Cadmium Tolerance and Accumulation in Indian Mustard Is Enhanced by Overexpressing γ-Glutamylcysteine Synthetase

Abstract

To investigate rate-limiting factors for glutathione and phytochelatin (PC) production and the importance of these compounds for heavy metal tolerance, Indian mustard (Brassica juncea) was genetically engineered to overexpress the Escherichia coli gshI gene encoding ␥-glutamylcysteine synthetase (␥-ECS), targeted to the plastids. The ␥-ECS transgenic seedlings showed increased tolerance to Cd and had higher concentrations of PCs, ␥-GluCys, glutathione, and total non-protein thiols compared with wild-type (WT) seedlings. When tested in a hydroponic system, ␥-ECS mature plants accumulated more Cd than WT plants: shoot Cd concentrations were 40% to 90% higher. In spite of their higher tissue Cd concentration, the ␥-ECS plants grew better in the presence of Cd than WT. We conclude that overexpression of ␥-ECS increases biosynthesis of glutathione and PCs, which in turn enhances Cd tolerance and accumulation. Thus, overexpression of ␥-ECS appears to be a promising strategy for the production of plants with superior heavy metal phytoremediation capacity.Heavy metals and metalloids such as Cd, Pb, Hg, As, and Se are an increasing environmental problem worldwide. Plants can be used to remove heavy metals by accumulating, stabilizing, or biochemically transforming them. This cost-effective and environment-friendly technology has been called "phytoremediation" (Salt et al., 1995). Hyperaccumulators-plant species that accumulate extremely high concentrations of heavy metals in shoots-offer one option for the phytoremediation of metal-contaminated sites. However, hyperaccumulators tend to grow slower and produce little biomass (Brooks, 1994). An alternative approach is to genetically engineer fast-growing species to improve their metal tolerance and metal-accumulating capacity. A suitable target species for this strategy is Indian mustard (Brassica juncea), which has a large biomass production, a relatively high trace element accumulation capacity (Dushenkov et al., 1995), and can be genetically engineered .Non-protein thiols (NPTs), which contain a high percentage of Cys sulfhydryl residues in plants, play a pivotal role in heavy metal detoxification. The reduced form of glutathione (␥-Glu-Cys-Gly, GSH) is one of the most important components of NPT metabolism. GSH may play several roles in heavy metal tolerance and sequestration. It protects cells from oxidative stress damage, such as that caused by heavy metals in plants (Gallego et al

Up-regulation of antioxidant and glyoxalase systems by exogenous glycinebetaine and proline in mung bean confer tolerance to cadmium stress

“…Several authors have shown that an elevated GSH content is correlated with ability of plants to withstand Cdinduced oxidative stress (Chen and Kao 1995;Zhu et al 1999;Reisinger et al 2008). GSH takes part in the control of H 2 O 2 levels and regeneration of AsA through the AsA-GSH cycle (Foyer and Noctor 2005).…”

A critical review on the bio-removal of hazardous heavy metals from contaminated soils: Issues, progress, eco-environmental concerns and opportunities

“…Studies on this field are increasing and its mechanism is more and more clear [33][34][35][36][37][38][39][40][41]. Using transgenic technology is a tendency in the future to create an ideal species purposely.…”

Phytoextraction: a review on enhanced metal availability and plant accumulation

“…However, overproduction of phytochelatin has played an important role in the attempts to genetically transform high biomass plants into efficient phytoremediators. For example, transgenic seedlings of Brassica juncea overexpressing γ-glutamylcysteine synthetase (γ-ECS), had higher concentrations of PCs than wild genotype seedlings (Zhu et al, 1999). As a consequence, the transgenic plants accumulated more Cd than the wild genotype and possessed shoot Cd concentrations 40% to 90% higher.…”

“…Both ligands are widely distributed in plants and form stable complexes with metals in the cytosol which can be subsequently sequestered into the vacuole (Zenk, 1996;Goldsbrough, 2000). Many physiological and genetic studies indicate that PCs and MTs are critical for metal tolerance and accumulation in plants (Howden & Cobbett, 1992;Zhu et al, 1999;Schmöger et al, 2000;Inouhe et al, 2000;Hartley-Whitaker et al, 2001;Van Hoof et al, 2001). A comprehensive review of PCs and MTs and their characteristics is found in Cobbett & Goldsbrough (2002).…”