Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

15.5 Microbial Enhancement of Metal Ion Removal Capacity of Water Hyacinthj295
therefore, be expected to modify the microbial ecology of rhizosphere [19–23,117].
This can be attained by various methods such as addition of organic amendments,
microbial inoculation, manipulation of plant genotypes and exposure to environmental
stresses such as heavy metal contamination.
15.5.2
Mechanisms of Metal Ion Removal by Plant Roots
Possible mechanisms of toxic metal ion removal by plant roots include extracellular
precipitation, cell wall precipitation and adsorption, intracellular uptake followed by
cytoplasm compartmentalization or vacuolar deposition (Figure 15.1) [130]. The
metal ion uptake and detoxification mechanisms will vary with different plant
species. Most metal ions enter plant cells by an energy-dependent, saturable process
through specific or generic metal ion carriers or channels [42]. Toxic metal ions may
employ the same mechanisms that are responsible for the uptake of essential ions.
Paganetto et al. [118] used the patch-clamp techniques to study the transport
properties of vacuolar ion channels from the roots of water hyacinth, Eichhornia
crassipes (Mart. Solms, Pontederiaceae). The vacuolar currents for the transport of
Ni 2+ and Zn 2+ were found to be different from other common ions such as Na + ,Ca 2+
and NH 4 + . However, membrane transport systems such as aqueous pores, ion efflux
pumps, ion selective channels and proton–anion contraports may fail to discriminate
among different metal ions that have similar ionic radii and the same ionic
charge [42,87].
Metabolically important cations from the external solution can accumulate in a
nonmetabolic step (Figure 15.3) [87]. Entry and association of metal ions with plant
cells may occur by a number of physical processes, including diffusion, ion exchange,
mass flow and adsorption. The cation-exchange occurs within the free
space of the roots where ions can penetrate without passage through a living
membrane. The exchangeable cations are electrostatically bound to negatively
charged functional groups, probably the free carboxyl groups of pectic cell wall
matrix substances [119–121]. Precipitation and exchangeable sorption remove metal
ions by forming insoluble compounds in the free space (Figure 15.3) [87]. Some
natural hyperaccumulators extend and proliferate their roots positively into patches
of high metal ion availability. In contrast, nonaccumulators actively avoid these
areas; this is one of the mechanisms by which hyperaccumulators absorb more
metal ions when grown in the same soil [122]. Also, phytochelatins play an important
role in the accumulation and detoxification of excess metal ions such as Cd 2+ in plant
cells [58,87,123–127]. This mechanism further assists phytofiltration technology, as
it increases the specificity of metal ions to binding domains in the plant root. Some
species of plants also have multiple binding capacities for metal ions owing to
induced formation of phytochelatins by different metal ions such as Cu 2+ ,Zn 2+ ,
Pb 2+ and Cd 2+ [87]. Indeed, the application of chelates has been shown to induce
substrate accumulation of metals such as Pb 2+ ,U 2+ and Au 3+ in the shoots of
nonhyperaccumulators by increasing metal solubility and root-to-shoot translocation
[122].