Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation

Abstract

Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils.

Keywords: heavy metals; hyperaccumulation; phytoremediation; plant genotype improvement; soil management.

Figure 1

The main aspects of phytoremediation: the main steps involved in phytoremediation of heavy metals, which include (a) metal (yellow dots) adsorption on soil particles or cell walls (induced by rhizosphere metabolism) and compartmentalization of metals into root cell vacuoles (blue circles inside cells), preventing transport to the shoot; (b) metal accumulation in aerial organs (e.g., in vacuoles or trichomes) upon root-to-shoot xylem transport; (c) for particular metalloids (e.g., Se and As), leaf metabolism allows volatilization of the toxic compound.

Figure 2

Main aspects of human intervention to enhance phytoremediation. Steps in which human activity can operate are, (a) the selection, through varietal choice, classical breeding (e.g., somatic hybridization), or transgenic approach, of the most useful plant species to be applied; (b) the enhancement of rhizosphere interconnections between plant growth promoting rhizo- and endophytic bacteria (PGPR and PGPE, respectively, blue dots), and mycorrhizal fungi (drawn in red -arbuscular mychorrizae, AM - and orange), exploiting ex-novo inoculum or the native microflora; (c) the management of the polluted site, in terms of both soil conditions and growth techniques, which can change metal availability, plant growth, and remediation effectiveness.