In order to fulfil their requirements of exclusively inorganic nutrients, plants must interact tightly with their adjacent soil while handling the challenges arising in this differently composed sphere. In order to understand the genetic basis of plant-soil interactions in an ecological and evolutionary context, we focus on the species Arabidopsis halleri, within which exceptionally diverse adaptations to local soil composition are known. Among the closest relatives of the model species A. thaliana, A. halleri is a zinc (Zn) and cadmium (Cd) hyperaccumulator, meaning that single individuals contained more than 10,000 µg g^-1 Zn or more than 100 µg g^-1 Cd in leaf dry biomass at their natural sites of growth in the field. Populations of A. halleri are naturally found on calamine metalliferous soils containing toxic levels of Zn, Cd and lead (Pb), but they are more widespread on pristine non-metalliferous soils. Hyperaccumulation of Cd was observed in a subset of populations on both types of soil, thus indicating that the enrichment of the non-essential metal Cd in leaves can be enormously efficient in A. halleri. We expect these diverse phenotypic characteristics to be of interest for the future improvement of phytoremediation and phytomining applications.

We combine fieldwork with genetic and genomic as well as molecular biology approaches in order to understand how diverse adaptations to local soils function and arise in A. halleri, focusing here on soil Cd levels. Cd is a comparably rare soft metal that generally lacks a nutritional function and has very high toxicity potential in biological organisms. Progressively enriched with successive trophic levels, elevated environmental Cd levels are an important human health concern. Similar to other cells, plant roots take up Cd²⁺ cations inadvertently through transmembrane transport proteins, for example of divalent nutrient cations Fe²⁺, Zn²⁺ and Ca²⁺, and they universally possess a molecular machinery capable of conferring low-level basal Cd tolerance. Elevated Cd tolerance of A. halleri accessions originating from geogenically or anthropogenically Cd-enriched environments indicate that Cd can be the source of substantial selective pressure in natural environments. We will discuss first candidate sequence variants found in populations on extremely Cd-contaminated soil through genome-wide association studies in A. halleri. These are beginning to provide information on the targets of Cd toxicity in natural plant populations. Finally, comparative transcriptomics of A. halleri populations from contrasting environments detail plants’ needs for nutrient balancing in Cd-rich environments. In summary, we have taken first steps towards understanding how opposing directions of selection in differing environments have led to a large extent of within-species phenotypic variation in Cd handling in A. halleri.