**2. Phytoremediation-based strategies**

#### **2.1 Phytodegradation (phytotransformation)**

A number of plant and microbial enzymes play a major role in degradating (metabolized) or mineralizing the contaminants which are hyper accumulated inside the plant cells. Phytoremediation mostly mediated by the group of enzymes are well documented. It is understood from Nitroreductases degradation of nitroaromatic compounds and glycosyltransferase that bioactivity of plant hormones are altered by glycosylation. This has been reviewed for plant hormones such as auxins, cytokinins, gibberellins and abscisic acid [6] and glutathione transferases (GSTs) that controls the internal cell pressure due to chemical-induced toxicity. It protects cell and provides tolerance by catalyzing S-conjugation between the thiol group of GSH and electrophilic moiety in the hydrophobic and toxic substrate [2].

Oxidases (Metal-modifying enzymes) which is involved in the assimilation of heavy metals into organic molecules (e.g., selenate is metabolized to dimethyl selenide), or in changing the oxidation state of metals e.g., toxic Cr (VI) is reduced to nontoxic Cr (III) [3]. Phosphatases, nitrilases and dehalogenases play a vital role in *Phytoremediatiation of Metal and Metalloid Pollutants from Farmland… DOI: http://dx.doi.org/10.5772/intechopen.98659*

the transformation and conjugation of explosives and dehalogenases degradation. These enzymes are involved in the transformation of toxic xenobiotic compounds such as explosives, pesticides, nerve gases, and halogenated organic compounds. Nitro reductases are involved in the degradation of nitroaromatic compounds, chlorinated solvents and pesticides. Many diverse organophosphates detoxify other contaminants by reducing either halogen groups or organically bound phosphate [7].

Many endophytes are resistant to heavy metals and are capable of degrading organic contaminants. The endophyte-assisted phytoremediation has been documented in formulating biofertilizers which are providing promising result for *in situ* remediation of contaminated soils accompanied by phosphate solubilizing, biosurfactant activity in degradation of oil-contaminated soil, siderophore production, and antimicrobial activity. In addition, plants and many microorganisms contain abundance of oxidases such as laccases (degradation of anilines) and peroxidases. These enzymes are involved in forming a defense layer in many plant processes. *Populus* species and *Myriophyllium spicatum* are examples of plants that have these enzymatic systems [8]. Phytoremediation essentially comprises of six different strategies, though more than one may be used by the plant simultaneously. They are as shown in **Figure 1** and **Table 1**.

### **2.2 Phytostabilization**

Metals are precipitated as insoluble forms by the direct action of roots which secrete phenolic and low molecular weight organic exudates subsequently trapped in the soil matrix as contaminants. Later when get accumulated organic or inorganic

**Figure 1.** *Schematic representation of phytoremediation.*


#### **Table 1.**

*Concise view of various phytoremediation strategies.*

pollutants are incorporated into the lignin of the cell wall of cells or in humus. The main intention is to cultivate plants like *Haumaniastrum*, *Eragrostis*, *Ascolepis*, and *Gladiolus* in polluted agricultural fields to limit the mobilization and diffusion of contaminants in the soil [16–18].

The plants are involved in absorbing many toxic elements from rock, soil, and polluted water by the root system. Plant exudates aggregate metals in the soil. Soil microbes which are symbionts can decrease the toxic effects of contaminants in the soil. For example, exudate peptides from the bacterium *Pseudomonas putida* and Arbuscular Mycorrhizal Fungi (AMF) have great potential in phytostabilization and in removing metal contaminants in the soil can decrease Cd toxicity in plants. Plants can also convert contaminants into less toxic forms as well as decrease their bioavailability [19].

Siderophores, organic acids, and phenolics secreted by the microbes associated with the roots of certain plants are natural chelating compounds that form complexes with metals in the rhizosphere. In addition, plants, and their associated soil microbes play a major role in releasing chemicals that act as biosurfactants in the soil that increase the uptake of hydrocarbon toxic pollutants. These contaminants are stabilized in natural and constructed wetlands through a process called phytofiltration. It includes rhizofiltration where metals are precipitated within the rhizosphere zone and in the root membrane. Metal uptake by plants that is generally active diffusion takes place by specific protein transporters (channel proteins) or H+ coupled carrier proteins located along the cell membrane of the root. For example,

the Fe regulated transporter (IRT1) allows the uptake of Fe. Uptake of other metals also occurs via IRT1 transporters, especially even in very low concentrations of Fe exist in the soil. By expelling the proton gradient, more ions are concentrated near the root zone. Inadvertent uptake of non-essential metals also takes place via other cell membrane transporters.
