HEAVY METAL PROCESSES WITHIN PLANTS
Heavy metals are normally toxic to plants but not to Thrift. Thrift sequesters heavy metals particularly copper, and to a lesser extent cadmium, mercury, zinc, nickel, iron and manganese in that order. It is a hyperaccumulator of heavy metals and can be usefully employed as a phytoremediator to clean up contaminated lands. The copper is picked up by the roots and is to be found in the roots and leaves, where it is preferentially bound to proteins. Because copper stresses the cell, heat shock proteins are involved. Thrift concentrates the copper by between 2000 and 4000 times greater than other plants growing in the same area, it is a hyperaccumulator of copper. The heavy metals can appear on the surface of the leaves as a precipitate. It is no coincidence that the Family Name (Plumbaginacaea) has the same roots as the latin name for lead (plumbum, chemical abbreviation Pb). It is able to grow in heavily contaminated soils where other plants may struggle to survive, such as salt marshes, serpentine rocks, very acidic soils and heavy metal mine tailings and waste heaps near lead and zinc mines. Those growing on serpentine rocks have a slightly different appearance and may be a different species, but authorities disagree.
Upon exposure to heavy metals, plants respond by synthesising a variety of compounds to deal with it. These include glutathione, phytochelatin, the amines spermine, spermidine, putrescine, nicotianamine and mugineic acid and the amino acid proline. Proline may be involved as the chelating agent to sequester the heavy metals, but many other mechanisms and compounds may be involved.
Glutathione is a tri-peptide present in all plant and animal cells and which is unusually sensitive to toxins within cells. It possesses an active thiol group (SH) and is an anti-oxidant, reducing any poisonous hydrogen peroxide. It is involved in the pathways against plant pathogens and plant defence signalling.
Glutathione is the pre-cursor to the generation of phytochelatins, which are of variable length (n). Phytochelatins are produced from glutathione by the action of the enzyme phytochelatin synthase. As the name implies, phytochelatins are effective chelators (sequesters) of any toxic heavy metals that may find their way into plants, such as lead or cadmium, etc. When heavy metal ions enter any cells, they bind to glutathione on the thiol (SH) group, blocking the active region. Because Phtyochelatin synthase uses glutathione in the blocked state to produce phytochelatin, more is produced when heavy metal ion concentration increases. When the phytochelatin has absorbed a heavy metal ion, it is then sequestered safely away into a vacuole, where it accumulates.
Nicotianamine occurs in all plant cells where it chelates both Fe3+ and Fe2+ ions and appears to be involved in the internal transport of iron and other metals. The scavenging of Fe3+ and of nickel (whose toxic concentration is high in soils made from serpentine rock) may be important in protecting the cell from oxidative damage. It is chemically similar to Mucineic Acid, which is also ubiquitous in all plants. Both possess a four membered ring. Mugineic Acid, another metal chelator, is an amino acid which is excreted by some grass plants when they are deficient in iron. Thus deposited in the soil, the Mucineic Acid forms a complex with the previously un-available iron in the soil, mobilising it and enabling its subsequent uptake by the roots. By this means the iron is made available to the plant. They are also able, by similar means, to transport wanted zinc into the plant when the plant is deficient in zinc. But this mechanism might also transport other and un-wanted heavy metals into the plant, which the plant will then have to deal with (by chelating it safely away). Shown is the Mugineic Acid complex with ferric iron ready for uptake by the roots.
Putrescine, Spermidine and Spermine are all poly-amines found in all plant cells. Both bind to the phosphate backbone of nucleic acids. The polyamines are crucial to cell migration, proliferation and differentiation in both plants and animals, so are tightly regulated within cells. Spermidine stimulates the enzyme T7-RNA polymerase. Spermine stabilises the helical structure of RNA, particularly of virii. Both Spermine and Spermidine were first discovered in human semen. Both are now used in skin-care beauty creams. Spermidine and Spermine are derivatives of Putrescine, which smells putrid and is excreted by cells as a means of discarding polyamines.
Proline is a ubiquitous amino acid with a five-membered ring. It is capable of chelating heavy metal ion, but it also plays a role in managing the osmotic pressure within plants particularly with regard to salt and sea-water. Thus Proline may confer salt-tolerance. Growing near the sea or on mine waste tips Thrift is one such halophyte (salt-tolerant plant) which needs to regulate its sodium uptake and water loss. However, salt tolerance is conferred not by any one molecule, but by a whole raft of differing mechanisms and molecules, such as proline, glycine, glycine betain, proline betain, tertiary amines, choline o-sulfate, di-methyl sulfonium propironate etc, etc... Thrift is more salt tolerant than most, but does not grow in the sea-water like some halophytes (Common Cord-Grass (Spartina anglica) for instance).
So, all in all, plants have devised means of importing required essential heavy elements like zinc from soil to plant, especially if they are deficient in that element. Unfortunately, that same mechanism also imports excess nutrients over and above what it requires whilst at the same time not discriminating between unwanted toxic heavy elements such as lead or cadmium. Most plants have a strategy of safely sequestering away these unwanted absorbed heavy elements, but only up to a certain point; they are un-able to cope with high toxic loads in the soils and cannot flourish in such soils. In effect, they poison themselves. However, some plants such as Thrift and Bladder Campion are able to chelate large amounts of heavy elements safely away into vacuoles and are thus capable of tolerating, or even thriving, on soils so heavily laden with toxic heavy metals that they are the only few plants able to colonise such areas. A worthwhile strategy. It has paid off.
It is possible to utilise the affinity for metals in plant hyperaccumulators in a process called phytomining. However, the harvesting of the plants and extracting of metals from the biomass of the plant is an expensive process and yields are lowish. It is not currently cost-effective to mine cheap metals such as lead, copper or zinc by phytomining, but it may be more profitable for higher value metals such as thallium, nickel or cobalt. Gold would be even more promising, but no hyperaccumulator of gold has yet been found, although certain coniferous trees can accumulate gold up to parts per billion in the plant tissue. Gold under natural conditions is highly insoluble in soil, and this limits its bioavailability to plants.
INDUCED HYPERACCUMULATION OF GOLD
But induced hyperaccumulation, by application of chemicals to the soil to promote bioavailability, can provide the basis for commercial extraction.
Indian Mustard (Brassica juncea) has been experimentally induced to accumulate gold in leaf tissues up to 57ppm by dry weight. Lucerne (Medicago sativa ssp. sativa) when used with the inducing agent
Thiourea can extract gold, but
Garden Radish (Raphanus sativus),
Beet (Beta vulgaris) and Wild Carrot (Daucus carota) have all been shown to accumulate gold up to 200ppm by dry weight. Other gold inducers are
ammonium thiosulfate. With the latter as the inducer
Wild Turnip (Brassica rapa ssp. campestris) hyperaccumulates gold up to 304ppm by dry weight. However, as far as the Author can ascertain, none of these methods is currently used commercially to phytomine gold or any other metal.
LIST OF OTHER METALLOPHYTES
Other heavy-metal tolerant plants (metallophytes) include:
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