Despite the wide diversity observed in metal hyperaccumulator plants in terms of species distribution and the nature of accumulated metals, most of the molecular studies on this complex trait focus on the hyperaccumulation of zinc and cadmium in hyperaccumulator species of the Brassicaceae family including Arabidopsis halleri and Noccaea caerulescens. We believe that it is now essential to obtain a broader picture of metal hyperaccumulation in plants using recent developments in elemental analysis and sequencing technologies.

To identify new hyperaccumulator species, we initiated a screen of plant specimens at the Herbarium of the French National Museum of Natural History using X-ray fluorescence. Our analyses focusing on regions that have been poorly investigated so far, including Central/South America and Madagascar, revealed putative new hyperaccumulator of zinc and manganese. In particular, we identified 22 species from Proteaceae, Phyllanthaceae and Salicaceae families containing more than 10,000 ppm manganese in leaves, thus significantly expending the number of known manganese hyperaccumulators.

To identify molecular mechanisms involved in metal hyperaccumulation in plants, we are also developing comparative RNA-Seq based strategies to identify genes whose expression is linked to the metal hyperaccumulation trait in several plant families. Differential gene expression analyses using pairs of hyperaccumulator and closely related non-accumulator species from diverse plant families coupled to annotation of Cluster of Orthologous Groups revealed mechanisms involved in nickel hyperaccumulation shared by different species. These analyses revealed that the high expression of Ferroportin/IREG transporters in leaves is a highly convergent mechanisms involved in nickel tolerance and hyperaccumulation. Accordingly, functional studies using transgenic N. caerulescens support that the NcIREG2 transporter located on the vacuole is implicated in nickel accumulation.

A better knowledge of these peculiar species and the underlying mechanisms will become instrumental to develop phytoextraction technologies to limit the impact of development on the environment and recycle valuable metals.