**2.6 Rhizodegradation**

Microbes that harbor inside plant parts like endophytic bacteria and fungal population that grows in roots are tend to promote the growth of plants and involve in degrading rhizosphere pollutants. They utilize exudates and metabolites of plants as a source of carbon and energy. In addition, plants provide biodegrading enzymes. The application of phytostimulation is targeted to organic contaminants [37]. The microbial community in the rhizosphere is diverged genetically and physiologically. This varies according to the spatial distribution of nutrients irrespective of factors [17].

There are other strategies, which are of rhizodegradation. These include:

#### *2.6.1 Hydraulic barriers*

Some large trees like *Populus* sp. have deep roots which have a major role in transpiration of groundwater in large quantities. Plant enzymes play a key role in eliminating contaminants after metabolized and vaporized together with water in plant tissues.

#### *2.6.2 Vegetation covers*

Herbs including grasses, shrubs, or trees planted on landfills, pits, trenches, or tailings minimize the infiltration of rainwater and the spread of pollutants. The roots facilitate soil aeration and in turn enhance the biodegradation, evaporation, and transpiration [44, 45]. Organic soil composed of sawdust, plant remains and NPKfertilizers promote plant growth which helps in phytoremediation. Many field trials are emphasized at the end of a single biological cycle with 76 different plant species including cereals, shrubs, fruit trees, and even large trees like oaks and pines.

#### *2.6.3 Constructed wetlands*

The components of ecosystems comprise of organic soils, microorganisms, algae and vascular aquatic plants. All are involved in the effluent treatment through evaporation, filtration, ion exchange, adsorption and precipitation [46]. Here all the components are interlinked to phytoremediation and the entire system is given a promising effect [47]. The advantages are good cleaning efficacy, less cost of designing along with easy operation and maintenance. It is widely focused in the treatment of domestic, agricultural, and industrial wastewater, and also for treating acid mine drainages [48, 49]. Herbs (grasses, shrubs) or trees planted on landfills or tailings are used to reduce the infiltration of rainwater which is loaded with pollutants from various areas. Since there are difficulties in establishing rooting in tailings some other techniques must be evaluated for future prospective. For example, plants like *Hungarian agronomists* (*Biological Reclamation Process, BRP*) are propagated to utilize residues of organic soil that is composed of sawdust, plant remains and NPK-fertilizers [50].

#### *2.6.4 Phytodesalination*

The cultivation of halophytes on salt-rich soil is to improve the productivity of the soil and to remove the excess salt from saline soil [51, 52]. The potential of *Suaeda maritima* and *Sesuvium portulacastrum* is used in removal and accumulation of NaCl from highly saline soil. The plants in saline soil accumulate sodium in shoots and aerial parts which is based on the soil nature and the climatic conditions. The upper horizon of the soil layer is leached by halophytes [53].

## **3. Recent advancements in phytoremediation**

To enhance the rate of phytoremediation, to improve the adaptation to various environmental conditions and to minimize their limitations such as slow-growth several strategies are developed through recombinant DNA technology to create transgenic plants or plant hybridization with fast-growing hyperaccumulators and microbe-associated phytoremediation. The hyperaccumulators can accumulate high levels of contaminants including heavy metals and other pollutants. Electrofusion is used for the fusion of protoplasts between two plants namely *T. caerulescens* which has a *Phytoremediatiation of Metal and Metalloid Pollutants from Farmland… DOI: http://dx.doi.org/10.5772/intechopen.98659*

high-level Zn accumulator and *Brassica napus* which has a biomass production capability. The resulted somatic hybrids have the properties like hyperaccumulation capability, tolerance derived from *T. caerulescens* and higher biomass production derived from *B. napus* [54]. Moreover in comparison with rhizosphere microorganisms, endophytes have close interaction with their host plants. Genetically modified endophytes can be used as bio-fertilizers that could more efficiently improve phytoremediation [28].

#### **3.1 Genetic engineering**

Genetic engineering is a tool for improving strains in industries and clinics. It also enhances the phytoremediation abilities of plants in removing heavy metals. To generate genetically modified plants, a foreign source of the gene of interest which can be obtained from an organism, such as a plant species or even bacteria or animals, is transferred and inserted into the genome of a target plant through a proper vector system. After DNA recombination, the foreign gene gets integrated and inherited that confers specific traits to the plants. Moreover, genetic engineering has tools to transfer desirable characters from hyperaccumulator source plant to sexually incompatible plant species, which is impossible through traditional methods including vegetative propagation [55].

