Ed van der Meijden

Defence and regrowth, alternative plant strategies in the struggle against herbivores

Differences in intensity or frequency of defoliation between species do not necessarily imply differences in ultimate effect: survival or growth. Several species appear to be very successful to overcome a high defoliation pressure because of their regrowth capacity. The hypothesis is formulated that defence mechanisms and regrowth capacity are alternatives. Given a limited amount of energy and nutrients available in the struggle against herbivores, there should be a trade off between these two mechanisms. Observations on the intensity of herbivory, experiments on regrowth capacity and on storage of reserves in roots agree with the predictions based on this hypothesis. Herbivory theory is discussed and it is concluded that a short population longevity and/or a short life cycle may be one of the ways to reduce the effect of herbivory. Differences in shoot/root ratio are discussed in this context.

Metabolomic Differentiation of Brassica rapa Following Herbivory by Different Insect Instars using Two-Dimensional Nuclear Magnetic Resonance Spectroscopy

The metabolic alterations of Brassica rapa (L.) leaves attacked by larvae of the specialist Plutella xylostella L. (Lepidoptera: Yponomeutidae) and the generalist Spodoptera exigua Hubner (Lepidoptera: Noctuidae) were investigated with nuclear magnetic resonance (NMR) spectroscopy, followed by a multivariate data analysis. The principal component analysis (PCA) of 1H NMR spectra showed that metabolic changes in B. rapa leaves induced by the 2nd and the 4th instars were different from each other. However, the congestion of the one-dimensional 1H NMR spectrum made it difficult to identify discriminating metabolites. To overcome the spectral complexity, several two-dimensional NMR techniques were applied. Of those evaluated, J-resolved spectroscopy, which affords an additional coupling constant, provided a wide range of structure information on differentiating the metabolites. Based on the J-resolved spectra combined with PCA, the major signals contributing to the discrimination were alanine, threonine, glucose, sucrose, feruloyl malate, sinapoyl malate, and gluconapin.

Diversity of pyrrolizidine alkaloids in Senecio species does not affect the specialist herbivore Tyria jacobaeae

Abstract. The evolution of the diversity of related secondary metabolites in plants is still poorly understood. It is often thought that the evolution of plant secondary metabolites is driven by specialist insect herbivores and under this coevolutionary model it is expected that related compounds differ in their effects on specialist herbivores. Here we focus on the diversity of pyrrolizidine alkaloids (PAs) in Senecio species and their effects on Tyria jacobaeae, a specialist moth on Senecio jacobaea. As a first step to determine the effects of related PAs on T. jacobaeae, we studied larval performance on plants from 11 S. jacobaea populations and eight Senecio species with different PA compositions. Although the populations of S. jacobaea differed in their PA compositions, there was no difference in larval performance among the populations. Larval performance differed among the eight species but we could not show a correlation with PA composition. Oviposition choice experiments showed a strong correlation between oviposition preference and larval performance on the eight species but oviposition preference did not seem to be correlated with PAs. We found no indications that related PAs differ in effects on the specialist T. jacobaeae; therefore it seems unlikely that T. jacobaeae is a driving force behind the evolution of the diversity of PAs. Alternatively, we propose that the evolution of the diversity of PAs is driven by selection pressure from generalist herbivores or that the diversity of PAs may even be selectively neutral.

Tritrophic interactions between aphids (Aphis jacobaeae Schrank), ant species, Tyria jacobaeae L., and Senecio jacobaea L. lead to maintenance of genetic variation in pyrrolizidine alkaloid concentration

SummaryWe hypothesize that the tritrophic interaction between ants, the aphid Aphis jacobaeae, the moth Tyria jacobaeae, and the plant Senecio jacobaea can explain the genetic variation observed in pyrrolizidine alkaloid concentration in natural populations of S. jacobaea. The ant Lasius niger effectively defends S. jacobaea plants infested with A. jacobaeae against larvae of T. jacobaeae. S. jacobaea plants with A. jacobaeae which are defended by ants escape regular defoliation by T. jacobaeae. Plants with aphids and ants have a lower pyrrolizidine alkaloid concentration than plants without aphids and ants. When these data are fitted to an existing theoretical model for temporal variation in fitness it is shown that varying herbivore pressure by T. jacobaeae in interaction with ants defending aphid-infested plants with a low pyrrolizidine alkaloid concentration can lead to a stable polymorphism in pyrrolizidine alkaloid concentration. Costs of the production and maintenance of pyrrolizidine alkaloids are not accounted for in the model.

Involvement of strictosidine as a defensive chemical inCatharanthus roseus.

