Phytoremediation

Definition

Phytoremediation is a process that uses vascular plants as a means of extracting inorganic and organic contaminants from soils (1). The strategies used in phytoremediation can be grouped into physical, chemical and biological methods for mitigating the effect subsurface pollutants have on the soil and groundwater.

Strategies

Phytodegradation

Phytodegradation utilizes the metabolic capability of plants in breaking down soil contaminants. The term “green liver” has been used to describe this process as the plant metabolizes xenobiotic compounds in an analogous way to that of the mammalian liver (2). In plants, the xenobiotic metabolism occurs over three phases; transformation, conjugation, and excretion. The result of these phases is detoxification of the xenobiotic as well as making them more inert and stored away from other cellular processes.

The first phase involves the chemical modification of the xenobiotic compound by either oxidation, reduction, or hydrolysis. This causes the xenobiotic to become more water soluble and thus be more biochemically reactive within the plant (3). Plants utilize many different enzymes to alter these compounds. Cytochrome P450 family enzymes act as mono-oxygenases towards a broad range of substrates and convert hydrophobic compounds into those which are more soluble in water (4). Carboxylesterases (CXEs) can convert carboxyl esters into carboxylic acids via hydrolysis which can go on to react with other molecules in the next phase (5).

The water soluble and reactive compounds formed from transformation are then conjugated to endogenous modules such as sugars or peptides. The joining ox xenobiotic fragments with larger endogenous compounds decreases the toxicity of the xenobiotic while also making it easier to shuttle them around the cell during the last phase of compartmentalization. Glycosyltransferases are a large family of enzymes that catalyze the joining of nucleotide-diphosphate-sugars (usually in the form of Uridine diphosphate glucose or UDP-glucose) to xenobiotic compounds (6). The glycosylation of these compounds results in a higher stability of the xenobiotic fragments as well as trapping them within the vacuole and preventing them from reacting further to form harmful substances (7). Glutathione S-transferases (GSTs) is another class of enzymes that attach to xenobiotics at a tripeptide glutathione region and facilitate transfer to the vacuole or cell wall (8).

The final phase sees the xenobiotics bound to larger macromolecules sequestered to specific locations within plant cells, namely the vacuole or cell wall. Most xenobiotics are incorporated into the cell wall after being bound to lignin molecules while enzymes called ATP-binding cassette (ABC) transporters facilitate xenobiotic transfer to the vacuole (9).

References

1. Reichenauer, Thomas G., and James J. Germida. “Phytoremediation of Organic Contaminants in Soil and Groundwater.” ChemSusChem, vol. 1, no. 8‐9, WILEY‐VCH Verlag, 2008, pp. 708–17, doi:10.1002/cssc.200800125. https://chemistry-europe-onlinelibrary-wiley-com.gate.lib.buffalo.edu/doi/full/10.1002/cssc.200800125#bib23.