Phytostabilization of nickel by the zinc and cadmium hyperaccumulator Solanum nigrum L. Are metallothioneins involved?

Some heavy metals (HM) are highly reactive and consequently can be toxic to living cells when present at high levels. Consequently, strategies for reducing HM toxicity in the environmental must be undertaken. This work focused on evaluating the Nickel (Ni) accumulation potential of the hyperaccumulator Solanum nigrum L., and the participation of metallothioneins (MT) in the plant Ni homeostasis. Metallothioneins (MT) are gene-encoded metal chelators that participate in the transport, sequestration and storage of metals. After different periods of exposure to different Ni concentrations, plant biometric and biochemical parameters were accessed to determine the effects caused by this pollutant. Semi-quantitative RT-PCR reactions were performed to investigate the specific accumulation of MT-related transcripts throughout the plant and in response to Ni exposure. The data obtained revealed that Ni induced toxicity symptoms and accumulated mostly in roots, where it caused membrane damage in the shock-treated plants, with a parallel increase of free proline content, suggesting that proline participates in protecting root cells from oxidative stress. The MT-specific mRNA accumulation analysis showed that MT2a- and MT2d-encoding genes are constitutively active, that Ni stimulated their transcript accumulation, and also that Ni induced the de novo accumulation of MT2c- and MT3-related transcripts in shoots, exerting no influence on MT1 mRNA accumulation. These results strongly suggest the involvement of MT2a, MT2c, MT2d and MT3 in S. nigrum Ni homeostasis and detoxification, this way contributing to the clarification of the roles the various types of MTs play in metal homeostasis and detoxification in plants.