In response to excess metal, higher plants produce metal-binding peptides ([yEC].G) whose biosynthesis is believed to be mediated by enzymes involved in glutathione (yECG) metabolism. In contrast, animals synthesize metallothioneins, gene-encoded low molecular weight cysteine-rich metal-binding proteins. In an investigation of copper-regulated genes in the copper-tolerant flowering plant Mimulus guttutus, we have isolated a series of cDNA clones identifying two genes which encode a protein with class I metallothionein domains. This represents the first description of a metallothionein gene in a flowering plant.
The review discusses some of the important aspects of the molecular biology of metal tolerances in animals, fungi and plants. First, results of classical ecological and genetieal studies are briefly outlined. The evidence for the occurrence and properties of metal-binding proteins (metallothioneins) and peptides (phytochelatins) in fungi and plants is described. It is concluded that at present there is no firm evidence to suggest that a protein homologous with the metallothioneins of animals and fungi occurs in plants. The discovery of phytochelatins, y-glutamyl peptides, containing only glutamic acid, cysteine and glycine, in plants is described and evidence for their role in heavy metal tolerance is assessed. The difference between sulphur metabolism in animals and plants and its relationship to heavy metal tolerances is discussed in terms of the occurrences of metallothioneins in animals and phytochelatins in plants. Future prospects for research in this area are outlined in terms of identification of plant genes coding for metallothioneins and for the enzymes involved in the synthesis of phytochelatins.
SUMMARY
The uptake of copper by non‐tolerant and tolerant clones of Agrostis stolonifera has been studied. In both clones the roots accumulated more copper than the leaves and the roots of the tolerant clone accumulated more copper than the non–tolerant one. When supplied at concentrations above 10 μM in culture solutions copper moved up into the leaves of the non‐tolerant clone more readily than the tolerant one. Uptake of oxygen by roots was only affected when they were pretreated with copper, the non‐tolerant ones being more sensitive to inhibition than tolerant ones. Over the concentration range tested copper had no effect upon l‐malate‐dehydrogenase (MDH) activity in root extracts of either clone. Extracts prepared from non–tolerant roots pretreated with copper were found to contain less MDH activity than those prepared from tolerant ones. It is suggested that this was due to an inhibition of protein synthesis in the roots of the non‐tolerant clone, which did not occur in the tolerant one.
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