A love of parasites

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Associate Susann Wicke with her test objects: tobacco plants which have been aff
Associate Susann Wicke with her test objects: tobacco plants which have been afflicted and damaged by broomrape. © WWU - Peter Leßmann
Broomrape, rattle, dodder. It’s not only the wonderful-sounding names that these plants have in common - it’s also the way they live, because they do so at the expense of other plants, robbing them of water and nutrients in order to secure their own existence. And, in doing so, they have exerted a fascination on Dr. Susann Wicke, an associate professor at the University of Münster. Susann Wicke, a biologist, has a preference for plants with a ‘certain something’, and one thing she is certain of is that, as she says, “We still don’t know enough about these fascinating species. How for example, genetically speaking, does a parasitic plant differ from a ‘normal’ plant? What evolutionary adaptations were necessary for it to be able to actually develop such a way of life?” These are questions which Susann Wicke, 37, has been studying since 2012 at Münster University, and as the leader of an Emmy Noether research group in the past two years. This year she was awarded the Junior Researcher Prize by the Universitätsgesellschaft (Friends of Münster University).

In most cases, it is agriculture which brings parasitic plants to the attention of humans. In sub-Saharan Africa and in the Mediterranean area they can afflict cereal and vegetable fields over a large area, resulting all over the world in failed harvests which cost billions. In Germany it is often the European mistletoe (Viscum album) that causes damage in forestry. Fundamental research of parasitic plant biology can help to develop appropriate measures to combat these parasites. However, although these plants are often viewed with suspicion, they do have their good features: in intact ecosystems they prevent any excessive profusion of individual species of plants.

Among the 4,500 species of parasitic plants that exist worldwide, it is broomrape - native to Europe - that especially fascinates Susann Wicke. And not because of its pretty flowers, but because of one of its particular properties: broomrape is a so-called holoparasite that germinates only when it is close to potential hosts. Then, it takes advantage of signals emitted by its host plants to actually attract mutually beneficial partners like mycorrhizal fungi or soil bacteria.

It comes as no surprise to find that many of these favourite parasites of Susann Wicke are among the 250 or so plants that she and her ten group members currently nurse in the cellar of the Institute of Evolution and Biodiversity. There, both host plants and parasites live under perfect conditions which match their needs. Behind the thick aluminium doors of climate chambers, they get precisely the moisture level, temperature and light that they need. An Egyptian broomrape, for example, spends its time at a constant and agreeable temperature of 25 degrees. The plants grow in so-called rhizotrons, which are flat containers made of glass and filled with soil, and which enable researchers to directly observe the activities and changes taking place at the roots. The background to this is that many parasites intertwine with the roots of their hosts in order to tap into them by means of specially formed suckers.

Even a non-specialist can see that the health of the infected host plants varies greatly. Some of the plants in the climate chambers display a lush green, while others have clearly lost some of their vigour. “It’s fascinating to see how the effect that the parasite has on a host plant can vary so much,” says Susann Wicke. “In some cases we have to provide additional nutrients in order for the host plant to actually survive. That shows clearly just how intensive the molecular interactions are between host and parasite.”

New experimental parasites are already waiting in around 39 plants, in pots in which they are being propagated for further studies. The Münster researchers get the seeds for this from abroad - mostly from Kenya, Israel or France. Another task facing the researchers is to bring the picky parasites together with their preferred hosts: clover, tomatoes, tobacco and thale cress are some of the host plants that the researchers use in their experiments. “By using genetically modified hosts, we try to find out what the essential genetic function is that favours parasitic behaviour,” Susann Wicke explains.

In evolutionary experiments stretching over several generations of plants and years, one of the things that Susann Wicke and her colleagues want to find out is what genetic adaptations a plant goes through when it mutates into a parasite. “Another highly interesting question is why host plants do not display any notable immune reactions to parasites,” Susann Wicke adds. Here she is talking about just one of the many puzzles which she - and the relatively small community worldwide of just a few hundred parasitic plant researchers - are still aiming to solve.