14.4 Assisted Phytoremediation
Assisted phytoremediation includes remediation with the help of plants and microflora which includes microbes, earthworms, fungi, and actinomycetes. Microflora
plays a crucial role in shaping the soil ecosystem. There are two types of processes:
one is direct in which solubilization of heavy metals and their accumulation takes
place directly with plants, whereas indirect process involves prevention form phytopathogens, enhanced growth of plants, and heavy metal accumulation with the
help of microflora.
14.4.1 Mechanism of Plant Growth-Promoting
Microorganisms in Phytoremediation
Rhizosphere (i.e., an area around the plant roots) of plants is a natural habitat for
microorganisms. Plants dwelling in the metal-polluted soil provide shelter to many
soil microorganisms which are able to tolerate the effects of metals in the soil and
play beneficial role in the bioremediation of heavy metals (Zarei et al. 2010). Plant
roots release ample of compounds known as root exudates which include carbohydrates, amino acids, carboxylic acids, etc. that serve as nutrient media for microbial
population. Microbial cells generate and sense the signal molecules and undergo the
process of quorum sensing (Ullah et al. 2015). Some soil microbiota has metalresistant bacterial population. They play effective role in providing nutrients and
reducing the heavy metal toxicity. Microbial degradation of organic and inorganic
pollutants has been well documented by many researchers (Gadd 2010; Khanna et al.
2019). Microbes secrete siderophores, organic acids, and plant growth regulators
which help in mobilization, binding of the heavy metal ions present in the soil, and
increase their bioavailability for plants (Fig. 14.2) (Visioli et al. 2014). A study
conducted by Braud et al. (2009) documented that P. aeruginosa enhanced the
bioavailability of Pb and Cr to the maize. In Alyssum serpyllifolium, increased Ni
uptake has been observed on inoculation of Pseudomonas sp. A3R3 (Ma et al.
2011b). Sulfur-oxidizing bacteria increase the Cu mobilization and its uptake in
Hordeum vulgare (Shi et al. 2012). In his study Glick (2012) documented that plant
growth-promoting bacteria have potential to produce various plant hormones like
auxin, cytokinins, and gibberellins which play effective role in stress amelioration.
Oladipo et al. (2018) documented that many filamentous fungi genera
(Trichoderma, Aspergillus, Mucor, Penicillium) have the ability to tolerate metal
toxicity. Various functional groups like amine, carboxylic, phosphate, etc. are
present on the fungal cell wall which generates overall negative charge that helps
in binding the metal (Ong et al. 2017). Arbuscular mycorrhizal fungi (AMF) are
important entity of soil microflora which have potential to withstand heavy metal
stress. They establish synergistic association with plants by increasing their root
14 PGPR and Earthworm-Assisted Phytoremediation of Heavy Metals
233
Assisted phytoremediation includes remediation with the help of plants and microflora which includes microbes, earthworms, fungi, and actinomycetes. Microflora
plays a crucial role in shaping the soil ecosystem. There are two types of processes:
one is direct in which solubilization of heavy metals and their accumulation takes
place directly with plants, whereas indirect process involves prevention form phytopathogens, enhanced growth of plants, and heavy metal accumulation with the
help of microflora.
14.4.1 Mechanism of Plant Growth-Promoting
Microorganisms in Phytoremediation
Rhizosphere (i.e., an area around the plant roots) of plants is a natural habitat for
microorganisms. Plants dwelling in the metal-polluted soil provide shelter to many
soil microorganisms which are able to tolerate the effects of metals in the soil and
play beneficial role in the bioremediation of heavy metals (Zarei et al. 2010). Plant
roots release ample of compounds known as root exudates which include carbohydrates, amino acids, carboxylic acids, etc. that serve as nutrient media for microbial
population. Microbial cells generate and sense the signal molecules and undergo the
process of quorum sensing (Ullah et al. 2015). Some soil microbiota has metalresistant bacterial population. They play effective role in providing nutrients and
reducing the heavy metal toxicity. Microbial degradation of organic and inorganic
pollutants has been well documented by many researchers (Gadd 2010; Khanna et al.
2019). Microbes secrete siderophores, organic acids, and plant growth regulators
which help in mobilization, binding of the heavy metal ions present in the soil, and
increase their bioavailability for plants (Fig. 14.2) (Visioli et al. 2014). A study
conducted by Braud et al. (2009) documented that P. aeruginosa enhanced the
bioavailability of Pb and Cr to the maize. In Alyssum serpyllifolium, increased Ni
uptake has been observed on inoculation of Pseudomonas sp. A3R3 (Ma et al.
2011b). Sulfur-oxidizing bacteria increase the Cu mobilization and its uptake in
Hordeum vulgare (Shi et al. 2012). In his study Glick (2012) documented that plant
growth-promoting bacteria have potential to produce various plant hormones like
auxin, cytokinins, and gibberellins which play effective role in stress amelioration.
Oladipo et al. (2018) documented that many filamentous fungi genera
(Trichoderma, Aspergillus, Mucor, Penicillium) have the ability to tolerate metal
toxicity. Various functional groups like amine, carboxylic, phosphate, etc. are
present on the fungal cell wall which generates overall negative charge that helps
in binding the metal (Ong et al. 2017). Arbuscular mycorrhizal fungi (AMF) are
important entity of soil microflora which have potential to withstand heavy metal
stress. They establish synergistic association with plants by increasing their root
14 PGPR and Earthworm-Assisted Phytoremediation of Heavy Metals
233