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Copper is one of the most toxic metals to any form of plant life. Most plants and algae are quite susceptible to even low levels of copper, and copper-resistant species usually have strategies to exclude it from their metabolism.

In that context, a study published in the latest issue of Plant Physiology reports the surprising finding of a plant being both resistant to copper and furthermore accumulating high levels of this metal – despite its toxicity.

Crassula helmsii, also known as swamp stonecrop or pigmyweed and originating from coastal wetlands of Australia and New Zealand, has been invading British and European wetland habitats for several decades now, outcompeting most native species and forming dense, almost impenetrable carpets.

The exceptional heavy metal resistance together with high tolerance against drought and salinity may contribute to the competitive success of this newcomer to the European flora when invading wetland habitats, because other plant species are known to accumulate toxic heavy metals as feeding deterrents against grazers.

The copper resistance of Crassula helmsii as well as the chemical speciation and toxicity of copper in the plants was compared to the copper-sensitive, cadmium/zinc-hyperaccumulating plant Thlaspi caerulescens, which is described in a companion study in the same issue of Plant Physiology. This comparison showed that metal detoxification mechanisms and binding of the metal are similar for hyperaccumulated metals (Cu in Crassula helmsii, Cd and Zn in Thlaspi caerulescens) but strongly differ between hyperaccumulated and non-accumulated metals (Cu in Thlaspi caerulescens investigated in the current study, and Cd or Zn in other non-accumulator species analysed earlier). Hyperaccumulated metals were always found to be bound by weak ligands (binding partners), while non-accumulated toxic metals were found to be bound by strong ligands.

Also unusual is the seasonal variation in copper resistance of Crassula helmsii – in the summer, when growing in an emerged state as wetlands tend to partly dry up, the plant is more tolerant to the toxic metal than in the winter, when submerged growth predominates. This correlates with a switch between different photosynthetic pathways in  the emerged compared to the submerged state (C3 in the emerged, CAM in the submerged state).

The study involved SAMS scientist Frithjof Küpper, Wolfram Meyer-Klaucke at the European Molecular Biology Laboratory, Ana Mijovilovich at the University of Utrecht, and was led by Hendrik Küpper at the University of Konstanz, Germany. In fact, the work had its roots in a project which had won the brothers Hendrik and Frithjof first prizes at the European Union Contest for Young Scientists and related competitions in 1992. At that time, they had first found Crassula helmsii at a regenerated coal mining site near their German home town, Marl. For the first years at that time and prior to having access to university or institutional laboratories, experiments were conducted in their private laboratory in the basement of their parents’ house.

Link to the original articles (open access):
Küpper H, Mijovilovich A, Götz B, Kroneck PMH, Küpper FC, Meyer-Klaucke W, 2009: Complexation and toxicity of copper in higher plants (I): Characterisation of copper accumulation, speciation and toxicity in Crassula helmsii as a new copper hyperaccumulator.- Plant Physiology 151,  702–714


Companion article on Thlaspi caerulescens:
Mijovilovich A, Leitenmaier B, Meyer-Klaucke W, Kroneck PMH, Götz B, Küpper H (2009) Complexation and toxicity of copper in higher plants (II): Different mechanisms for Cu vs. Cd detoxification in the Cu-sensitive Cd/Zn hyperaccumulator Thlaspi caerulescens (Ganges ecotype). Plant Physiology 151, 715-731


Research group of Hendrik Küpper