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How a water plant can save its “skin”

Biologist Dr. Elisabeth Groß is investigating the complicated defence mechanisms of a water plant (water milfoil). The plant uses these defence mechanisms to save its “skin” from herbivore attacks and to make life difficult for competing algae.

Dr. Elisabeth Groß, biologist at the University of Constance © Peter Schmidt

The water milfoil uses tannins, substances consumed by human beings in fruit, tea and wine, for example. Tannins are a weapon that plants use to ward off enemies. However, they are only a small cog in the complex interactions between plants and herbivores, plants and bacteria as well as between plants themselves. It is silent and peaceful in the small room which is submerged in bright light. Only the buzzing of the air conditioner disturbs the silence. On the shelves along the walls, there are long rows of glass containers with water plants, carefully wrapped in silver aluminium foil and clearly labelled. An unreal atmosphere for many visitors, but not for biologist Dr. Elisabeth Groß who is in her element here.

In this room, the scientist stores plants which she uses for her scientific projects: the Eurasian water milfoil, a submerged aquatic plant with slender stems up to 4 m long. The flowers are produced on a spike held above the water. The fact that the flowers look like ears of corn is what gives the plant its German name - Ähriges Tausendblatt (Engl. Eurasian water milfoil).

In the flowering season, the flowers remain above the water where they are pollinated by insects. The pinnae resemble fine small green feathers. The Eurasian water milfoil, which originates in both Europe and Asia, is very common in many lakes of the Northern Hemisphere, but is on the retreat in Lake Constance since the lake has become poorer in nutrients. Ten years ago, there were still real milfoil forests in Lake Constance," said Groß.

Despite its delicate appearance, the plant is nevertheless very efficient in fending off herbivores and algae that are competing for light, for the carbon dioxide it needs for photosynthesis, and for nutrients. "Lack of light and the competition for nutrients with other plants leads to inefficient photosynthesis and hence a lack of opportunity to grow. The plant has to use efficient strategies to ensure its survival," said Groß explaining the interaction between the plants. The plant uses tannins (polyphenols) to fend off herbivores. These tannins are frequently found in food, fruit and plants, for example in the bark of oak and chestnut trees. Tannins are also found in green and black tea as well as in red wine.

Tannins have become an important subject of study due to their antioxidative effect. Substances with an antioxidative effect are naturally present in food and in the human organism and can protect the organism against potential damage to cell nuclei and cell membranes. Tannins are not only an efficient weapon against algae but also against herbivores such as the caterpillars of the Small China-mark moths. This is a family of moths of which one species spends its larval stage under water. The caterpillars live in the water and the short-lived adult moths above the water, for example on the flowers of the Eurasian water milfoil.

The Small China-mark caterpillars are strongly attracted to the milfoil leaves. However, eating the tannin-containing leaves retards the growth of the caterpillars. "But the tannins cannot just have negative effects; they must also have positive effects, otherwise the larvae would not eat the young plant tips that are particularly rich in tannins. It could be that the tannins keep certain pathogenic bacteria or parasites at bay, and that is why they are so popular with the caterpillars," said Groß explaining the complicated interplay of advantage and disadvantage. Groß also assumes that certain bacteria help the animals to digest the tannins effectively. The caterpillars take up the bacteria along with the leaves. In this way, they eat an all-in-one package, consisting of leaves and a bacteria cocktail.
 
The researchers determine the composition of the bacteria community on the plant using DNA. The bacteria are removed from the leaves and their DNA separated using a special method, DGGE (denaturing gradient gel electrophoresis). "This provides us with a kind of fingerprint of a particular bacteria community and enables us to compare the identity of the individual bacteria with data in large databases - just as the police do," said the biologist describing the complex procedure. Groß and her team have also examined how the biofilm, i.e. bacteria communities, on milfoil, pondweed and inanimate material differ from each other. The researchers found that the bacteria community of a tannin-producing plant looks completely different from the bacteria on plants that do not produce tannins.

But how does the complicated mechanism between tannins and bacteria work? To find this out, Groß and her team fed the bacteria with tannins that had been isolated from the plant. The tannins are extracted from the freeze-dried plant using solvents. "The bacteria culture cannot eat "à la carte", or choose their favourite food, but are forced to live on tannins that are added to the test tube," said Groß with a smile. "This enabled us to isolate some strains that degrade tannins. The problem is that only a few bacteria cultures grow in the test tube. We are now working on the development of methods that will help us investigate the degradation of tannins in bacteria that cannot be cultivated. The caterpillars will benefit the most from the bacteria when the bacteria help them to keep the tannins in check or when the tannins destroy those bacteria that are harmful for the caterpillars.

Plants, tannins and bacteria seem to interact with each other by way of a very ingenious system. "The bacteria do not eat all the tannins, leaving the plant with enough tannins to protect itself against herbivores and algae," said Groß explaining how the system balances itself out. A large quantity of tannins is found in the young leaves of the plant where its role is to protect what is most valuable to the plant. "In order to gain a greater understanding of the complex interactions, we have established a system that enables us to specifically add certain bacteria to "clean" plants." In order to achieve this, the researchers remove all bacteria and algae from the plants, resulting in so-called axenic plants, which will then be stored in the aforementioned air-conditioned room.

"We initially switch off the complexity in the laboratory and then add it again in small doses in order to reproduce the real situation as closely as possible," said the biologist. Groß also hopes that the Bioimaging Centre at the University of Constance will provide totally new opportunities, she is hoping to gain access to equipment that will provide her with even more precise information on how the different bacteria are arranged in the biofilm on the plant and what influence this has on the release of the bioactive tannins.

Further information:

Dr. Elisabeth Groß
Faculty of Biology
University of Constance
Tel.: +49 7531 88-3112
Fax: +49 7531 88-4136
E-mail: elisabeth.gross(at)uni-konstanz.de

Website address: https://www.biooekonomie-bw.de/en/articles/pm/how-a-water-plant-can-save-its-skin