biological activity (Gadd 1992). Hence, these toxic metal pollutants have to removed
or be made unavailable for biological systems by rendering them in a safe way.
PGPR encounters HMs in the ecosystem and make them inactive through various
processes of mobilization, immobilization, and transformation, through which they
are adopted to survive in such stressed conditions (Nies 1999). Some of the mechanisms are given here: (1) exclusion, a process by which metals are excluded from
the targeted sites; (2) extrusion, in which absorbed metals are pushed out from the
cells via chromosomal/plasmid functioning; (3) accommodation, or conjugation
with metal-binding proteins such as metallothioneins and other low molecular
weight proteins (Kao et al. 2006; Umrania 2006) or other components of the cell;
(4) biotransformation, the conversion of toxic to less toxic forms; and (5) methylation
and demethylation. These methods lay a platform for survival ability and being
metabolically active in such stress conditions.
6.7 Synergistic Interaction of PGPR and Plants in HeavyMetal Remediation
Plant growth promotion by PGPR is well documented (Reed and Glick 2004;
Babalola et al. 2007; Babalola 2010). Recent reports say this interaction not only
enhances growth but also reduces environmental stress to the plant where it grows;
achieved by the synergic association with the plant root beyond many beneficial
effects such as improving soil quality, fixing atmospheric nitrogen, enhancing plant
performance, and also nutrient availability (Tinker 1984; Babalola et al. 2007; Li
et al. 2013).
6.8 ACC Deaminase and Plant Stress Reduction from
Ethylene
The ACC (1-aminocyclopropane) deaminase of certain PGPR enhances the uptake
of inorganic contaminants in a site by plants via modifying their root architecture and
their uptake abilities; this was achieved through modulating stress-induced production of ethylene by plants (Macek et al. 2000; Arshad et al. 2007). Plant growth
inhibition and reduction in biomass is effectively achieved by stress ethylene
biosynthesis, especially for roots (Glick et al. 2007). The PGPR with ACC deaminase is the only successful strategy to overcome the challenges in remediation by
plants (Glick 2003; Gerhardt et al. 2006; Meplan 2011), which hydrolyzes the
precursor to ethylene, 1-aminocyclopropane-1-carboxylic acid, thus lowering the
ethylene biosynthesis rate (Glick et al. 1998; Ma et al. 2001), without being affected
by giving space for the plant for the key defense response through a little burst of
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M. K. Awasthi et al.
or be made unavailable for biological systems by rendering them in a safe way.
PGPR encounters HMs in the ecosystem and make them inactive through various
processes of mobilization, immobilization, and transformation, through which they
are adopted to survive in such stressed conditions (Nies 1999). Some of the mechanisms are given here: (1) exclusion, a process by which metals are excluded from
the targeted sites; (2) extrusion, in which absorbed metals are pushed out from the
cells via chromosomal/plasmid functioning; (3) accommodation, or conjugation
with metal-binding proteins such as metallothioneins and other low molecular
weight proteins (Kao et al. 2006; Umrania 2006) or other components of the cell;
(4) biotransformation, the conversion of toxic to less toxic forms; and (5) methylation
and demethylation. These methods lay a platform for survival ability and being
metabolically active in such stress conditions.
6.7 Synergistic Interaction of PGPR and Plants in HeavyMetal Remediation
Plant growth promotion by PGPR is well documented (Reed and Glick 2004;
Babalola et al. 2007; Babalola 2010). Recent reports say this interaction not only
enhances growth but also reduces environmental stress to the plant where it grows;
achieved by the synergic association with the plant root beyond many beneficial
effects such as improving soil quality, fixing atmospheric nitrogen, enhancing plant
performance, and also nutrient availability (Tinker 1984; Babalola et al. 2007; Li
et al. 2013).
6.8 ACC Deaminase and Plant Stress Reduction from
Ethylene
The ACC (1-aminocyclopropane) deaminase of certain PGPR enhances the uptake
of inorganic contaminants in a site by plants via modifying their root architecture and
their uptake abilities; this was achieved through modulating stress-induced production of ethylene by plants (Macek et al. 2000; Arshad et al. 2007). Plant growth
inhibition and reduction in biomass is effectively achieved by stress ethylene
biosynthesis, especially for roots (Glick et al. 2007). The PGPR with ACC deaminase is the only successful strategy to overcome the challenges in remediation by
plants (Glick 2003; Gerhardt et al. 2006; Meplan 2011), which hydrolyzes the
precursor to ethylene, 1-aminocyclopropane-1-carboxylic acid, thus lowering the
ethylene biosynthesis rate (Glick et al. 1998; Ma et al. 2001), without being affected
by giving space for the plant for the key defense response through a little burst of
98
M. K. Awasthi et al.