Uninhibited biosynthesis and release of phytosiderophores in the presence of heavy metal (HM) favors HM remediation

Production of phytosiderophore (PS) has been causally related to iron-deficiency tolerance in cereals. However, PS can also chelate heavy metal and thus may represent a viable phytoextraction strategy on contaminated soils. Two separate experiments were conducted to assess the affect of heavy metal on phytosiderophore biosynthesis and their release in the rhizosphere of wheat. Root exudates were collected from 10-day-old wheat seedlings raised on Fe-deficient nutrient solution in the presence of 2.5, 5.0, and 10 mM concentration of different heavy metals (Cd, Pb, and Ni) for 3-day period, for the phytosiderophore and the heavy metal analysis. Plant uptake of respective heavy metal was positively correlated with the heavy metal concentration of the nutrient solution. Phytosiderophore release was positively influenced in the presence of the heavy metal. Increasing concentration of Cd, Pb, and Ni showed positive correlation with the PS release until 5 mM concentration followed by a decline at 10 mM. However, a higher induction of PS release was measured in wheat seedlings treated with Cd and Pb than Ni. Further, transcript expression analysis of nicotianamine synthase (NAS) and nicotianamine amino transferase (NAAT), involved in phytosiderophore biosynthesis, was done in roots of 10-day-old Fe-deficient wheat subjected to 2.5, 5.0, and 10 mM of Cd, Pb, and Ni. Both NAS and NAAT were expressed not only under Fe deficiency but also in the presence of Cd, Pb, and Ni. Sequencing of partial cDNA of NAS revealed a nucleotide length of 998 bp, while multiple sequence alignment of NAS with HvNAS revealed 92% sequence similarity. This study irrevocably shows that phytosiderophore biosynthesis and release are not impaired in the presence of heavy metal and that phytosiderophore mediates the uptake of toxic heavy metal.