Metallothionein multigene family expression is differentially affected by Chromium (III) and (VI) in Solanum nigrum L. plants

Solanum nigrum L. plants were grown in hydroponic systems exposed to Cr (III) or Cr (VI) under two different exposure regimes (a 4-week prolonged treatment with lower concentrations and a 1-week shock treatment with higher concentrations), to determinate the effects of Cr exposure on plant metabolism and growth and to define the Cr phytoremediation potential of this plant species. The data obtained shows that only Cr (VI) induced the appearance of visible symptoms of metal toxicity (reduction of root and shoot growth and fresh mass), while none of the Cr oxidation states caused induction of lipid peroxidation, in terms of malondialdehyde content, neither affected total chlorophyll and carotenoids content. An increase of free proline content in shoots from plants exposed to both Cr (VI) treatments and to a prolonged 375 μmol L−1 Cr (III) exposure was found, as well as in roots from shock treatments with both metals. The levels of Cr (III) and Cr (VI) quantified in these plants' tissues do not allow this plant species to be considered as a hyperaccumulator for Cr, but these plants can be used to phytostabilize this pollutant. The metallothioneins (MT) mRNA accumulation analysis showed that Cr (III) induced an increased accumulation of MT2a-related transcripts in both roots and shoots, and of MT1- and MT2d-related transcripts only in roots, whereas Cr (VI) induced the accumulation of MT2a- and MT2d-related transcripts only in roots from the shock treatment, and the de novo accumulation of the MT2c-related transcripts only in shoots, suggesting that these MTs are related to the Cr homeostasis in S. nigrum.