Title: Chemical information exchange in soil; tritrophic interactions between conifers, herbivores and predators

Primary Researcher: Marieke Prins
Email address: m.w.prins@uva.nl
Submitted on: October 3, 2006

Start date: 01 September 2004
End date: 01 September 2008

Introduction:

In the past two decades it has been well documented that plants respond to damage and herbivory with the emission of a bouquet of volatiles. Besides it has been documented for the last decade that these volatiles, which are emitted by herbivory-damaged plants, are attracting carnivorous enemies of the herbivores. The blends emitted by herbivory-damaged plants, semiochemicals or infochemicals, contain fatty-acid derivatives and terpenoids, but also nitrogeneous compounds, sulphur containing compounds and phenolics, such as methyl silicate are frequently found.
However roots are a vital, yet vulnerable part of the plants and the role of natural enemies in suppressing the population of root-feeding insects in natural ecosystems have been shown. A study by van Tol was first to prove that plants actively protect their roots by attracting these natural enemies. In a tritrophic system consisted of a conifer (Thuja occidentalis), vine weevil larvae (Otiorhynchus sulcatus) and entomopathogenic nematodes (EPN) (Heterorhabditis megidis), EPN’s were more attracted towards the conifer upon attack by the weevil larvae compared to a undamaged conifer.

The information obtained by van Tol has led to an interest of the chemical interactions at this tritrophic level. It has led to much attention in reviews in Trends in Ecology and Evolution (Van der Putten, Vet et al. 2001), Annual Reviews in Entomology (Blossey and Hunt-Joshi 2003) and in the Forum section of Ecology (Bardgett and Wardle 2003) (Van Der Putten 2003). Not only the results were of interest, but the outcome of these results coincides with new approaches in the field of soil ecology, in which the soil is no longer considered a black box, and focus of the research is on underlying processes.

Aim:

This PhD research proposal is a prolongation of the work of Van Tol. The main aim is;

Elucidate the environmental chemistry of the chemicals involved in the tritrophic system of the conifer, vine weevil larvae and EPN’s.

This project has, besides a chemical component, a biological/ecological component, since only chemicals to which the nematodes response (determined by bio-assays) are of interest. Therefore this project is collaboration between the University of Amsterdam, which will provide the chemical expertise, and Plant Research International, focussing on the ecological part of this project.

The research concerns three aspects of the tritrophic system in which these chemicals, so called semiochemicals are concerned;
1) the emitter (the plant )
2) the receiver (the entomopathogenic nematode)
3) the pathway of the semiochemical between the producer and receiver (occurs in the soil)

These three aspects has led to the three following research questions;
1) qualitative and quantitative characterisation of the semiochemicals released by conifer roots.
(2) The qualification and quantification of responses of nematodes towards different doses of the selected semiochemicals and the influence of soil parameters on this response.
(3) The transport and transport routes of the semiochemicals through soil including the possible abiotic and biotic transformations.

Research:

Qualitative and quantitative characterisation of the semiochemicals

In this sub objective the semiochemicals to which the nematodes respond are identified. At first a qualitative characterisation is made, since the identity of the semiochemicals which are causing an induced response of the nematodes, is of interest. In order to know whether the chemicals that are used in the identification experiments are semiochemicals, bio-assays are conducted to check the preference of the EPN's for the chemicals.
Quantitative research is necessary to unravel the mehcanism undelrying the production of these semiochemicals by the conifer and describe the production itself. Items of interest is whether the production is a systematic response of the plant, which signal-transduction pathway is involved, how fast and how long will the plant respond upon herbivore attack with releasing semiochemicals.

Qualification and quantification of the response of the nematodes towards the semiochemicals

With the semiochemicals selected from the former sub objective doses-response relationships are investigated. The foraging strategy of the nematodes is investigated as well. Items that are involved are the distance on which these tritrophic interactions act, the influence of the semiochemical and or the larvae on the foraging behaviour, the detection of the semiochemical by the nematode.
This latter item may be of great importance in a later phase of this project. The structure of the semiochemicals involved in other tritrophic systems with EPN’s can be predicted by knowing which chemical structures can be detected by the nematodes in the tritrophic system with vine weevil larvae and Thuja.

Transport and fate of the semiochemicals in the soil

The transport of the semiochemicals from the producer (conifer) to the receiver (EPN’s) is a highly complicated process depending on many abiotic and biotic factors. Transport of chemicals through soil may take place through diffusion and convection in the gaseous and water phases. This transport will be influenced by the partitioning of the chemicals between air, water, organic matter (dissolved or solid) and other phases and by adsorption to surfaces such as mineral surfaces. Research on terpenes, at the department of the UVA, show that terpenes have a low Henry coefficient which will cause them to partition mainly into the water phase of the soil. Preliminary to this research proposal several experiments were performed on the tritrophic system with the conifer, vine weevil larvae and EPN’s. The compounds released by Thuja are mainly terpenes as have also been found by Nickavar (Nickavar, Amin et al. 2003). Though, it is not excluded that other chemicals are involved as well. This may increase the complexity of this project, since the chemicals will probably have different chemical properties and will therefore be transported differently through soil.
Since the structure of the chemical which will finally be received by the nematodes is of interest, transformations, either biotical or abiotic, must be included as well. Transformations may lower the activity of the compounds or result in a potential structure with a higher mobility. These transformations process highly depend on soil characteristics like acidity, organic matter content, inorganic molecules (nutrients), temperature, etc. Each of the characteristics has their own spatial and temporal variation. All of the influencing parameters must be included in this research proposal in order to investigate the fate of the chemicals in the soil.

Future Research

Depending on the outcome of the former objectives a few objects are of interest as well. The objectives described above are only outlining the below ground interactions. However, in the field belowground and aboveground processes interact. For example infestation on the stem, might induce a release of compounds by the roots or the other way around. Diffusion of induced compounds released by the stem or leaves into the soil might be possible as well. Besides, this intra-phytological interactions several inter phytological interaction can be expected on a community level. A second interesting subject is the intra-specific difference in a strain of nematodes. Van Tol found (personal communication) that not all nematodes of the same star are capable of infesting the larvae. It seems to differ with the outcome time of juveniles ones they leave the larvae and search for a new host. Besides, it is interesting to pay further attention on the selection of host plants for nematodes.

Funding via University

Yes