How do Plants Cope with Heavy-Metal Stress?

The following article will guide you about how plants cope with heavy-metal stress.

Heavy-Metal Stress in Plants:

A heavy-metal is any one of a number of high atomic weight elements which has the properties of a metallic substance at room temperature. There are several different definitions concerning as to which elements fall in this class designation. According to one definition, “heavy-metals are a group of elements between vanadium and bismuth on the periodic table of the elements having specific gravities greater than 4.0”.

As has been discussed earlier in chapter 8, the plants require relatively small number of elements for their normal growth and development. Many other elements present in the environment especially in soil, do find their entry into plants in-spite of selectivity of root cell membranes. Some of these elements, especially the heavy-metals such as lead, cadmium, mercury, chromium and silver or metalloids such as arsenic, may be highly toxic to them.

Beside these, some essential micronutrients which are also heavy-metal elements such as nickel, copper and zinc, may be injurious to plants when present in excess quantities. Therefore, the plants growing in soils contaminated with higher concentrations of heavy-metals have to undergo heavy-metal stress.

Although, these heavy-metals are universally present in environment-soil, water and air in trace amounts, but the global anthropogenic (i.e., due to human activity) heavy-metal inputs to ecosystem through soil, water and air have increased substantially over the last century and are increasingly becoming a problem in agriculture and to human health.

Unrestricted mining, manufacturing, municipal waste disposal practices and deposits from atmospheric emissions, all contribute to increased levels of heavy-metals in the environment; Natural geochemical processes may also cause unusually high levels of heavy-metals in some localized regions. According to an estimate, the total toxicity of heavy-metal pollutants added to the environment every year exceeds the combined toxicity of all organic and radioactive waste.

Heavy-Metal Toxicity:

The toxicity of many heavy-metals results from their ability to cause oxidative damage to tissues that include:

(i) Increased lipid peroxidation in membranes,

(ii) DNA damage and

(iii) Oxidation of sulfhydryl groups of proteins.

Heavy-Metal Resistance in Plants:

Plant species differ greatly in their sensitivity to heavy-metals. Some plant species are sensitive to heavy-metals and therefore, are susceptible to injury while others may withstand increased concentration of heavy-metals without much damage.

The plants cope with heavy-metal stress in two ways:

(i) Heavy-metal avoidance and

(ii) Heavy-metal tolerance.

Heavy-Metal Avoidance:

Some plant species may thrive well on soils rich in heavy-metals by avoiding uptake of heavy-metals due to normal selectivity of root cell membranes.

Heavy-Metal Tolerance:

Some plant species absorb heavy-metals and accumulate them in their cell vacuoles without much damage to a level that would be lethal to sensitive or non-tolerant species. Such plants are called as accumulators or super-accumulators or heavy-metal tolerant plants. For example, a number of plant species endemic to nickel-rich soils (serpentine soils), may accumulate nickel in excess of 1000 µgg^-1 dry weight, compared with normal concentration of about 0.05 µgg^-1 dry weight.

Thlaspi goesingense (family Brassicaceae) may accumulate nickel to 9.5 mg/g or about 1% of its dry weight. Viola calaminaria (family Violacere) is known ₍to accumulate zinc and lead in excess of 10,000 µgg^-1 dry weight. Some species of Astragalus (family Fabaceae) and Stanleya pinnata (family Brassicaceae) may accumulate selenium up to 10% of dry weight of their seeds. Possibilities are being explored actively at present for potential use of hyper-accumulators in phytoremediation for reclamation of heavy-metal contaminated soils in agriculture.

Mechanism of Heavy-metal Tolerance:

Plants adopt two main strategies to cope with excess heavy-metal ions, accumulated in their cells by detoxifying them which involve:

(i) Combination or complexation of the toxic heavy-metal with organic compounds and

(ii) Their compartmentation within the cell vacuoles.

Following organic compounds are usually produced by plants that are capable of forming complexes with heavy-metal ions:

(i) Organic Acids:

The cell vacuoles of heavy-metal treated plants often contain high concentrations of metals complexed with organic acids, Citrate and malate for example, are known to bind with cadmium and zinc respectively.

(ii) Amino Acids:

Amino acids such as histidine may also form complexes with metal ions. Kramer et al (1996) have shown nickel to be complexed with histidine in nickel hyper-accumulator plant species Alyssum lesbiacum (family Brassicaceae). According to them, histidine synthesis is important mechanism for hyper-accumulation of nickel.

(iii) Phytochelatins:

These are any of a group of sulphur-rich metal binding polypeptides found in plants. They are unusual peptides with general formula (-glutamic acid-cysteine)_n– glycine where n = 2 to 11. (The unusual structure is due to the fact that the peptide bond between glutamic acid and cysteine involves -carboxylic group of glutamic acid instead of its α-carboxylic group which is characteristic of proteins see Fig. 23.4)

Phytochelatins are synthesized from the tripeptide glutathione (GSH), the most abundant thiol (-SH) compound in plants. (For structure of GSH, see Chapter 10). It is believed that phytochelatins serve to shuttle toxic metal ions from cytoplasm to the cell vacuole where they are sequestered by organic acids. (Phytochelatins accumulate in cell vacuoles of plants that are exposed to excess heavy- metals and are considered to be indicators of heavy-metal stress).

(iv) Metallothioneins:

These are small metal binding proteins which may protect cellular constituents from Oxidative damage due to heavy-metals. However, their exact role in metal tolerance is not yet clear.