Plant metallothioneins

In plants, two kinds of metal-binding peptides or proteins are synthesized. Plant metallothioneins are inducible cysteine-rich entities very like those found in animals. Differential expression (induction) of metallothionein genes can be due to both variation of external heavy metal concentrations and the influence of various environmental factors. The principle role of plant metallothioneins seems to be in homeostasis rather than in metal detoxification. Plants are also known to have so-called phytochelatins, which are non-protein thiols specifically induced upon exposure to heavy metals. A close positive relationship between the concentrations of cadmium and phytochelatins in the plant shoot material has been observed and linked to the degree of growth inhibition (Keltjens and Van Beu-sichem, 1998). These observations make the use of phytochelatins promising for the assessment of heavy metal effect on plants. 

The current classification divides metallothioneins into 15 groups and is based on taxonomic parameters and the pattern of distribution of cysteine residues along the metallothionein sequence. Family 15 contains the plant metallothioneins later classified into four different types depending on the distribution of their cysteine residues and cysteine-devoid regions. 

Metalloporphyrins, studies of, 78 591 Metalloreceptors, 76 787 Metallothermic magnesium, 75 343 Metallothermic reduction, rare-earth-metal production by, 74 643 Metallothioneins, as natural defense against silver, 22 655, 657, 681 Metal lubricant, indium and, 74 195 Metallurgical (smelter) plants, 23 792 Metallurgical additives... 

Goldsbrough P., 2000, Metal tolerance in plants The role of phytochelatins and metallothioneins, in Phytoremediation of contaminated soil and water, N. Terry, G. Banuelos, eds, Lewis Publishers, Boca Raton. 

The metabolism of sulphur plays an important role in metal response, since most chelators, including metallothioneins and phytochelatins, contain this element. APSl encodes an ATP sulfurylase and Brassica juncea has been engineered to constitutively express this gene (Pilon-Smits et al., 1999). The plants showed increased S assimilation, higher glutathione (GSH) levels and were more tolerant to Se, accumulating 2-fold higher Se levels in the shoots. 

Since zinc ions have no color their presence has often been overlooked. Zinc ions will doubtless be found in many more places within cells. Zinc is usually the major component of the bound metals in the metallothioneins (Box 6-E). These small 6.6-kDa proteins which contain 33% cysteine and bind as many as six ions of Cd2+, Hg2+, Cu2+, or Zn2+ per molecule are present in all animal tissues as well as in plants and some bacteria.)... 

The metallothioneins have been found in several vertebrate species and in marine invertebrates.1452 Prinz and Weser purified a copper-containing metallothionein from Saccharomyces cerevisiae.1453 Another copper-binding protein was isolated from Neurospora crassa.1452 The first unequivocal demonstration of a metallothionein in a vascular plant was recently reported.1455 The amount of metallothionein in different species and tissues is variable. The concentration has been reported to increase up to 40-fold by the induction of its biosynthesis by certain metals such as cadmium or zinc. In new-born rat liver (one to four days old) the concentration of Zn- and Cu-metallothionein is 20 times that in 70-day-old adult rats.1456 There are several recent reports and reviews in this active area.1243,1467a k... 

The addition of copper, zinc, cadmium or mercury to animals results in the synthesis of a cysteine-rich protein called metallothionein.1147-1149 These proteins have been isolated from a number of sources, and have molecular weights in the range 6000 to 12 000 with a cysteine content of about 30-35% of the total amino acid content. They have also been found in microorganisms and plants. These proteins are thought to play an important role in the storage of zinc and copper, and as a result of their storage capacity, are able to bind and detoxify cadmium and mercury. 

Grill, E., Winnacker, E.-L. Zenk, M.H. (1987). Phytochelatins, a class of heavy metal binding peptides from plants are functionally analogous to metallothioneins. Proceedings of the National Academy of Sciences (USA) 84, 439-43. 

Kawashima, I., Inokuchi, Y., Chino, M, Kimura, M. Shimizu, N. (1991). Isolation of a gene for a metallothionein-like protein from soybean. Plant Cell Physiology 32, 913-16. 

Cobbett, C. and Goldsbrough, P. 2002. Phytochelatins and metallothioneins Roles in heavy metal detoxification and homeostasis. Annual Reviews in Plant Biology, 53 159-82. 

Sanders et al. (1983) have also shown that the effects of Cu(II) on the growth of crab larvae and on their metallothionein with copper chelate buffer systems must be interpreted on the basis of free Cu ion activity. The data obtained reveal predictable relations between [Cu ] in seawater and processes at the cellular and organismic levels. Similarly, the uptake of metal ions by plants (e.g., of aluminum) is usually related to free metal-ion activity. Others have shown that the chelation of a variety of metals reduces the toxicity of metals to organisms for example, a reduction in the uptake of mercury by fish in the presence of EDTA and cysteine a reduction in copper and/or zinc toxicity to... 

In response to heavy metal exposure, certain plants and yeasts produce PCs, analogous to the sulfur-rich protein metallothionein made by vertebrates. PCs are sulfur-rich peptides of the general chemical makeup (y-Glu-Cys) -Gly where = 2 -11 (Fig. 7.7). PCs are... 

Plants contain phytoferritins, which accumulate in non-green plastids in conditions of iron loading. They are targeted to the plastids by a putative transit peptide at their N-terminal extremity, and possess the specific residues for ferroxidase activity and iron nucleation, found in mammalian H-type or L-type ferritin subunits. We already mentioned the presence of metallothioneins in photosynthetic cyanobacteria, and it comes as no surprise that metallothioneins as well as phytochelatins are found in plants where they probably function by protecting from toxic metals. 

Lefebvre, D., B. Miki, and J. Laliberte, Mammalian metallothionein functions in plants. Bio-technology, 1987 5 1053-1056. 

Tolerance to metal stress relies on plant capacity to detoxify metals having entered cells. The postulated mechanisms involve biochemical detoxification, for example by binding to organic acids (especially citrate) or proteins like ferritin, metallothioneins and phytochelatins, and finally compart-mentalization of the metal within the cell. In most plant cells the vacuole comprises more than 80-90% of the cell volume and is acting as a central storage compartment for ions (Briat et al. 1999). 

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