Arabidopsis thaliana and Arabidopsis halleri are two sister species. Genetically very close, the two plants are nonetheless distinguished by particular behaviour. Whilst the first does not tolerate strong concentrations of heavy metals, the second grows in soils which are rich in them. In a study published in Nature(1), German researchers at the Max-Planck Institute and Belgian researchers at the University of Liège have uncovered the crucial role of a gene, HMA4, in the evolution of metal hyper-accumulating plants (2).

Arabidopsis thaliana, or mouse ear cress, is well known in the scientific world. This small flowering plant effectively constitutes a model organism within the field of plant biology. This Brassicae owes its success notably to its genome, which is of a small size. The latter, composed of some 28,000 genes, has been completely sequenced since 2001.

Another noticeable quality of A.thaliana is that it is very easily ‘transformable’. Scientists can thus create transgenic plants in which the target genes can be deliberately switched off or activated. Moreover, this plant's life cycle is very rapid. In effect, it only needs two months to pass from a seed into a plant and then a new seed (compared to six months for corn, for example).

Finally, mouse ear cress is self-fertilising. This characteristic means that homozygous seedlings with the desired mutation can be obtained easily. In effect a gene's two alleles, in this case derived from a single and unique individual, will be identical. Genetically modified Arabidopsis thus express their mutation(s) fully.

Each to their own path

Five million years ago Arabidopsis thaliana and Arabidopsis halleri took divergent evolutionary paths. If that seems like an eternity on the scale of a human lifespan, in the history of our planet it consists of a very recent event. This youthful separation between the two species is moreover marked by the great similarity of their genomes: they share 94% DNA identity within their coding sequences.

But despite being such sisters, as if to distinguish themselves from each other, these Arabidopsis developed very specific 'personalities'. The habitats they colonise and their behaviour are in fact very different. A.halleri lives on soils contaminated by zinc and cadmium. It accumulates these metals and stores them in its leaves. This type of plant is called 'metallophyte' or 'hyper-accummulating'.

Metallic substances can thus constitute up to 3% of the weight of A.halleri whilst for A.thaliana this percentage does not exceed 0.01%. The latter is a non-accumulating plant and does not tolerate strong concentrations of zinc and cadmium. Cadmium has, as far as is known today, no vital function for the plant. This metal can moreover be very toxic if the plant does not have the required adaptations to be able to tolerate it. Zinc, on the other hand, is a micronutriment which has its place in the flow of metals essential for plant nutrition.

(1) Marc Hanikenne, Ina N. Talke, Michael J. Haydon, Christa Lanz, Andrea Nolte, Patrick Motte, Juergen Kroymann, Detlef Weigel & Ute Krämer, Evolution of metal hyperaccumulation required cis-regulatory changes and triplication of HMA4, in Nature 453, 391-395 (15 May 2008).

(2) This work results from a collaboration between the laboratories at the Max-Planck Institution (MPI-Plant Molecular Physiology, Postdam ; MPI-Developmental Biology, Tübingen and MPI-Chemical Ecology, Jena), at the University of Heidelberg (Heidelberg Institute of Plant Sciences ; and "BioQuant Center, the new center for systems biology at the University of Heidelberg") and at the University of Liège.