154
(PGPR) perform vital role by increasing the production of growth regulators (e.g.
gibberellic acid, indole acetic acid, cytokinins and ethylene). Rhizobacteria also
produce phosphate-solubilizing agents, antibiotics, hydrocyanic acid, siderophores,
etc. that escalate metal bioavailability and root absorption (Davies Jr et al. 2001;
Gupta et al. 2016a).
6 Uranium Uptake by Plants
Uptake of several essential elements from soil is a natural phenomenon in plants
(Fig. 1). However, during the process of uptake, non-essential analogous elements
that are present in the environment enter into the plant’s body by comparable mechanisms. Some factors including agroclimatic conditions, soil properties, pH, mineral and organic constituents, soil microbial communities, chelating agents, etc.
influence the uptake process and accumulation of any element (Walther and Gupta
2015). Nevertheless, decontamination of polluted environment using plants is of
great interest in recent time (Gupta et al. 2016a). Behaviour of U in soil is an important factor for understanding phytoremediation of this element as is having different
chemical species, which are of diverse solubility and biological accessibility patterns (Mitchell et al. 2013; Newsome et al. 2014). For example, in oxic milieus,
hexavalent U(VI) usually occurs in the hydrated state (UO 2
2+
soluble uranyl ion,
schoepite, etc.), while less soluble compounds are formed in anoxic environments
(Ebbs et al. 1998; Boghi et al. 2018). Although soil organic matters like clay mineral
particles adsorb UO 2
2+
, solubility increases if specific ligands like carbonates are
present in solution (Davis et al. 2004; Bryan et al. 2012; Cumberland et al. 2016).
Regulation methods of U uptake and translocation by plants are not clearly understood. Effective phytoremediation for ameliorating U-contaminated soil depends
upon various factors like solubilization, absorption, transportation and accumulation within plants. It was observed that low pH enhances the root-to-shoot uptake
and translocation of U, which varies amongst species (Saenen et al. 2015; Favas
et al. 2016). But large accumulation of U within shoot is toxic for plants creating
oxidative stress (Saenen et al. 2015). Mycorrhizal fungi play an important role in U
uptake and translocation. Due to solubilization by mycorrhizal fungi at rhizosphere,
although plant uptake of U increases, though, impediments of root-to-shoot translocation have also been reported (De Boulois et al. 2008; Davies et al. 2015; Boghi
et al. 2018). Gupta et al. (2016b) working with hydroponically grown Pisum sativum plant reported that H 2 O 2 production was higher in experimental concentration
of [U] = 25 and 50 μM, with higher U accumulation in roots than shoots. However,
damage to the membranes of both root and leaves was recorded due to the accumulation of U signifying oxidative stress and related toxic mechanisms (Gupta et al.
2016b).
D. K. Gupta et al.
(PGPR) perform vital role by increasing the production of growth regulators (e.g.
gibberellic acid, indole acetic acid, cytokinins and ethylene). Rhizobacteria also
produce phosphate-solubilizing agents, antibiotics, hydrocyanic acid, siderophores,
etc. that escalate metal bioavailability and root absorption (Davies Jr et al. 2001;
Gupta et al. 2016a).
6 Uranium Uptake by Plants
Uptake of several essential elements from soil is a natural phenomenon in plants
(Fig. 1). However, during the process of uptake, non-essential analogous elements
that are present in the environment enter into the plant’s body by comparable mechanisms. Some factors including agroclimatic conditions, soil properties, pH, mineral and organic constituents, soil microbial communities, chelating agents, etc.
influence the uptake process and accumulation of any element (Walther and Gupta
2015). Nevertheless, decontamination of polluted environment using plants is of
great interest in recent time (Gupta et al. 2016a). Behaviour of U in soil is an important factor for understanding phytoremediation of this element as is having different
chemical species, which are of diverse solubility and biological accessibility patterns (Mitchell et al. 2013; Newsome et al. 2014). For example, in oxic milieus,
hexavalent U(VI) usually occurs in the hydrated state (UO 2
2+
soluble uranyl ion,
schoepite, etc.), while less soluble compounds are formed in anoxic environments
(Ebbs et al. 1998; Boghi et al. 2018). Although soil organic matters like clay mineral
particles adsorb UO 2
2+
, solubility increases if specific ligands like carbonates are
present in solution (Davis et al. 2004; Bryan et al. 2012; Cumberland et al. 2016).
Regulation methods of U uptake and translocation by plants are not clearly understood. Effective phytoremediation for ameliorating U-contaminated soil depends
upon various factors like solubilization, absorption, transportation and accumulation within plants. It was observed that low pH enhances the root-to-shoot uptake
and translocation of U, which varies amongst species (Saenen et al. 2015; Favas
et al. 2016). But large accumulation of U within shoot is toxic for plants creating
oxidative stress (Saenen et al. 2015). Mycorrhizal fungi play an important role in U
uptake and translocation. Due to solubilization by mycorrhizal fungi at rhizosphere,
although plant uptake of U increases, though, impediments of root-to-shoot translocation have also been reported (De Boulois et al. 2008; Davies et al. 2015; Boghi
et al. 2018). Gupta et al. (2016b) working with hydroponically grown Pisum sativum plant reported that H 2 O 2 production was higher in experimental concentration
of [U] = 25 and 50 μM, with higher U accumulation in roots than shoots. However,
damage to the membranes of both root and leaves was recorded due to the accumulation of U signifying oxidative stress and related toxic mechanisms (Gupta et al.
2016b).
D. K. Gupta et al.