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 Mugineic 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 Mugineic 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.
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. It might be no coincidence that Spermidine and Spermine are present in semen because that also needs a supply of zinc to function efficiently, and both
Spermine, being polyamines, are capable of binding to zinc.
Spermidine and 24-epi-Brassinolide (aka Brassinolide) are plant-growth regulators (but Brassinolide is not universally present in all plants, only a select few such as those belonging to the Cabbage) family (Brassicaceae). The toxic effects of salt or zinc metal stress on
Mung Bean Vigna radiata (a Cabbage Family member) plants are completely overcome by the combination of these two plant-growth regulators. Zinc is in itself an essential micronutrient in all plants responsible for their normal growth and development, and all plants are capable of easily absorbing it. However too much zinc in the soil or water results in zinc toxicity in many plants not equipped to deal with this toxic influx. Plants unable to deal with an excess of zinc in the soil or water suffer inhibition of root-growth and stem-growth resulting in chlorosis and necrosis of leaves, damage to the photosynthetic machinery, significantly altered mitotic cell division as well as altered membrane integrity and permeability. Zinc kills cells in plants ill-equipped to deal with unwanted high concentrations in the soil or water. The presence of Spermidine or other polyamines and 24-epi-Brassionolide (a phytosterol present in many Brassicaceae plants) helps some plants to deal with excess zinc. Many others succumb to the toxic effects of zinc and will not grow in its excess presence.
Polyamines play a large role in managing stress in plants posed by adverse environmental conditions such as drought-stress or stress due to water salinity. They also protect the plant from damaging heavy elements in the soil. There are other polyamines present in (some) plants, namely
NorSpermidine, which are slightly shortened versions of each, with one CH2 moiety removed from the (CH2)4 section making all of the sections (CH2)3.
ThermoSpermine is yet another polyamine and is a positional (aka structural) isomer of Spermine, but whereas Spermine has the four CH2 moieties in the centre, ThermoSpermine has it near one of the ends. It is a new type of
plant growth regulator with widespread presence in the plant kingdom. Unlike Spermine, ThermoSpermine is essential for the normal development of plants. Spermidine can be converted within plants to ThermoSpermine by the enzyme Thermospermine Synthase (TSPMS).
ThermoSpermine is involved in the regulation of stem elongation in (at least) Thale Cress (upon which most research is now based since it has a very small genome), and without it severe dwarfism and over-proliferation of xylem vessels results. The growth hormone Auxin and ThermoSpermine operate against each other, in yet another negative-feedback balancing arrangement. The levels of the polyamines within plants is also highly regulated by the plant, and only increases in response to environmental stress. The hormone Abscisic Acid (ABA) also plays a role in managing stress due to drought conditions.
The interconnectedness of all things is all cross-connected in accordance with the writer Douglas Adams.
Xylemin (CH2)4NH2 (CH2)4NH2 promotes xylem differentiation by the act of inhibiting the synthesis of ThermoSpermine.
Polyamines tend to associate themselves with the macro-molecule RNA until they are required to perform some purpose. There is still a great deal yet to discover about polyamines and their multitudinous roles in both plants and animals. The above just scratches the surface of the plethora of ways in which they might interact with other metabolites.
Proline is a ubiquitous amino acid with a five-membered ring. It is capable of chelating heavy metal ions, 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. List of Salt Tolerant Species. 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).
Halophytes come in two divisions: salt-resistant species (
facultative halophytes) and salt-tolerant species; those which must have salt to thrive and those which can tolerate salt but would just as rather not have to deal with it. Both Annual Sea-blite (Suaeda maritima) and Common Glasswort (Salicornia europaea) belong to the salt-resistant category, in fact, it must have high concentrations of salt in order to germinate properly - it only grows in the tidal zone (or maybe near salt-mines?). It is claimed that all other genera of halophytes are merely salt-tolerant and grow better without the burden of salt in their watery diet, they just tolerate it, but maybe they might have missed out some lesser-known salt-resistant species such as perhaps also Prickly Saltwort (Salsola kali) or
Saltmarsh Grasses (Puccinellia), Sea Kale (Crambe maritima), Yellow Horned-Poppy (Glaucium flavum),
Sea-Lavenders (Limonium), Sea Aster Aster tripolium and many other salt-marsh species but possibly only some
Atriplex) [which can grow well away from the salt-marshes] (?)
