Biomolecular approaches to understanding metal tolerance and hyperaccumulation in plants

Trace metal elements are essential for plant growth but become toxic at high concentrations, while some non-essential elements, such as Cd and As, show toxicity even in traces. Thus, metal homeostasis is tightly regulated in plants. Plant species colonising metalliferous soils have evolved mechanisms to hypertolerate metals and, in rare cases, can hyperaccumulate them in excess amounts in their shoots. The molecular mechanisms of metal hypertolerance and hyperaccumulation are likely derived from alterations in the basic mechanisms involved in general metal homeostasis. Genes involved in metal transport, synthesis of metal chelators and oxidative stress responses are constitutively and highly expressed in metal hypertolerant and hyperaccumulator species. Plant specialized metabolites and cell wall components have been proposed as major players in these mechanisms. In addition, the high intra-specific natural variation of metal hypertolerance and hyperaccumulation suggests that various molecular mechanisms might be involved in the evolution of these traits. To date, the potential of wild plant populations as systems to study metal tolerance and hyperaccumulation has not been fully exploited. The advent of next-generation sequencing (NGS) has enabled the study of non-model species, providing an opportunity to study natural populations and new tolerant and/or hyperaccumulating species, and will provide new insights into metal tolerance and hyperaccumulation. In this review we highlight background knowledge about metal tolerance and hyperaccumulation in plants and the current state-of-the-art techniques to study and identify the underlying mechanisms of metal hypertolerance and hyperaccumulation. We also outline for the reader the importance of the multidisciplinarity of this research field and how the integration of multiomic approaches will benefit facing the future scientific challenges.