the highest glucosinolate concentrations had a higher body weight. Both natural enemies showed a positive performance-preference correlation. The predator preferred the ecotype with the lowest emission of volatile glucosinolate breakdown products in each test combination, whereas the parasitoid wasp preferred the A. thaliana ecotype with the highest emission of these volatiles. The study shows that there are differential herbivore-mediated effects of glucosinolates on a predator and a parasitoid of a specialist aphid that selectively sequesters glucosinolates from its host plant

Characterization of the natural variation in Arabidopsis thaliana metabolome by the analysis of metabolic distance

Metabolite fingerprinting is widely used to unravel the chemical characteristics of biological samples. Multivariate data analysis and other statistical tools are subsequently used to analyze and visualize the plasticity of the metabolome and/or the relationship between those samples. However, there are limitations to these approaches for example because of the multi-dimensionality of the data that makes interpretation of the data obtained from untargeted analysis almost impossible for an average human being. These limitations make the biological information that is of prime importance in untargeted studies be partially exploited. Even in the case of full exploitation, current methods for relationship elucidation focus mainly on between groups variation and differences. Therefore, a measure that is capable of exploiting both between- and within-group biological variation would be of great value. Here, we examined the natural variation in the metabolome of nine Arabidopsis thaliana accessions grown under various environmental conditions and established a measure for the metabolic distance between accessions and across environments. This data analysis approach shows that there is just a minor correlation between genetic and metabolic diversity of the nine accessions. On the other hand, it delivers so far in Arabidopsis unexplored chemical information and is shown to be biologically relevant for resistance studies

Root Herbivore Effects on Aboveground Multitrophic Interactions: Patterns, Processes and Mechanisms

In terrestrial food webs, the study of multitrophic interactions traditionally has focused on organisms that share a common domain, mainly above ground. In the last two decades, it has become clear that to further understand multitrophic interactions, the barrier between the belowground and aboveground domains has to be crossed. Belowground organisms that are intimately associated with the roots of terrestrial plants can influence the levels of primary and secondary chemistry and biomass of aboveground plant parts. These changes, in turn, influence the growth, development, and survival of aboveground insect herbivores. The discovery that soil organisms, which are usually out of sight and out of mind, can affect plant-herbivore interactions aboveground raised the question if and how higher trophic level organisms, such as carnivores, could be influenced. At present, the study of above-belowground interactions is evolving from interactions between organisms directly associated with the plant roots and shoots (e.g., root feeders - plant - foliar herbivores) to interactions involving members of higher trophic levels (e.g., parasitoids), as well as non-herbivorous organisms (e.g., decomposers, symbiotic plant mutualists, and pollinators). This multitrophic approach linking above- and belowground food webs aims at addressing interactions between plants, herbivores, and carnivores in a more realistic community setting. The ultimate goal is to understand the ecology and evolution of species in communities and, ultimately how community interactions contribute to the functioning of terrestrial ecosystems. Here, we summarize studies on the effects of root feeders on aboveground insect herbivores and parasitoids and discuss if there are common trends. We discuss the mechanisms that have been reported to mediate these effects, from changes in concentrations of plant nutritional quality and secondary chemistry to defense signaling. Finally, we discuss how the traditional framework of fixed paired combinations of root- and shoot-related organisms feeding on a common plant can be transformed into a more dynamic and realistic framework that incorporates community variation in species, densities, space and time, in order to gain further insight in this exciting and rapidly developing field

Consequences of intra-specific metabolic diversity in plants for soil organisms: A baseline approach for evaluating ecological effects of genetic modifications

