PPlants often use toxins to protect themselves from hungry mouths. These substances, which include caffeine and nicotine as well as the glycosides of the foxglove and the alkaloids of the opium poppy, usually come from our own production. Haller’s foam cress is one of the plants that instead extract toxic heavy metals from the ground (Arabidopsis states). Unlike the related thale cress (Arabidopsis thaliana), the preferred research object of geneticists, it forms offshoots on which – similar to strawberries – new plants grow. Instead of reproducing sexually, it can reproduce as a clone.
When Haller’s cress is nibbled, it sends its roots to places that are rich in cadmium and absorbs considerable amounts of the toxic heavy metal from the soil. The plants then not only arm their leaves, but also pass on the metal via their runners, as botanists from the University of Tübingen have discovered. In the greenhouse, Anubhav Mohiley and his colleagues each placed two plants connected by runners – that is, genetically identical – plants in different flower pots: One pot contained normal soil with a low cadmium content, the other the soil was enriched with the heavy metal.
No wonder that the cress thrive there poorly, after all, the metal also damages the plant metabolism. These plants kept the cadmium content of their leaves at just as moderate a level as their twins, who obtained water and nutrients from uncontaminated soil.
With chemical armament against parasites
To simulate an attack by hungry insects, the researchers poked holes in the leaves and applied jasmonic acid. This is a plant hormone that, among other things, encourages the defense against attackers. The connected potted plants reacted with chemical armament: Both tripled the cadmium content of their leaves.
It made no difference which plant was originally challenged by the simulated attack, as the researchers write in the Proceedings of the Royal Society B.. Apparently, Haller’s cress can not only use its foothills to inform about a threat, but also send other parts of the clone the heavy metals they need to defend themselves, which spoil the appetite of all herbivores.
Some populations of Haller’s cress grow in former mining areas, for example in the Harz near Clausthal-Zellerfeld, on soils that are particularly rich in heavy metals. There, evolution has ensured through natural selection that the plants get along well where conspecifics from unpolluted locations would wither.
Less productive but tougher, they can tolerate relatively high levels of cadmium. Planted in soil with a corresponding heavy metal content, such a cress accumulated noticeably large amounts of cadmium in its leaves – regardless of whether it was left undisturbed or whether Mohiley and colleagues pretended to be attacking voracious insects.
However, if its twin was attacked, who could only get a little cadmium from the earth, the plant shared the accumulated heavy metal with him. The communication between sprouts, which are connected by runners, also seems to work in the wild. Likewise, neighborhood help within a clone when it comes to effective defense.