In 2015 it was discovered that Melatonin not only occurs in mammals, but is also, much to the astonishment of everyone, ubiquitous in plants. Melatonin is capable of chelating heavy metal elements forming complexes, such as with cadmium and many others. Melatonin is also a highly multi-functional agent which offers environmental protection to the plant from all sorts of enemies such as pests, heat-stress, cold-stress, damaging ultra-violet light and almost anything else - see Hormones and Signalling Molecules.
So, all in all, plants have devised a variety of differing 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.
PHYTO-REMEDIATION OF HEAVY-METAL CONTAMINATED LAND
Some hyper-accumulators of heavy metals are used for the Phytoremediation of land contaminated by heavy-metals. By growing plants such as Water Fern on contaminated waters or
Stinking Fleabane on contaminated land, harvesting the plant every season, and safely disposing of the harvested material over several years it is possible to reduce the burden of heavy metals in contaminated water/land, but it is a slow iterative process sometimes requiring decades. The contamination will never be reduced to zero, since it is subject to the law of exponential decay, like reverberating sound in a church hall or the radioactive decay of radioisotopes.
LIST OF OTHER METALLOPHYTES
Other heavy-metal tolerant plants (metallophytes) include:
[Abbreviations : HyperAcc = HyperAccumulator; Acc = Accumulator;]
[Abbreviations : HyperAcc = HyperAccumulator; Acc = Accumulator;]
- Arrow-head (Sagittaria sagittifolia) [Cd, Cs-137]
Tufted Hair-grass (Deschampsia cespitosa)
- Bladder Campion (Silene vulgaris) [Zn]
- Sea Campion (Silene uniflora)
- Water Fern (Azolla filiculoides) [Acc: Al, As, Cr, Cu, Fe, Pb, Mn, Ni, Zn]
Spring Sandwort (Minuartia verna)
Alpine Pennycress (Noccaea caerulescens) [Mo; Acc: Cr; HyperAcc: Cd, Co, Cu, Ni, Pb, Zn 10gm/kg]
- Field Penny-Cress (Thlapsi arvense)
Alpine Catchfly (Silene alpestris)
- Shetland Mouse-Ear (Cerastium nigrescens)
- Pyrenean Scurvygrass (alpine sub-species of) (Cochlearia pyrenaica ssp. alpina)
Sheep's Fescue (ophioliticola sub-species of) (Festuca ovina ssp. ophioliticola) [Zn]
- Mountain Pansy (Viola lutea)
Moonwort (Botrychium lunaria)
- Maize (Zea mays)
- Dune Helleborine (Epipactis dunensis)
- Mountain Pansy (Viola lutea) [Zn]
- Ribwort Plantain (Plantago lanceolata)
- Ragweed (Ambrosia artemisiifolia) [HyperAcc: Pb]
- Annual Sunflower (Helianthus annuus) [As, Zn, Cs-137, Sr-90]
- English Scurvygrass (Coclearia anglica)
- Pyrenean Scurvygrass (Cochlearia pyrenaica)
- Hydrangea (Hydrangea macrophylla) [Al]
- Broad Bean (Vicia faba) [Al 0.1gm/kg]
- Chives (Allium schoenoprasum) [Cd]
- Oil-Seed Rape (Brassica nappa ssp. oleifera) [Ag, Cr, Hg, Pb, Se, Zn]
- Bilberry (Vacciumium myrtillus [Cr, leaves]
Common Bent (Agrostis capillaris [Acc 0.1gm/kg: Al, Mn, Pb, Zn]
Stinking Fleabane (Dittrichia graveolens) and other Dittichia spp. [esp. Zn]
- Jounama Snow Gum [Au]
- the lichen Usnea flammea
- Yarrow [Cd]
- Bird's-foot Trefoil (Lotus corniculatus) [Zn]
- Kidney Vetch [Zn]
- Harebell [Zn, Pb]
- Thyme (Wild) [Zn, Pb]
- Common Sorrel [Cd]
- The alien Ribbon Fern Pteris vittata [Se, As (27g/kg dry)]
- The alien Sebertia acuminata [Ni (up to 26% Ni dry-weight)]
Barley (Hordeum vulgare) [Al (1g/kg]
Osier species [Cr, Hg, Se, Cd, Pb, U, Zn, organic solvents]
Esthwaite Waterweed Hydrilla verticillata [RRR] [HyperAcc: Cd, Hg, Pb]
- Lucerne (Alfalfa) (Medicago sativa) [Cr]
Giant Duckweed (Spirodela polyrhiza) [HyperAcc: Cd, Ni, Pb; Acc: Zn]
