On looking a little closer at these genes' sequences the scientists noticed a region that varied considerably from one species to the other: the promoter. This region, situated close to a gene’s coding sequence is indispensable for its transcription. It is this that enables a gene's expression to be regulated, the gene being HMA4 in this case.
‘These regions turned out to be a lot more active in A.halleri', Marc Hanikenne picks up the story. ‘If one can compare this promoting region to a car engine, it’s a bit as if one had placed a Ferrari engine in place of that of a Fiat’.

Other factors under the magnifying glass!

The secret’s of Arabidopsis halleri’s overexpression having been revealed, it just remained to test the Ferrari motor in the Fiat! The scientists thus introduced an A.halleri HMA4 promoter in place of an A.thaliana promoter. The results did not take long to show themselves: once controlled by its sister's promoter, the expression of the HMA4 gene rose and A.thaliana became a metal hyper-accumulater! But if its ability to absorb zinc increased appreciably, it nonetheless could not tolerate it. In effect, zinc makes it fragile as it does not possess the adaptations necessary to store this metal in its leaves.
Thus, HMA4 on its own allows a plant to be given the label 'metal accumulator'. The researchers now hope to put their finger on the gene or genes responsible for Arabidopsis halleri’s hyper-tolerance.

As was mentioned above, this work could not only allow for great strides to be made in the fields of phytoremediation and biofortification, two new technologies aiming at improving the environment and health respectively. But the discovery revealed by the present study is above all a great advance in fundamental research in the field of metal homeostasis. It in effect enables a greater understanding of how one species is transformed into another and of how it has acquired the new characteristics that have allowed it to colonise new habitats.