Metal tolerance and hyperaccumulation: Costs and trade-offs between traits and environment.

Metal hyperaccumulation is a trait present in over 450 higher plant species. Hyperaccumulators are also tolerant to metals, but hyperaccumulation and tolerance are genetically independent traits. The ecological and biological significance of hyperaccumulation is not clear yet. To provide new insights, this review examines recent literature, in particular focusing on the Cd and Zn hyperaccumulator species Arabidopsis halleri (L.) O’Kane and Al Shehbaz and Thlaspi caerulescens J. et C. Presl. in comparison with the model plant species Arabidopsis thaliana (L.) Heynh. The main aspects considered in the discussion on hyperaccumulation and tolerance involve: (i) uptake of metals, (ii) vacuolar sequestration, (iii) xylem loading, and (iv) chelation with ligands. The review discusses the advancement of knowledge obtained through genetic analysis and molecular biology, together with the use of transgenic approaches and transcriptomics. The most important genes which have been correlated to hyperaccumulation and tolerance in plant species are described and discussed. From the in depth analysis of published results, the main topics for future research are highlighted. Ecological relevance of the hyperaccumulation and tolerance traits in the environment is discussed, with the advantages they can confer to individuals, the possible disadvantages, and the trade-offs between these genetic traits and the environmental conditions.