Plant intra-specific variation, i.e. variation within a plant species, is known to affect organisms that are directly associated to plants. These effects may be due to for example differences in nutritional quality or defensive metabolites. Plant intra-specific variation can also affect higher trophic level and organisms not directly associated to the plants, i.e. non-target organisms. These effects occur via differences in the quality of herbivores serving as host or prey, due to differences in the rates of attractiveness for higher trophic level organisms, differences in decomposition rates of litter, or differences in root exudates. Intra-specific variation occurs naturally in wild plant populations and humans have used this to select plants for agricultural use. Breeding and artificial selection for plant traits that were desirable for agricultural practices resulted in novel varieties, adding to intra-specific variation in these species.<br /> In addition to natural variation and variation between cultivars, plant genetic modification can result in crop varieties with novel traits such as increased productivity or insect resistance. These traits may also affect non-target organisms. The question is whether these effects fall within or outside the range of non-target effects observed for conventionally bred varieties. To answer this question, one first needs to determine the range of non-target effects caused by conventional varieties, a so-called &lsquo;baseline&rsquo;.In this thesis, I examined the baseline effects that conventional white cabbage varieties have on soil organisms, the mechanisms behind these effects, and the consequences for interactions between below-and aboveground organisms. The range of effects observed in the conventionally bred varieties can then serve as a baseline for evaluating the effects of genetic modifications.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br /> I started by examining intra-specific variation in glucosinolate concentrations and profiles in white cabbage cultivars. Glucosinolates, a group of circa 120 secondary plant compounds predominantly present in the Brassicaceae, have been shown to affect not only aboveground organisms but also belowground organisms. Glucosinolate profiles in both roots and shoots of white cabbage cultivars showed significant intra-specific variation. The root glucosinolate profiles were more diverse than in the shoots. The variation in root glucosinolate profiles between four of the cultivars was used to evaluate the effects of glucosinolates on a range of soil organisms from different trophic levels, which differ in their degree of association with plant roots. In the field I recorded that plant-parasitic nematodes were affected by the differences in the root glucosinolate profiles, whereas non-target organisms were not. The latter observation might be explained by the reduced intra-specific variation in the glucosinolate profiles of the root exudates compared to those of the roots. Even though total glucosinolate concentrations in roots and root exudates correlated positively, the number of individual glucosinolates that were recorded in the root exudates did not match those found in the roots. My experiments show that this may be due to different degradation rates of the individual glucosinolates in the soil.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br /> By adding different soil communities to sterilized soils I examined whether belowground organisms can affect aboveground organisms via their shared host plant. For this, I used two cabbage cultivars that were highly divergent both in their effects on soil organisms and glucosinolate profiles. Microorganisms added to the soils promoted aphid population growth. The addition of nematodes tended to decrease aphid population growth. However, the effect of the soil organisms on aboveground organisms was similar for both cultivars, indicating that the outcome of below-aboveground interactions was not affected by intra-specific differences.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br /> Genetic modification of plants could also indirectly affect plant growth, for example if the modification would affect the soil communities associated with the plants, which could subsequently affect aerial plant parts and aboveground processes. I explored this possibility by reviewing the recent literature on genetic modified plants, focusing on two case studies; rice plants modified to tolerate drought and salt stress and plants transformed to enhance their capacity to accumulate pollutants. Indeed, feedback loops between plants and the rhizosphere can result in both positive and negative feedback effects of the modified gene on aboveground plant properties. This may have unexpected consequences for the net effect of the genetic modification and eventually annihilate the positive effect of the modification on desired plant properties such as yield.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br /> In this thesis I also showed that when evaluating genetically modified plants in greenhouse studies, effects on soil organisms are limited when compared to the field. I propose that this is due to the fact that greenhouse studies use relatively simple soils that lack the complex plant-soil interactions that can be present in the field. This is an indication that greenhouse studies have a limited predictive value for field effects on soil organisms. Greenhouse studies are nevertheless useful for selecting a suitable and manageable set of varieties that are representative for the range of variation present in the full set of available cultivars. The selection of this sub-set can be made using the appropriate statistical tools, such as multivariate statistics. The selected sub-set can then be used as a baseline for more extensive studies assessing effects on (non-target) organisms in the field. I found that for root glucosinolate profiles this was a useful approach. Whether this holds true for other traits is yet to be assessed.<br /> Before one can evaluate whether effects of genetically modified plants fall within or outside the effects of conventional plants, a good knowledge of the range of effects that can be observed for conventional varieties is required. In this thesis, I have provided the basis for this knowledge for white cabbage, especially for belowground interactions and to some extent on belowground-aboveground interactions. Conventional varieties already differ in their effects on soil organisms and these effects can potentially result in altered below-aboveground interactions, as was simulated by the addition of specific organism groups to the soil. This range of effects can serve as a baseline to determine whether effects of genetically modified white cabbage plants fall inside or outside the range of effects that can be observed for these conventional varieties

