Maize and its scents

Indirect defence

Christine Zipfel and her colleagues want to discover whether the release of scents in Bt maize is different from conventional maize. The scientists are particularly interested in these substances because scent is a means of communication for the plant. For instance, when a maize plant starts to be eaten by a caterpillar it releases volatile substances which attract the antagonists of the caterpillar. If it is lucky, a parasitic wasp will respond to the signal and parasitize the caterpillar by laying eggs in it. This interplay between plant, butterfly and parasitoid could be affected by a change in the release of signal substances in Bt maize.

Scent production is triggered by a substance in the pest’s saliva. Each pest provokes a different scent composition.

“The part of the plant attacked by the caterpillar is also significant”. Christine Zipfel points to the stem of a maize plant: "The European corn borer for instance eats this part and triggers fewer scent emissions than other caterpillars, which tend to eat the leaves."

There are three main scents that are triggered when caterpillars eat the maize plant: caryophyllene, bergamotene and farnesene. To what extent is the formation of these substances triggered by various caterpillars and in what composition? Much fundamental research is still necessary before this question can be answered. Within this project, various caterpillars are tested in a climate chamber, in which the temperature, air humidity, light and other environmental factors can be controlled. They are placed on maize plants and the emitted scents are then collected and analysed.

Individual scents

Each maize variety has its own particular maize-typical basic scent pattern and each plant has its own variant of this pattern, which it acquires over the course of its life and which is constantly subject to change. A young maize plant does not emit scents. It starts developing scent patterns only in the intensive growth phase, depending on the type of location and to what extent it is attacked by pests and pathogens. A plant’s recent life experiences can therefore be identified from its scent.

Not much happens with old plants. The scent pattern is no longer altered if the plant is attacked by caterpillars because the leaves are too tough to be eaten or because there is more attractive fodder in the vicinity.

Scent emissions change over the course of the day as well. Plants emit the largest amount of scent at midday and in the early afternoon. This is why the team always carries out three hours of measurements around lunchtime. During the heat wave however, measurements had to be limited to one hour because otherwise the pumps overheated.

Measurements were carried out on this field once in July and once in August. Two average-looking plants were selected at random for measurement from each of the eight plots of the trial field. When selecting plants for today’s measurement, however, it is just a question of finding two plants with a bit of green on them. At this stage Christine Zipfel is happy to get at least one good, analysable measurement per plot.

A whole range of influencing factors

In the field there is, of course, an abundance of influencing factors that can affect a measurement result and are difficult to assess. For instance, although the composition of scents among plants of the same variety is generally the same – at least as regards the primary components – there can be considerable differences in the amount of scent released. Plants that look very similar, have a similar weight and are infested with roughly the same number of caterpillars can still emit very different amounts of scent. Maybe because their basic constitution is different, or maybe the scent balance is affected by fungal infection, root pests or other factors.

For this reason, preliminary tests were carried out in the laboratory to determine the scent patterns of the different pairs of varieties in the absence of site and infestation influences, and so that they could later be compared with the field data. In addition to the field measurements, molecular tests are also being conducted. Leaf samples are taken from the individual varieties on the different plots and examined to see whether certain genes responsible for individual scent components are disabled.

Preliminary results

So far, the measurements in the laboratory and in the field have not shown any verifiable Bt effect. Differences were found in one variety pair – between the transgenic variety Novelis (Mon 810) and its isogenic parent variety Nobilis – both in terms of the scent spectrum and the amount of scent emitted. However, Christine Zipfel sees this as a varietal effect, i.e. due to the fact that these two varieties differ in more aspects than just the genetic modification.

This year, and last year too, the scent composition suddenly changed in late July/early August from one measurement to the next for all transgenic and isogenic varieties under investigation. Whether and to what extent this may have been caused by the extreme weather conditions (the flooding last year, heat wave this year) remains a mystery for the moment.