Role of mycorrhization in the phytoremediation of heavy metals in urban soils

Urbanisation, together with industrial activities and road traffic, represent key sources of pollution in urban ecosystem. They can lead to accumulation of heavy metals in the soils of urban areas, which represents a serious threat to human and animal health. Phytoremediation, and in particular phytoextraction, can be adopted in urban areas in order to remove harmful pollutants from the soil and to reduce the environmental footprint of urban activities. Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that develop a close mutualistic symbiotic association with the roots of a living host plant. Besides facilitating and enhancing plant nutrient uptake, they protect plant roots from heavy metal toxicity and may play a pivotal role in remediation of metal-polluted soils. This study investigated the phytoextraction capacity of two ornamental varieties of the energy crops Brassica juncea (L.) Czern. and Helianthus annuus L. in urban soils contaminated by heavy metals (Co, Cr, Cu, Ni, Pb, and Zn). The presence of fungal symbiosis was examined through morphological and molecular analysis and its possible contribution to pollutant extraction was evaluated. Heavy metals were mainly stored in the root system of the two examined species and sunflower displayed the highest phytoextraction capacity. Mycorrhizal colonization was high in sunflower and low in mustard, which is notoriously a nonhost plant. Interestingly, high levels of mycorrhizal colonization corresponded to increased accumulation of Co and Cu in the upper plant parts, and to low accumulation in the roots, suggesting an active role of AMF in the translocation of pollutants. The combination of AMF and ornamental host plants characterised by high harvestable biomass production (e.g., sunflower) can lead to an efficient phytoextraction system, with high potential for urban green infrastructures, which are subject to contamination by harmful pollutants.