Why does Thlaspi goesingense Hálácsy (Brassicaceae) Accumulate Metals?

Despite their great potential, a multitude of questions remain to be addressed before hyperaccumulators can be used to clean up contaminated sites. we used a scanning electron microscope and light microscopy to address the question how Tlaspi goesingense accumulates metals. Hyperaccumulation of nicekl by T. goesingense was confirmed: the recorded concentration was ten times beyond the treshold that defines a nickel hyperaccumulator. The pathogen/herbivore defence hypothesis can indirectly be confrimed because cuticular striations increased when the nickel concentration decreased. The large, elongated epidermis cells in T. goesingense indicate that metals are sequestered and immobilized because these cells correlated with elevated nickel concentrations. These cells are less frequently encountered in T. arvense, a non-hyperaccumulator. Exceptions were recorded in the leaves of the inflorescence axis: the nickel concentration here was relatively high but only a few elongated cells were present. The high amount of nickel and zinc in the plant confirms the metal tolerance of T. goesingense. The disposal-from-the-Plant-Body Theory can also be confirmed because the leaves of the leaves inflorescence axis, which are lost after flowering, accumulated high amounts of nickel. Detoxification is by disposal of nonessential plant organs. Other mechanisms of disposal are unlikely because no trichomes or other adaptations were recorde

Activated carbon addition affects substrate ph and germination of six plant species

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Activated carbon addition affects soil pH and germination of six plant species

Activated carbon (AC) is widely used in ecological studies for neutralizing allelopathic compounds. However, it has been suggested that AC has direct effects on plants because it alters substrate parameters such as nutrient availability and pH. These side-effects of AC addition may interfere with allelopathic effects. In this study we analyzed three widely used commercial AC brands and analyzed their effect on pH, their ability to retain glucosinolates, and their effect on the germination of six plant species. AC brands differed significantly in their effect on pH values when added to different substrates. Glucosinolates were completely adsorbed by all brands, indicating that AC is suitable as adsorbent for this compound class. Finally, AC addition to substrates had differential effects on seed germination of Arabidopsis thaliana, Plantago lanceolata, Solidago canadensis, and Lotus corniculatus, whereas no effect was found on the germination of Lactuca sativa and Brassica oleracea. We suggest that scientists using AC should always include an experimental control to test for direct effects of AC addition on both substrate parameters and plant performance.

Activated carbon addition affects soil pH and germination of six plant species

Activated carbon (AC) is widely used in ecological studies for neutralizing allelopathic compounds. However, it has been suggested that AC has direct effects on plants because it alters substrate parameters such as nutrient availability and pH. These side-effects of AC addition may interfere with allelopathic effects. In this study we analyzed three widely used commercial AC brands and analyzed their effect on pH, their ability to retain glucosinolates, and their effect on the germination of six plant species. AC brands differed significantly in their effect on pH values when added to different substrates. Glucosinolates were completely adsorbed by all brands, indicating that AC is suitable as adsorbent for this compound class. Finally, AC addition to substrates had differential effects on seed germination of Arabidopsis thaliana, Plantago lanceolata, Solidago canadensis, and Lotus corniculatus, whereas no effect was found on the germination of Lactuca sativa and Brassica oleracea. We suggest that scientists using AC should always include an experimental control to test for direct effects of AC addition on both substrate parameters and plant performance