Arbuscular mycorrhiza in metal hyperaccumulating plants

Abstract

Metal hyperaccumulating plants are in the focus of research in recent decades due to their applicability in phytoremedial techniques. They are characterized by the ability to accumulate very high concentrations of certain elements, far in excess of normal physiological requirements (if any), and far in excess of the levels found in the majority of other species tolerant of metalliferous soils. Until recently, they were widely accepted as non-mycorrhizal. Members of the Brassicaceae, however, seem to be colonized by arbuscular mycorrhizal fungi (AMF), at least during the reproductive phase of development. Despite their high metal tolerance, symbioses with indigenous AMF in general decrease metal accumulation and improve mineral nutrition, with an obvious impact on the improvement of plant performance on metal polluted sites. Despite the low AMF spore diversities and densities commonly observed in metal enriched soils, the existing fungal colonisers are presumably the best suited to cope with the existing microclimatic and microedaphic conditions. Reduced mycorrhizal colonization levels frequently seen in metal enriched soils presumably arise from both the composition of the plant communities, usually comprised of plant species with low to moderate mycorrhizal colonization levels, and low levels of mycorrhizal colonization of the specimens of particular plant species, which are however still within the limits for that plant species, thus maintaining the consistent hierarchy. In addition, intense formation of intraradical fungal spores is frequently found. All these characteristics of mycorrhizal colonization may be seen as a plant mycorrhizal strategy on highly metal contaminated environments. Therefore the interactions of compatible AMF with hyperaccumulating plants deserve further attention in the contemporary phytoremedial biotechnology.