High concentrations of Zn and Cu alter rooting, metallothionein gene transcription and polyamine response in poplar.

Poplar is a good candidate for use in phytoremediation. In order to acquire knowledge on the processes underlying metal detoxification and tolerance in this plant, we analysed the expression profiles of 3 poplar metallothionein (MT) genes, types 1, 2 and 3, in in vitro-grown P. alba cv “Villafranca” after exposure to various concentrations of ZnSO4 and CuCl2. In shoots, MT1 and MT3 mRNA levels were enhanced by Zn, while MT2 transcripts were not affected; on the contrary, Cu treatments did not alter MT transcription. Based on leaf symptoms and root formation, 0.5-1 mM Zn was transiently toxic while 2–4 mM Zn concentrations were increasingly toxic. In shoots, polyamines (PAs) accumulated in a dose-response manner and transcription of their biosynthetic genes was early enhanced by all Zn concentrations. In Zn-treated adventitious roots, PA levels also rose, although their reciprocal ratios were different from those of leaves. Cu caused severe leaf toxicity and strongly impaired adventitious root formation and no changes in PA levels were detected until culture end. The different timing of MT and PA response to Zn and Cu is discussed in relation to the extent of metal toxicity.