Plant Physiology - Musienko M. M. 2001

Adaptation and Plant Resistance Mechanisms
Plant Resistance to Heavy Metal Pollution

Recently, due to increasing anthropogenic pressure, environmental pollution by heavy metals has grown significantly. The term "heavy metals" refers to metals with a density exceeding 5 g/cm³ or an atomic number greater than 20. Among chemical elements, heavy metals are extremely toxic, second only to pesticides. The phytotoxicity of heavy metals depends on such chemical properties as valence, ionic radius, and complexing capacity.

The Cell wall and Plasmalemma serve as the first barrier for heavy metals entering The Cell. Alterations in membrane permeability induced by heavy metals disrupt ion balance, Water relations, enzyme activity, Photosynthesis, and other processes in the plant Organism. Plants most frequently accumulate toxic ions of such elements as cadmium, lead, copper, mercury, zinc, and nickel. The toxicity of these elements in the Cytoplasm is based on their high affinity for sulfhydryl groups, resulting in the inactivation of Enzymes containing SH-groups. In animals and humans, metallothioneins (MTs)—specific low-molecular-weight Proteins with a high Cysteine content—serve to bind these ions. To test whether the MT Gene could function in higher plants, a cadmium tolerance gene encoding an MT protein was transferred from the Chinese hamster genome into the genomes of field mustard and tobacco. This was performed at the Plant Research Centre (Canada) using a cauliflower mosaic virus-based vector, marking the first instance of a mammalian gene functioning in a higher plant genome. As a result, the cadmium-binding capacity of the transformed plant Cells increased by nearly 4-fold.

It has been established that certain plant species can accumulate significant concentrations of zinc, lead, and copper (exceeding 1% of dry mass). The high tolerance of such plants is due to the Synthesis of specific proteins called phytochelatins:

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These are Polypeptides consisting of 1–10 linear chains of glutamic acid and cysteine, terminating in Glycine, and possessing the general formula:

Phytochelatins, like metallothioneins, bind heavy Metal Ions via cysteine residue groups into stable thiolate complexes (ranging from 3 to 10 kDa in mass) with the participation of the enzyme phytochelatin synthase. This system operates in the cytoplasm and can function as a metabolic regulatory cycle to prevent critical (toxic) metal concentrations within the cell. It is universal and widespread in both Higher Plants and Algae. In addition to metallothionein- and phytochelatin-type proteins, plants may also induce the synthesis of phytoalexins in response to elevated heavy metal concentrations.

Compartmentalization of heavy metals in cell walls and vacuoles also plays a protective role.

Another resistance mechanism may involve alterations in Enzyme Structure or The Emergence of alternative metabolic pathways, which help the plant survive under excessive heavy metal concentrations.

A pressing issue today is The Study of heavy metal accumulation in soils and their migration through food chains, as they exert a chronic toxic effect on All living organisms, including humans. In the course of evolution, plant organisms from various geochemical provinces have formed biochemical adaptation mechanisms to extreme conditions caused by the heterogeneous geochemical composition of their native soils. Consequently, plants often exhibit interspecific, interpopulational, and intrapopulational differences IN RESISTANCE to heavy metal pollution. Detailed research into such differences will help unravel the mechanisms of Plant resistance to heavy metals and enable The Development of bioindication and biomonitoring Methods for the environment. For instance, indicators of lead and cadmium pollution include Potamogeton crispus, Elodea canadensis, and Potamogeton lucens, whereas Oligotrichum hercinicum serves as a monitoring organism for copper and nickel pollution, among others.

Based on their ability to absorb heavy metals from soils with varying degrees of contamination, plants are divided into three groups:

· accumulator plants, which accumulate metals in their aboveground parts;

· indicator plants, which regulate the Uptake and Transport of metals to the aboveground parts such that their internal concentration reflects the presence of heavy metals in the soil;

· excluder plants, in which the metal content in the aboveground parts remains constant and low over a wide range of soil concentrations.

Accumulator plants can be used for soil detoxification, indicator plants for assessing environmental pollution, and excluder plants for identifying and exploring the mechanisms of plant resistance to heavy metal excess.