**4. Phytoremediation and its mechanism**

In phytoremediation, plants absorb contaminants from the surrounding atmosphere and then degrade or detoxify them using a variety of mechanisms [61, 62]. It is currently unclear how specific air pollutants are captured by plants for subsequent degradation, metabolization, or sequestration. Additionally, phytoremediation has been studied for plant propensity to filter ambient air and exchange gases with their surroundings [63]. The large biologically active surface areas of plants have added advantage in capturing different air pollutants through absorption, transport, and deposition of organic pollutants in the rhizosphere and phyllosphere [64]. For the elimination of air pollutants, the photosynthetic systems of C3, C4, crassulacean acid metabolism (CAM), and facultative CAM plants have been studied under various circumstances [65]. C4 plants posses high intensity in exchange of gases as compared to C3 plants and may exchange more CO2, especially during the day. CAM plants exchanges the gases during the night which makes them highly efficient in phytoremediation specially when they are placed in indoors [66]. Microorganisms that make up a phyllo-microbiome colonize on leaf surfaces and have potential to degrade a variety of organic contaminants [67]. Even soil microorganisms have ability to eliminate gaseous air pollutants when they are associated with plants [68]. Various mechanisms in phytoremediation are discussed below (**Figure 2**).

Phytoextraction refers to the process of taking up of pollutants from soil and transporting them to above ground plant parts for further degradation. Thus, in phytoextraction both phyllosphere and rhizosphere are involved [69]. Efficacy of phytoextraction process is dependent upon mobility and availability of heavy metals in the root zone [70–72]. Phytovolatilization is transport of contaminants into the phyllosphere of plants through the epidermis by diffusion across the cuticle and also through open stomata. These contaminants are further degraded into volatile components which are further released in the atmosphere through transpiration from the stomata [73]. Transpired volatile components either stay in the atmosphere as an air pollutant or they may break down by the action of hydroxyl radicals [74]. Phytovolatilization process is observed for number of contaminants both inorganic and organic forms. In phytovolatilization, pollutants are absorbed by plants through phyllosphere and transformed into volatile compounds. Eventually, these degraded volatile substances transpired into the atmosphere via stomata [75].

#### **Figure 2.**

*Mechanisms of phytoremediation.*

In phytodegradation, organic contaminants including polycyclic aromatic hydrocarbons (PAHs), total petroleum hydrocarbons (TPHs), polychlorinated biphenyls (PCBs), and inorganic contaminants like atmospheric nitrogen oxides and sulfur oxides are taken up and transformed by plants into simpler less toxic forms [9]. The obtained products of phytotransformation are incorporated into phyllosphere and rhizosphere. Specific plant enzymes such as nitroreductases, dehalogenases, laccases, and peroxidase play vital role in the phytotransformation process [73]. The immobilization of contaminants takes place in the rhizosphere during phytostabilization and is known as phytoimmobilization. Lignin, which is found in the cell wall of plant roots, is able to adsorb pollutants which then precipitates into insoluble compounds and stores them in the root zone [71]. Rhizodegradation refers to the biodegradation of contaminants in the soil by edaphic microbes enhanced by the inherent character of the rhizosphere itself. The roots of plants offer an additional surface area which allows for the transmission of oxygen and the growth of microorganisms. Plants emit biodegradable enzymes and metabolites into the rhizosphere, where microbes can use them to develop and break down contaminants [76]. Root exudates have many potential uses such as enhancing plant defense mechanism, boosting nutrient availability in the root zone, and even phytoextraction of heavy metals. The ability of bacteria to degrade pollutants is *Bacillus*, *Acinetobacter*, *Arthrobacter*, *Diaphorobacter*, *Enterobacter*, *Flavobacterium*, *Phanerochaete chrysosporium*, *Polysporus*, *Pseudomonas*, *Pseudoxanthomonas*, *Rhodococcus wratislaviensis*, *Sphingomonas*, and *Stenotrophomonas*.
