Coordinated responses of phytochelatin synthase and metallothionein genes in black mangrove, Avicennia germinans, exposed to cadmium and copper.

To evaluate the role of phytochelatins and metallothioneins in heavy metal tolerance of black mangrove Avicennia germinans, 3-month-old seedlings were exposed to cadmium or copper for 30 h, under hydroponic conditions. Degenerate Mt2 and PCS primers were synthesized based on amino acid and nucleotide alignment sequences reported for Mt2 and PCS in other plant species found in GenBank. Total RNA was isolated from A. germinans leaves and two partial fragments of metallothionein and phytochelatin synthase genes were isolated. Gene expression was evaluated with reverse transcripatase-polymerase chain reaction (RT-PCR) amplification technique. Temporal analysis showed that low Cd2+ and Cu2+ concentrations caused a slight (but not significant) increase in AvMt2 expression after a 16 h exposure time, while AvPCS expression showed a significant increase under the same conditions but only after 4h. Results strongly suggest that the rapid increase in AvPCS expression may contribute to Cd2+ and Cu2+ detoxification. Moreover, we found that A. germinans has the capacity to over-express both genes (AvMt2 and AvPCS), which may constitute a coordinated detoxification response mechanism targeting non-essential metals. Nonetheless, our results confirm that AvPCS was the most active gene involved in the regulation of essential metals (e.g., Cu2+) in A. germinans leaves.