ethylene production when exposed to stress; rather, it only reduces the deleterious
levels of stress ethylene by plants (Glick et al. 2007; Nagajyoti et al. 2010).
7 Mechanisms of Plant–Metal Interaction
in the Rhizosphere
The plant–microbe interaction responsible for biogeochemical cycling of metals and
applications of phytoremediation came to light after the discovery of some plant–
microbe–metal interactions (Fig. 4.2). Microorganisms are ubiquitous in nature:
found everywhere, they grow well in all ecosystems such as soil, atmosphere, and
water, even in some extreme conditions (Vidali 2001). One of the important habitats
of all kinds of prokaryotic and eukaryotic microorganisms is the rhizosphere, which
seems to have a good interaction with the nearby vegetation in various ways
(Azevedo et al. 2000; Hao et al. 2012). Various metals for plant bioavailability are
restricted because of some hindrance such as solubility levels and high affinity
toward the soil, but some PGPR, root-colonizing, and mycorrhizal microorganisms
enhance the bioavailability of metals to the plant systems. For example,
rhamnolipids, a biosurfactant released by some bacteria, modifies the hydrophobic
pollutants to more hydrophilic forms that are known to be released (Qiu et al. 1994;
Volkering et al. 1998).
The absorption of various metal ions such as Fe
2+ , Mn
2+ , and Cd
2+ by the root
system is facilitated by organic exudates of microorganisms via improving the
bioavailability. Root exudates in turn feed the microbes by being good sources of
carbohydrates, lipophilic compounds, and natural chelators such as citric, acetic, and
other organic acids, which are important in increasing the mobility of metal ions or
promoting the bio-surfactant-producing class of microorganisms (Fig. 4.2). The
many feedback mechanisms by plant roots and a rhizosphere microorganism allow
them to adapt to the conditions of their environmental habitats; for example, the root
exudates of plants in a phosphorus-deficient habitat contain large amounts of citric
acid as an attempt for mobilizing the phosphorus present in soils. In some cases, a
few of the rhizosphere-dwelling organisms secrete growth hormones for the plant to
increase the plant root growth, which in turn provides its nutrient root exudates.
7.1 Metal Bioavailability for Plant Roots
To remediate a pollutant from an environment, the plant or microorganisms should
establish a close contact and have the ability to act on it; hence, the bioavailability of
the pollutant is important for its remediation. Soil and environmental conditions and
the pollutant physiochemical properties decide bioavailability. The soil has more
sites for metal ions, especially in CEC, if it has small particle size (clay) by which it
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
99
levels of stress ethylene by plants (Glick et al. 2007; Nagajyoti et al. 2010).
7 Mechanisms of Plant–Metal Interaction
in the Rhizosphere
The plant–microbe interaction responsible for biogeochemical cycling of metals and
applications of phytoremediation came to light after the discovery of some plant–
microbe–metal interactions (Fig. 4.2). Microorganisms are ubiquitous in nature:
found everywhere, they grow well in all ecosystems such as soil, atmosphere, and
water, even in some extreme conditions (Vidali 2001). One of the important habitats
of all kinds of prokaryotic and eukaryotic microorganisms is the rhizosphere, which
seems to have a good interaction with the nearby vegetation in various ways
(Azevedo et al. 2000; Hao et al. 2012). Various metals for plant bioavailability are
restricted because of some hindrance such as solubility levels and high affinity
toward the soil, but some PGPR, root-colonizing, and mycorrhizal microorganisms
enhance the bioavailability of metals to the plant systems. For example,
rhamnolipids, a biosurfactant released by some bacteria, modifies the hydrophobic
pollutants to more hydrophilic forms that are known to be released (Qiu et al. 1994;
Volkering et al. 1998).
The absorption of various metal ions such as Fe
2+ , Mn
2+ , and Cd
2+ by the root
system is facilitated by organic exudates of microorganisms via improving the
bioavailability. Root exudates in turn feed the microbes by being good sources of
carbohydrates, lipophilic compounds, and natural chelators such as citric, acetic, and
other organic acids, which are important in increasing the mobility of metal ions or
promoting the bio-surfactant-producing class of microorganisms (Fig. 4.2). The
many feedback mechanisms by plant roots and a rhizosphere microorganism allow
them to adapt to the conditions of their environmental habitats; for example, the root
exudates of plants in a phosphorus-deficient habitat contain large amounts of citric
acid as an attempt for mobilizing the phosphorus present in soils. In some cases, a
few of the rhizosphere-dwelling organisms secrete growth hormones for the plant to
increase the plant root growth, which in turn provides its nutrient root exudates.
7.1 Metal Bioavailability for Plant Roots
To remediate a pollutant from an environment, the plant or microorganisms should
establish a close contact and have the ability to act on it; hence, the bioavailability of
the pollutant is important for its remediation. Soil and environmental conditions and
the pollutant physiochemical properties decide bioavailability. The soil has more
sites for metal ions, especially in CEC, if it has small particle size (clay) by which it
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
99