The involvement of the glucoalkaloid strictosidine in antimicrobial and antifeedant activity inCatharanthus roseus leaves was investigated. Strictosidine and its deglucosylation product, specifically formed by the enzyme strictosidine glucosidase, were shown to be active against several microorganisms. In contrast, neither the intact glucoside, nor the aglycone product(s) was found to exhibit antifeedant activity againstSpodoptera exigua larvae, as was found for intactC. roseus leaves and leaf extracts. Besides alkaloids further downstream in the biosynthesis pathway, a more apolar, yet unidentified compound may be involved in this activity.

Extreme differences in pyrrolizidine alkaloid levels between leaves of Cynoglossum officinale

Abstract Optimal defence theories predict that the most valuable parts of a plant should be best protected. This theory was tested with pyrrolizidine alkaloids (PAs) in the leaves of rosette plants of Cynoglossum officinale . The youngest leaves contained up to 190 times higher levels of PAs than older leaves. This skewed distribution of PAs may be a strategy to divert herbivores from the photosynthetically active young leaves and thus to optimize growth.

Pattern of storage and regrowth in ragwort.

Ragwort plants were damaged experimentally by removing the whole shoot. Within about 1 month the original allocation pattern of biomass to root and shoot was reestablished to a large extent. If left undisturbed for a longer period, plant growth accelerated into compensatory growth. Intraspecific variation in storage and tolerance (shoot weight), 1 month after damage, was significant. We could not detect a trade-off between storage or tolerance and relative growth rate of control plants. Consequently there are no indications for costs involved in storage of resources or in tolerating damage. Although tolerance is thought to be dependent upon storage of resources, we detected no effect of storage on tolerance after one event of damage. Storage is genotype specific, but at the same time highly plastic. We hypothesize that the value of storage in ragwort only becomes evident after repeated disturbances. Competition, history of herbivory and change of season all affected storage radically.

Induced responses in three alkaloid-containing plant species

In this paper we test three plant species for the inducibility of their alkaloid production. The plants were heavily damaged by cutting off 50% of their leaf surface using a pair of scissors. The cut-off leaf tips were used as controls for possible diurnal fluctuations. After 3, 6, 12, 24 and 48 h, respectively, the leaf bases of the damaged plants were harvested and the alkaloid concentration was measured. In Senecio jacobaea the pyrrolizidine alkaloid (Pa) concentration in damaged plants decreased within 6–12 h after damage. Within 24 h after damage the Pa concentration of Cynoglossum officinale doubled compared to control values. Indole alkaloid production in Catharanthus roseus was found not to be induced in this experiment. The responses are discussed in a functional context. We hypothesize that the nature of the response is not a feature of the type of secondary metabolite, but is related to whether the plants are damaged mainly by generalist or by specialist herbivores.

Tritrophic Metapopulation Dynamics: A Case Study of Ragwort, the Cinnabar Moth, and the Parasitoid Cotesia popularis

Publisher Summary This chapter analyzes whether, and to what extent, interactions within a tritrophic system are affected by the spatial distribution of habitat patches. The chapter reviews long-term data (two decades) on the relationships between the plant ragwort ( Senecio jacobaea ), its most important herbivore, the monophagous cinnabar moth ( Tyria jacobaeae ), and the specialist parasitoid of the herbivore, Cotesia popularis . Ragwort is a facultative biennial plant. It is native to Europe and has invaded overgrazed areas throughout the world. Its weedy character is largely due to its extremely powerful reproductive potentials. This chapter begins by giving an outline of the population dynamics of the three organisms, and describes how they interact with each other. The parameters describing the degree of synchrony between local populations and the metapopulation are then calculated to reveal to what extent the dynamics of local populations differ from each other. Attention is paid to spatial correlations within and between the three species. The chapter discusses the mechanisms that appear to play a role in the persistence of the plant, the herbivore, and the parasitoid in their patchy environment.

Oviposition by the cinnabar moth, Tyria jacobaeae, in relation to nitrogen, sugars and alkaloids of ragwort, Senecio jacobaea

Recent studies have demonstrated that relationships between nitrogen levels and oviposition, food selection, larval survival and/or damage to host plants are not simple. We studied the effect of different levels of nitrogen on oviposition in the cinnabar moth in relation to the concentrations of alkaloids, that are positively correlated with nitrogen, and of soluble carbohydrates that are negatively correlated with nitrogen. We could not demonstrate that total alkaloid concentration was important as a proximate factor in the selection of food plants. Cinnabar moth selected plants with a high concentration of organic nitrogen and sugars. Plants poor in both, as well as plants rich in only one of these substances received fewer egg batches.