Overexpression of PtPCS enhances cadmium tolerance and cadmium accumulation in tobacco

Abstract

Phytochelatins chelate heavy metal ions to decrease their toxicity. The chelates are then transferred to, and stored in, the vacuole. Phytochelatin synthase (PCS), which is involved in phytochelatin synthesis, is thought to be a key enzyme for phytoremediation. In this study, a PCS gene encoding phytochelatin synthase was cloned from poplar (Populus tomentosa Carr.), a widely grown model woody plant that accumulates high levels of heavy metals, especially cadmium. Poplar is considered to have potential applications in phytoremediation. The full-length PtPCS cDNA (1512-bp) encoded a polypeptide of 503 amino acid residues. The PtPCS cDNA was transferred into tobacco by Agrobacterium-mediated leaf disk transformation. The transgenic and wild-type (WT) lines of tobacco were subjected to a one time Cd treatment (90 μmol Cd2+) for 30 days, and then evaluated to determine their Cd tolerance. We evaluated morphological and physiological indices including leaf relative electrolyte leakage, malondialdehyde content, total superoxide dismutase activity, chlorophyll content and root activity. Compared with WT plants, the transgenic plants expressing PtPCS grew better in the Cd treatment and showed significantly higher Cd tolerance. Compared with WT plants, the transgenic lines accumulated higher concentrations of Cd (1.7 to 3.0-fold higher Cd concentration in roots; 1.24 to 2.28-fold higher Cd concentration in leaves). However, the transfer coefficient was lower in the transgenic lines than in wild type. We concluded that PtPCS encodes a functional PCS that may be involved in Cd tolerance and accumulation, but not in Cd transport.