Plants: Pollution Fighters — How Are They Made?

Plants have been used to clean up pollution from the soil and the atmosphere. This process, called phytoremediation, involves the use of plants to extract and break down pollutants into less toxic substances or harmless byproducts. Phytoremediation is a potentially effective and low-cost method for cleaning up polluted air and water, as well as restoring terrestrial and aquatic ecosystems. Some plants can absorb and break down pollutants through their roots, stems, and leaves, or by releasing them into the air. For example, transgenic trees and plants can break down pollutants like TCE, a common industrial contaminant, and RDX, a military explosive. Aquatic plants are also used to remove metal pollutants from water through rhizofiltration and biosorption. Large-scale plantings can also help to increase oxygen content and reduce greenhouse gases like carbon dioxide and methane.

Transgenic plants and trees

Phytoremediation is a cost-effective technology that uses plants to remediate contaminated sites. It is particularly effective in dealing with large areas of mine tailings with shallow contamination of organic, nutrient, and metal pollutants. Transgenic plants can be used to enhance the process of phytoremediation by improving the capacity of plants to remediate environmental pollutants. For example, Brassica juncea was genetically engineered to have increased tolerance to cadmium and higher concentrations of phytochelatins, glutathione, and total nonprotein thiols compared to wild-type seedlings.

Transgenic poplar plants have been shown to be effective in breaking down trichloroethylene (TCE), the most common groundwater contaminant at Superfund sites in the US. The plants are able to metabolize TCE into harmless byproducts at rates 100 times that of control plants. They do this by producing enzymes that break down TCE into chloride ions (harmless salt that the plant sheds) and recombining the carbon and hydrogen with oxygen to produce water and carbon dioxide.

Transgenic plants can also be used as sensors of environmental pollution genotoxicity. They can be intentionally altered to be more sensitive to particular pollutants and can be grown at the site of pollution or potentially dangerous sites. This allows for the quick and cheap assessment of contamination and its potential danger to the environment and humans.

However, some people see transgenic trees as risky due to the concern that they might spread into regular forests. Federal regulations in the US allow the growing of transgenic trees in greenhouses and controlled field trials for research purposes but do not allow their commercial growing.

Phytoremediation

• Phytostabilization: This process involves retaining contaminants in the soil by reducing their bioavailability. Microorganisms in the rhizosphere, such as bacteria and mycorrhiza, can assist in this process by absorbing metals onto their cell walls and increasing plant root depth and surface area.
• Phytodegradation: This process involves converting organic contaminants into less harmful substances.
• Phytovolatilization: This process involves converting contaminants inside plants into a gaseous state and releasing them into the atmosphere through evapotranspiration.
• Phytoextraction (or phytoaccumulation/phytosequestration): This process involves the use of plants or algae to remove contaminants from soil or water and concentrate them in the harvestable plant biomass. Phytoextraction is particularly useful for heavy metals and other inorganics.

The use of aquatic plants in phytoextraction is a significant advancement in water treatment. These plants have molecular antennae that capture nutrients and metal pollutants from the water. Additionally, the roots of dead plants can be crushed and transformed into plant powder, which is then used to treat polluted water.

Genetic engineering is a powerful method for enhancing the natural phytoremediation capabilities of plants or introducing new capabilities. For example, researchers have developed a transgenic hybrid aspen tree that can remove contaminants from water much faster than unaltered aspens.

Natural phytoextraction

The effectiveness of natural phytoextraction depends on the plant's biomass production and the content of contaminants in the harvested biomass. Fast-growing crops with high biomass production, such as willow, poplar, pine, and eucalyptus, are promising candidates for phytoextraction. Additionally, genetic engineering techniques can be employed to enhance the phytoremediation capabilities of plants, making them more efficient at absorbing and detoxifying contaminants.

Overall, natural phytoextraction is a promising green alternative to traditional physical and chemical methods of pollution remediation. It offers a cost-effective, environmentally friendly solution to restoring polluted environments and has gained popularity worldwide over the last two decades.

Plant-based technology

Air pollution is a pressing global issue that poses a significant threat to human health and well-being. While various measures have been implemented to mitigate this problem, many of these methods are costly and require specialized equipment. As a result, there is a growing interest in exploring plant-based technologies as a potential solution.

Plants have the inherent ability to assimilate, degrade, or modify toxic air pollutants into less harmful forms. This process, known as phytoremediation, offers a simple, inexpensive, and easily implemented approach to combating air pollution. Phytoremediation technology utilizes the natural processes in plant cells, particularly the leaves, to act as a sink for air pollutants and particulates. The leaves of plants can adsorb or absorb these pollutants, while the habituated microbes on the leaf surface and within the leaves (endophytes) play a crucial role in biodegrading or transforming these pollutants into less toxic or non-toxic molecules.

To enhance the effectiveness of phytoremediation, researchers have proposed a two-pronged approach involving genetic engineering and bacterial interventions. By introducing additional genes into the plant's metabolism, scientists can coax the production of specific enzymes, enabling the plant to utilize volatile organic compounds (VOCs) as carbon sources, resulting in increased pollution-fighting capacity. In parallel, specific strains of symbiotic bacteria can be introduced into the plant's soil to convert harmful pollutants like formaldehyde and BTEX (benzene, toluene, ethylbenzene, and xylene) into harmless sugars and amino acids.

The development of "Neoplants" by companies like Neoplant P1 is a prime example of this approach. By combining genetic modifications with microbial interventions, they have created plants with enhanced air-cleaning capabilities. While the microbiome currently handles the majority of the air-cleaning function, the potential for improvement in both plant and microbial components offers exciting prospects for the future.

Phylloremediation is another plant-based technology that specifically focuses on the use of plant leaves and leaf-associated microbes to remediate air pollutants. With advancements in omics technologies, scientists can deepen their understanding of the molecular mechanisms involved in pollutant reduction, leading to the development of effective phylloremediation protocols for both indoor and outdoor environments. The selection of plant species and microbes from air-polluted areas, followed by rigorous testing for synergistic effects, is key to the success of phylloremediation technologies.

Pollutant absorption and purification

Plants have the ability to absorb, break down, and modify toxic pollutants in the air, water, and soil into less harmful substances. This process, known as phytoremediation, is a potentially effective and low-cost technology for combating pollution.

Plants can absorb pollutants through their roots, leaves, and stems, and transport them to other parts of the plant, such as the leaves, where they are stored in high concentrations. This process is known as phytoextraction. For example, in the Gard region of France, common ragweed stores more than 17,000 ppm of zinc in its leaves. In addition, plants can also absorb pollutants directly from the air through their foliar surfaces, such as the leaves of cotton, which can absorb formaldehyde, benzene, trimethylamine, and xylene.

Some plants are particularly effective at accumulating and breaking down certain types of pollutants. For example, stinkweed (Noccaea caerulescens) and trees such as Geissois pruinosa and Grevillera meisneri are tolerant of and can hyper-accumulate metallic elements. Transgenic trees, such as a French hybrid of the aspen tree, have been created to break down pollutants like trichloroethylene (TCE) in water, and RDX in the soil. Genetically modified poplar plants have also been shown to be effective at absorbing and breaking down pollutants like TCE, chloroform, carbon tetrachloride, vinyl chloride, and benzene.

The use of dead plants has also been explored in water treatment. By crushing the roots of plants and transforming them into plant powder, metals can be sequestered and removed from water. This method has been successfully used to treat mining effluents in France polluted with zinc, iron, and arsenic.

Overall, the use of plants for pollutant absorption and purification offers a promising, low-cost, and environmentally friendly solution to combat pollution in both terrestrial and aquatic ecosystems.

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