Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

Abstract

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance.The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils.If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.Key-words: glucosinolates; glutathione; hypertolerance; hypotolerance; metallothionein; phytochelatin; zinc/ cadmium ratio.Abbreviations: EC50, effect concentration of a metal(loid) which diminishes plant performance by 50%, often measured as root elongation and/or plant biomass.

Cadmium stress tolerance in crop plants

“…S has occupied an important place after N, P and K. It is the most important macronutrient for normal growth and development of plants with numerous biological functions 56,57 after N, P and K. S is known for its role in the formation of sulfur-containing amino acids (Cys and Met), and synthesis of proteins, vitamins, chlorophyll and GSH involved in stress tolerance. [58][59][60][61] It has been reported that adequate supply of S enhances the photosynthetic potential and growth of crop plants.…”

Glutathione and Transition-Metal Homeostasis inEscherichia coli

“…The sequestration of heavy metal cations by phytochelatin and the consecutive transport of the bis-glutathionato complexes into vacuoles seem to be the main route of heavy-metal detoxification in these organisms. Phytochelatins can be produced successfully in transgenic bacteria, leading to enhanced metal accumulation in the bacterial cytoplasm (3,11,28,69,72) without the compartmentation ability of a vacuole.Why do bacteria detoxify heavy metals mainly by efflux (49, 50) even though they should be able to sequester metals to the phytochelatin educt GSH? In GSH, two cysteine residues and four carboxyl groups should be able to form octahedral bisglutathionato complexes that are very stable (76).…”

Cadmium Toxicity in Glutathione Mutants ofEscherichia coli

“…The main cellular thiol in cyanobacteria and proteobacteria is glutathione (GSH), which is absent in many other prokaryotes that contain other thiol compounds like mycothiol and ergothioneine (14,39). GSH (L-␥-glutamyl-Lcysteine-glycine) is essential in Saccharomyces cerevisiae for full cadmium resistance (18) and is the main substrate for the synthesis of the cadmium-binding polypeptide phytochelatin in many plants and fungi (11,13). GSH is synthesized from glutamate, cysteine, and glycine in two ATP-consuming steps (involving GshA and GshB) via the intermediate ␥-glutamylcysteine (␥EC).…”