Reduced Silver Nanoparticle Phytotoxicity in Crambe abyssinica with Enhanced Glutathione Production by Overexpressing Bacterial γ-Glutamylcysteine Synthase

Silver nanoparticles (Ag NPs) are widely used in consumer products, and their release has raised serious concerns about the risk of their exposure to the environment and to human health. However, biochemical mechanisms by which plants counteract NP toxicity are largely unknown. We have previously engineered Crambe abyssinica plants expressing the bacterial gamma-glutamylecysteine synthase (gamma-ECS) for enhancing glutathione (GSH) levels. In this study, we investigated if enhanced levels of GSH and its derivatives can protect plants from Ag NPs and AgNO3 (Ag+ ions). Our results showed that transgenic lines, when exposed to Ag NPs and Ag+ ions, were significantly more tolerant, attaining a 28%-46% higher biomass and 34-49% more chlorophyll content, as well as maintaining 35-46% higher transpiration rates as compared to those of wild type (WT) plants. Transgenic gamma-ECS lines showed 2-6-fold Ag accumulation in shoot tissue and slightly lower or no difference in root tissue relative to levels in WT plants. The levels of malondialdehyde (MDA) in gamma-ECS lines were also 27.3-32.5% lower than those in WT Crambe. These results indicate that GSH and related peptides protect plants from Ag nanotoxicity. To our knowledge, this is the first direct report of Ag NP detoxification by GSH in transgenic plants, and these results will be highly useful in developing strategies to counteract the phytotoxicty of metal-based nanoparticles in crop plants.