Arbuscular mycorrhizal fungi as a tool to ameliorate the phytoremediation potential of poplar: biochemical and molecular aspects

Poplar is a suitable species for phytoremediation, able to tolerate high concentrations of heavy metals (HMs). Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with the roots of most land plants; they improve nutrient uptake and enhance phytoextraction of HMs while alleviating stress in the host plant. This review summarizes previous results from field and greenhouse studies conducted by us and dealing with this topic. In a field trial on a highly Zn- and Cu-contaminated site, differences in plant survival and growth were observed among 168 clones originating from natural populations of Populus alba L. and Populus nigra L. from northern Italy. After two and a half years from planting, the density, activity and metabolic versatility of the culturable fraction of the soil bacteria in the HM-polluted field was higher in the soil close to where larger poplar plants were growing, in spite of comparable HM concentrations recorded in these soils. One well-performing clone of P. alba (AL35), which accumulated a higher concentration of both metals and had high foliar polyamine (PA) levels, was used for further investigation. In a greenhouse study, AL35 cuttings pre-inoculated with AMF (Glomus mosseae or Glomus intraradices) and then transferred to pots containing soil, collected from the HM-polluted site, displayed growth comparable to that of controls grown on unpolluted soil, in spite of higher Cu and Zn accumulation. Such plants also showed an overall up-regulation of metallothionein (MT) and PA biosynthetic genes, together with increased PA levels. A genome-wide transcriptomic (cDNA-AFLP) analysis allowed the identification of a number of genes, mostly belonging to stress-related functional categories of defense and secondary metabolism, that were differentially regulated in mycorrhizal vs. non mycorrhizal plants. A proteomic analysis revealed that, depending on sampling time, changes in protein profiles were differentially affected by AMF and/or HMs. It is concluded that soil-borne microorganisms affect plant performance on HM-polluted soil. In particular, mycorrhizal plants exhibited increased capacity for phytostabilization of HMs, together with improved growth. Their greater stress tolerance may derive from the protective role of PAs, and from the strong modulation in the expression profiles of stress-related genes and proteins.