Common Duckweed (Lemna minor) [Acc: Zn; HyperAcc: Cd, Pb]
Least Mallow (Malva parviflora) [Acc:Zn, Cd, Cu]
French Mallow (Malva nicaeensis) [HyperAcc: Zn, As]
- Common Mallow (Malva sylvestris) [Acc: Ni: HyperAcc: Pb, Hg]
Chinese Mallow (Malva verticillata) [Acc: Cd, Pb]
- Water Hyacinth (Eichornia crassipes) [Cs, Sr, U; Acc: Zn; HyperAcc: Cd, Cr, Hg, Pb]
- Buckwheat (Fagopyrum esculentum) [HyperAcc: Pb 4.2g/kg dry or, for soil treated with the biodegradable Methyl Glycine DiAcetic Acid (MGDA), this increases to 21g/kg dry]
Indian Pokeweed (Phytolacca acinosa) [HyperAcc: Mn 19.3g/kg]
Horsetails (Equisetum) species. Many are accumulators of heavy metals, especially for Zinc which they can accumulate to >7000ppm. [HyperAcc: Silica in Field Horsetail - up to 20% to 32% by dry weight! Average Si 35g/kg (dry weight) with September being the month with highest concentration (in Slovakia)]
- The aquatic Moss Warnstofia fluitans which grows in Northern Sweden (where water is often containated with high levels of arsenic) is capable of removing 90% of any Arsenic in an hour from arsenic-polluted drinking water. The moss will need replacing regularly for any use in water treatment plants.
See full List of Salt Tolerant Species (Ellenberg S values 0 to 9)
The metallophyte nature in many of these plants is genetically-adapted; where there are heavy metals present in the soil plants adapt themselves over a considerable time period to cope with the metals. Conversely, where they are no heavy metals present, they don't bother to genetically adapt: it would take effort to so do.
The above list have various affinities for the heavy metals listed on their right. Some are hyperaccumulators, whilst others absorb much less. A hyperaccumulator for Zinc is defined as one which can store 1% of zinc (dried plant, weight for weight) whereas a hyperaccumulator for cadmium is defined as one which can store greater than 0.01% (dried, w/w) of cadmium. Worldwide there are approximately 400 plants which are classed as hyperaccumulators (for one metal or another). Hyperaccumulators of lead seem to be either rare or non-existent because of the extremely-low mobility of lead and therefore not easily accumulated by plants. Other plants are hyperaccumulators only for a few specific heavy metals such as
Alpine Penny-cress (Noccaea caerulescens) which hyperaccumulates zinc and cadmium. The alien Ribbon Fern Pteris vittata which is found in a colliery tip in West Gloucestershire accumulates Selenium from soils which contain it (whether or not the soil in Gloucestershire contains selenium is not known).
The present record breaker hyper-accumulator is the foreign plant Sebertia acuminata which belongs to the Sapotaceae plant family and grows in New Caledonia - it can accumuluate up to 26% by dry weight of the heavy metal Nickel, which it stores as a two low-molecular weight water-soluble organometallic complex in the latex of the plant with 99.4% being complexed by Citrate the rest by
Nicotinamine. However, another source says that the NiII in Sebertia acuminata is coordinated to Methylated Aldaric Acid.
All metallophytes in the UK and Europe are merely metal tolerant, rather than metal-resistant; that is, they would rather not have to deal with heavy metals, and only grow in heavy-metal contaminated areas because there is less competition from other plants which are not heavy-metal tolerant at all. There do not seem to be any plants in the World which are heavy-metal resistant.
This list is incomplete...