Response of legumes, rhizobia and their symbiosis to toxic heavy metals

Abstract

Pollution of soils with toxic heavy metals is increasing over the world due mainly to anthropogenic activities. Natural soils containing high levels of heavy metals also exist. In polluted soils, plant production for food and feed is a risk for human and animal health. To overcome this problem two strategies have been proposed: soil bioremediation with plants and their associated microorganisms, and culture of plants with low capability to translocate heavy metals to the edible parts. Legumes are promising candidates for these proposals; in fact, legumes inoculated with rhizobia isolated from heavy metal-polluted soils have been successfully used in polluted soils. Plants and microorganisms display mechanisms of response to toxic heavy metal stress, which included uptake, translocation and accumulation of the metal to avoid toxic effects of the metals. Structural and ultrastructural alterations of cells and tissues are also responses to heavy metals. In bacteria and plants upon heavy metal stress, common responses to another abiotic stresses (imbalance of the redox homeostasis, hormonal alteration, etc.) and more specific responses (transporters, detoxification by proteins and enzymes, etc.) can be found. Studies of transcriptomic, proteomic, metabolomic, ionomic, etc. are revealing new factors involved in these responses. These new approaches might help to draw the network of molecular, biochemical and physiological changes that allow the maintenance of the cellular homeostasis and the metabolic reprogramming. So far, the responses of legumes, rhizobia and their symbiosis to toxic heavy metals have not been extensively studied. A better knowledge of these responses will allow the selection of more tolerant rhizobialegume partners. We will present an overview of the responses of legumes and their microsymbionts to heavy metals, mainly focused on cadmium and mercury. We will present and discuss our results on Medicago, Lupinus, Ensifer and Bradyrhizobium and on their symbiosis.