Therefore, creating transgenic plants with the desired gene expression in traits has attracted the researchers in the field of phytoremediation. Genetically modifying traits are fast-growing and high-biomass species with high tolerance against heavy metals. Their accumulation ability is more desirable than hyperaccumulators because sometimes hyperaccumulation may be harmful for biomass. Therefore, the selection of genes for genetic engineering should be based on heavy metal tolerance, construction of metabolic pathways in detoxifying heavy metals and accumulation mechanisms in plants. As heavy metals accumulation may create oxidative stress due to excessive Reactive Oxygen Species (ROS), a defense system provide heavy metal tolerance. To increase heavy metal accumulation through genetic engineering, genes are put under the control of strong promoter. The signal sequences facilitate in the uptake, translocation, and sequestration of heavy metals in elevated levels [56].

As metal chelators act as metal-binding ligands to improve heavy metal bioavailability, they promote heavy metal uptake and root-to-shoot translocation, as well as mediate intracellular sequestration of heavy metal ions in organelles. By over expression of genes encoding natural chelators, heavy metal uptake and translocation can be improved [57]. For example the supply of histidine is a nickel chelating agent and when it is supplied to plants which are originally non-accumulating species for metals greatly increases both its nickel tolerance and nickel transport to the shoot. It indicates the role of histidine in the hyper accumulation of nickel in *Alussum* plants [58].

Although the genetic engineering approach is a promising one, a few setbacks are there when the concentration is toxic to cells. On other hand, construction of all desired genes (that involved in mechanisms of detoxification and accumulation of heavy metals) is a time as well as effort consuming process and hence it is not providing a promising effect in the present scenario. Moreover it is difficult to get approval for the cultivation of genetically modified plants in test fields due to its toxicity, allergic levels and risk factor to ecosystem. Therefore, the researchers focus on alternate approaches to improve plants' role in phytoextraction.

#### **3.2 Role of endophytes in phytoremediation**

The role of plant-associated microorganisms (rhizospheric microorganisms) can be considered as an alternative approach to improve plant performance for phytoremediation as expressed in **Figure 2** and **Table 2**. The microbial communities

#### **Figure 2.**

*Role of endophytes in sustainable ecological perspective.*


#### **Table 2.**

*Documented records of role endophytes- based phytoremediation.*

#### *Phytoremediatiation of Metal and Metalloid Pollutants from Farmland… DOI: http://dx.doi.org/10.5772/intechopen.98659*

have symbiotic association with rhizosphere stimulating root proliferation and thus, promoting plant growth. They have increased heavy metal tolerance and plant fitness among the flora in local biosphere [69]. Plant growth-promoting rhizobacteria (PGPR) have a key role in phytoremediation. PGPR can promote plant growth via IAA production, antimicrobial activity, increase plant tolerance against heavy metals and improved nutrient uptake through diffusion as well as uptake of heavy metal from contaminated soil, translocation etc., [22]. This is achieved by producing various compounds such as organic acids for organic pollutant degradation, iron chelating siderophores, antibiotics, various enzymes involved in phytoremediation and growth promoting phytohormones [22]. PGPR can degrade the ethylene precursor ACC by synthesizing the 1-aminocyclopropane-1-carboxylate (ACC) deaminase. PGPR minimizes ethylene production and thus in turn promotes plant growth [70, 71].

Plants inoculated with PGPR containing extensive root and shoot densities result in enhanced uptake of heavy metals by the influence of ACC deaminase which promote phytoremediation efficiency [70, 72]. PGPR induces the formation of lateral root and root hair development, thus promoting plant growth and improving phytoremediation with bacterial indole acetic acid (IAA) [73]. Arbuscular mycorrhizal fungi (AMF) are the vast group of fungi, an important microbial community are predominant in soil profile that support plants for phytoremediation. AMF in rhizospheres increases the surface area for root absorption with an extensive hyphal network. They improve the uptake of water, nutrients and heavy metal bioavailability [74]. AMF can also produce phytohormones to promote plant growth and biosurfactant aids in phytoremediation [75].

A plant employs various strategies to enhance heavy metal bioavailability for better absorption. Root exudates promote desorption of heavy metals by making insoluble complexes of contaminants to free ions, by decreasing soil pH, which thus facilitate the accumulation of heavy metals in the soil for easy absorption near the roots [76]. Plants secrete metal-mobilizing compounds such as phytosiderophores, carboxylates, and organic acids in rhizosphere. According to the bioavailability heavy metals/metalloids in the soil are classified as high, moderate and low bioavailable heavy metals/metalloids. The high bioavailable are Cd, Ni, Zn, As, Se, Cu, moderately bioavailable heavy metals are Co, Mn, Fe, and least bioavailable are Pb, Cr [77].
