Molecular and Cellular Aspects of Contaminant Toxicity in Plants: The Importance of Sulphur and Associated Signalling Pathways

Environmental contamination with metals and organic compounds poses a serious threat to human health. Investigating plant responses to these contaminants at the molecular and cellular level is crucial to optimize phytoremediation strategies to clean up contaminated soils. Two key players in plant stress responses are the sulphur-containing amino acids cysteine and methionine. Cysteine is an important constituent of the metal-chelating metallothioneins and is also the precursor for glutathione and subsequent phytochelatin synthesis. During stress conditions, glutathione is involved in (1) metal chelation, (2) xenobiotic detoxification and (3) antioxidative defence. The activated form of methionine, S-adenosylmethionine, is involved in the synthesis of ethylene and polyamines, both playing important roles in signal transduction. This review provides an overview of sulphur uptake and assimilation and its conversion into basic metabolites essential for detoxification and signal transduction during metal and organic contaminant exposure in plants. Furthermore, the cross talk between these pathways and their relation to the contaminant-induced oxidative challenge are discussed.