Updated: Nov 30, 2022
Yagnit Vadhel and Meghna Vinodan
St. Xavier’s College (Autonomous),
Mumbai, Maharashtra, India
In this era, the big brains have provided us with the technologies for rapid industrialization. However, the co-existence of nature and anthropogenic activities is questionable. These activities have led to the degradation of land mainly due to the introduction of tons of heavy metals. We humans have turned a deaf ear to this environmental tragedy. Unlike us, plants are non-mobile because of which they do not have the option to live in denial. To deal with it they have evolved special anatomical, physiological and genetic adaptations for survival. The toxic environment tends to have a direct impact on their growth and development at both cellular and molecular levels of the organization. The metals are retained in their ionic form inside the rhizosphere either as free metal ions or in combination with other non-metals like sulphur, nitrogen and phosphorus.
In recent times many techniques of land restoration have been adopted. However, they have not produced the required levels of satisfaction with respect to the efficiency of soil detoxification. One of the most promising ways to restore land is phytoremediation. Phytoremediation is deploying green plants and associated microorganisms to remediate the environment by detoxifying and/or sequestering the pollutants. This method has proven to be highly efficient and economical. The plants can be genetically modified to improve the efficiency of their phytoremediation properties. The toxic component in its surrounding could be inorganic elements that include heavy metals such as arsenic, cadmium, chromium and uranium. The toxins can even be organic compounds like polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) etc. The organic components must be converted into simpler components for efficient remediation.
The strategies that the plants have evolved for heavy metal removal include accumulation and/or exclusion. The accumulators translocate the metals into their tissues and this process is known as phytoextraction. Plants that can accumulate more than 1000 µg g-1 of copper, cobalt, chromium, nickel and lead or more than 10,000 µg g-1 manganese and zinc in their tissues above the ground are known as hyperaccumulators. The excluders phytostabilize the metals, i.e., restrict the metals in the ground and do not accumulate them into their tissues above the ground. They immobilize the metal thereby avoiding environmental issues caused by the leaching of the pollutant into the groundwater. In the long run, microbial communities establish themselves in the rhizosphere which improves the growth of the plants and prevents soil erosion.
Plant roots play a crucial role in remediation as they absorb, precipitate and even accumulate the metals in their tissues depending on the plant species. This process is known as rhizofilteration and is accompanied by the microorganisms in the rhizosphere. When the heavy metals are at the root-soil interface, the organic acids secreted by the external root epidermis acts as an electron donor into the atomic orbitals of the metals. This results in a mutual chemical entity known as a metal complex through the process known as chelation. In absence of chelation, the plants actively transport the metals from the cytosol towards the external environment. The roots precipitate the water-soluble heavy metal ions into a water-insoluble heavy metal entity which the plant’s roots cannot absorb.
Despite all these attempts, sometimes the toxin still manages to traverse through the plant roots by passive absorption. To deal with this, over time the plants have evolved a range of phytochemicals like phytochelatins and metallothioneins. Organic acids like citric, oxalic and malic acids are responsible for the metal’s transport from the cell’s cytoplasm to the central vacuole that is considered as the trash bin of plants. Initially, the heavy metals are complexed by a relatively weaker complexing acid which is transported from the cytoplasm to the central vacuole. The acid is then replaced by a stronger complexing organic acid like oxalic acid, amino acid oligomers containing sulphur, and softer binding groups. Phytochelatins and metallothioneins are employed to act according to the concentration of the heavy metals present inside the plant cell. The plant’s natural chelators also consist of cysteine subunits along with glutathione’s units that contain electron-donating groups which form a bond with heavy metals present inside the host plants. Hyperaccumulators tend to accumulate a large capacity of metals like nickel, copper and zinc inside various parts like roots, fruits, growing flower buds and leaves.
Let us dive deeper into the biochemical and genetic mechanisms underlying this fact. Here the role of the enzymes and phytohormones comes into play. The entire process of heavy metal detoxification by plants revolves around a single enzyme that phytochelatin synthase. The presence of any heavy metal is detected through complex biochemical interactions. Later, as instructed by the genetic code the incoming heavy metal is linked to the free glutathione subunits present in the cytosol. This results in the protein phytochelatin that is responsible for its detoxification from the plant’s cell. The mode of action is governed by a gene known as PC11 which is present in all plants but is silenced in all except those growing in soils rich in heavy metals. Some plants can also convert highly volatile metals like Hg through various unknown biochemical pathways from its ionic state into elemental Hg that can be transpired by the plants through the small openings present on leaf surfaces termed as stomata.
In recent times, advancements in the field of biological sciences have resulted in our understandings of the complex mechanisms and adaptations that have led to plant developments in conditions not thought by mankind. It is a significant step for land restoration of sites polluted by heavy metals.
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