Towards the identification of mechanisms underlying the high expression of HMA4, an essential gene for zinc hyperaccumulation in Arabidopsis halleri.

To maintain metal homeostasis within physiological limits independently of metal concentrations present in soil, plants developed a complex network including metal uptake, chelation, trafficking, and storage processes. In this network, transporters have a central role.
To study the molecular basis of zinc homeostasis in plants, we are using A. halleri, a zinc- and cadmium-tolerant and zinc-hyperaccumulating species representing the extreme end of natural variation in terms of metal homeostasis. Indeed, A. halleri stores high amount of metals in vacuoles of above-ground tissues (>1% Zn or 0.01% Cd in shoot dry weight).
Cross-species transcriptomic studies identified about thirty genes which are constitutively over-expressed in A. halleri compared to its non-tolerant and non-hyperaccumulator sister species, Arabidopsis thaliana (1-3). HMA4 is one of these genes. It encodes a Zn/Cd plasmamembrane pump involved in xylem loading/unloading which is a key step in the hyperaccumulation process (4-5). The HMA4 overexpression is determined by a combination of gene triplication and cis-regulatory changes increasing transcription levels of each three HMA4 copies (5).

We aim to identify the cis-elements underlying high expression of HMA4 in A. halleri using promoter-reporter constructs.