Heavy metal-binding peptides and proteins in plants. A review

In plants, two kinds of specific metal-binding peptides or proteins are synthesized. Plant metallothioneins (MTs) and MT-like proteins are cysteine-rich translation products of genes inducible in tissue-specific manner during embryogenesis and plant development. In addition, differential expression of MT-like protein genes could be due to variation of external heavy metal concentrations (especially of Cu2+ and Fe2+), influence of various stress factors (heat shock, sucrose starvation, oxidative stress, wounding, plant pathogens). The principal role of plant MTs and MT-like proteins seems to be in homeostasis of essential transition metals rather than in metal detoxification. Phytochelatins (PCs) have general structure (gamma-Glu-Cys)(n)-Xaa, where n = 2-11 and Xaa amino acids Gly, beta-Ala, Ser, and Glu which depend on the species; the des-Xaa forms of PC also exist. PCs are synthesized in plants and some yeasts by a constitutive enzyme phytochelatin synthase (active only in the presence of free heavy metal ion) from glutathione or its anologue. Despite the PC capability of forming complexes with transition metal ions (their rule in metal homeostasis could not be excluded) and virtually prominent role in Cd2+ detoxification within plant cell, there is no evidence that elevated production of PCs may contribute to differential tolerance and/or could be responsible for the resistance to toxic metals. A review with 172 references.