Source: Max Planck Institute for Chemical Ecology, Newsletter 05/March 2006
Original newsletter in PDF format: http://www.ice.mpg.de/main/newsletter/Newsletter05_en.pdf

Scientists of the Max Planck Institute for Chemical Ecology are investigating how the information transfer based on chemical volatiles between neighboring plants functions. They analyzed the defense reactions of the wild tobacco Nicotiana attenuata to herbivory after the plant had received volatile signals from a wounded, neighboring sagebrush (Artemisia tridentata). They found that the defense reaction in tobacco plants that had been able to “eavesdrop” on sagebrush was faster and more efficient in comparison to the reaction of plants that did not have the information. This phenomenon is called “priming”. Amazingly, the tobacco plants boosted their defense only when they were actually attacked and not just after they had received the signals of the wounded neighboring plant. This reaction makes sense from an economic point of view: Transforming valuable resources into defense molecules just after receiving volatile signals could harm the plant unnecessarily if no attack occurs. The defense substances studied in these experiments are the so-called proteinase inhibitors (TPIs); these inhibit the digestion of herbivorous insect larvae.

The biologists place a premium on combining lab experiments and field trials. The conditions in the lab should be as close to nature as possible. In the laboratory, plants are often put into narrow glass containers for scent analysis. The problem is that the concentration of gaseous molecules is artificially increased in such “in vitro” experiments. On top of that the plants – hermetically sealed in the glass containers – suffer from a lack of CO2. To compensate for this deficiency the plants open their stomata. As a consequence not only more CO2 but also more other volatile compounds reach the inner leaf.

“The reaction of the receiver plant can then be artificially intensified or even falsified”, explains Anja Paschold. In her recently published paper the scientist studied the volatile communication among Nicotiana attenuata plants under “realistic” conditions on the one hand, and with transgenic “mute” plants on the other hand. She found that neither the complete scent profile of wild-type plants nor the scent profile of genetically modified plants, in which some volatile molecules had been knocked out, influences the known defense mechanisms in receiver plants: The concentration of nicotine, jasmonic acid and proteinase inhibitors hardly changed. A “priming” effect could also not be shown. However, analysis of gene expression patterns using micro-arrays showed that there were significantly more genes regulated in the receiver plant when the mixture of scents did not include “green leaf
volatiles” (i.e. specific alcohols and aldehydes - the typical smell of a freshly mowed lawn!). When the incomplete scent bouquet was complemented by adding synthetic leaf alcohols and aldehydes the genes were “un-regulated”. Certain gene groups seem to be switched on by volatile signals from plant to plant within one species, other gene groups, however, seem to be switched off. The function of most of these genes is still unknown and will be analyzed next.

by Priv. Doz. Dr. Jan-W. Kellmann, Max Planck Institute for Chemical Ecology, AP

Original publications:
[1]
Priming of plant defense responses in nature by airborne signaling between Artemisia tridentata and Nicotiana attenuata
André Kessler, Rayko Halitschke, Celia Diezel, Ian T. Baldwin
Oecologia, online first, DOI 10.1007/s00442-006-0365-8.

[2]
Using “mute” plants to translate volatile signals
Anja Paschold, Rayko Halitschke, Ian T. Baldwin
The Plant Journal 45, 275 - 291 (2006).