Induction of peroxidase isoforms in the roots of two Verbascum thapsus L. populations is involved in adaptive responses to excess Zn2+ and Cu2+

Abstract

To investigate metal specific responses of root class III peroxidases (POD, EC 1.11.1.7), two populations of Verbascum thapsus L. were exposed to excess Zn2+ or Cu2+ for three weeks in hydroponic culture. One population originating from an uncontaminated area (NMET) and one from an industrial disposal area for jarosite residues from zinc refining (MET) were chosen to test the capacity of V. thapsus to adapt to excess metal in the soil. Exposure to 60 μM Zn2+ led to increased levels of protein carbonyl groups only in the roots of NMET, which was accompanied by higher increase of POD activity and NADH-oxidase activity compared with MET plants. New anionic and cationic POD isoforms were induced in the roots of both populations in response to Zn2+ treatment, while IAA-oxidase activity decreased. On the other hand, root growth was more affected by Cu2+ than Zn2+ in both populations, which was correlated with increased auxin-oxidase (IAA-oxidase) activity. Cu2+ induced an increased activity of anionic POD isoforms in the roots of both populations, yet the ratio of NADH-oxidative to peroxidative POD activity remained higher in NMET than in MET plants. Overall results show differential effects of Zn2+ and Cu2+ on POD activity in the roots of V. thapsus L. In addition, higher tolerance to Zn2+ in MET plants than in NMET indicated that these plants have developed an adaptive mechanism to cope with Zn2+ excess.