Detection of Phytochelatin in the Xylem Sap ofBrassica napus

Abstract

Phytoremediation uses plants for environmental cleanup. However, various shortcomings have prevented routine adoption and use of this technology by engineers. For the purposes of heavy metal extraction, one such shortcoming has involved insufficient knowledge of plant metal trafficking biochemistry. This research focuses on the xylem sap concentrations of thiols such as phytochelatins (PCs), which are small peptides with the sequence (γ-Glu-Cys)n-Gly and are a major class of heavy metal-binding peptides in plants. Despite their importance in heavy metal binding and sequestration, their possible role in root-to-shoot transport is not well understood and is the source of some conflicting reports in the research literature. However, understanding root-to-shoot transport is critical for rational design and successful use of phytoremediation. In this study, we used a flowing culture system to grow Brassica napus and subsequently analyze its xylem sap for the presence of PCs and other thiols in response to low, but environmentally relevant, Cd concentrations ranging from 0.5 to 5.0 μM. Results indicated that PCs and cysteine were detectable in the B. napus xylem sap within 24 h of initial Cd exposure and responded to Cd in a time- and dose-dependent manner. This work is the first to focus on low Cd concentrations and demonstrate temporal and concentration-dependent PC responses in the xylem sap, which may ultimately help in developing better plants for heavy metal extraction.