Two mulberry phytochelatin synthase genes confer zinc/cadmium tolerance and accumulation in transgenic Arabidopsis and tobacco

Phytochelatin synthase (PCS) is an enzyme involved in the synthesis of phytochelatins, cysteine-rich peptides which play a key role in heavy metal (HM) detoxification of plants. Mulberry (Morus L.), one of the most ecologically and economically important tree genera, has the potential to remediate HM-contaminated soils. However, genes involved in HM detoxification in Morus, such as the PCS genes, have not been identified and characterized. In this study, we identified two Moms notabilis PCS genes based on a genome-wide analysis of the Morus genome database. Full-length MnPCS1 and MnPCS2 cDNAs were 1509 and 1491 bp long, respectively. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that, under 200 mu M Zn2+ or either 30 or 100 mu M Cd2+ stress, the relative expression of each of the two MaPCSs (from Mows alba) was induced in root, stem and leaf tissues within 24 h of exposure to the metals, with Cd2 inducing expression more strongly than did Zn2+. Based on the analysis of total root length and fresh weight of seedlings, overexpression of MnPCS1 and MnPCS2 in Arabidopsis and tobacco enhanced Zn2+/Cd2+ tolerance in most transgenic individuals. The results of transgenic Arabidopsis lines overexpressing MnPCS1 and MnPCS2 suggest that MnPCS1 play a more important role in Cd detoxification than MnPCS2. Zn2+/Cd-2+ concentrations in both shoots and roots of the transgenic Arabidopsis seedlings were higher than in wild type (WT) seedlings at two Zn2+/Cd2+ concentrations. In addition, there was a positive correlation between Zn accumulation and the expression level of MnPCS1 or MnPCS2. Our results indicated that the Morus PCS1 and PCS2 genes play important roles in HM stress tolerance and accumulation, providing a useful genetic resource for enhancing tolerance to HMs and for increasing the HM phytoremediation potential of these plants.