14.3.1 Mechanism of Heavy Metal Detoxification in Plants
Various cellular and molecular mechanisms are involved in detoxification and
tolerance and maintaining the homeostasis of metal-tolerant plant species (Hall
2002; Bhargava et al. 2012). Metal uptake by plants depends upon their bioavailability in soil which varies due to their low solubility in water and strong interaction
with soil particles. Metal present in soil first binds with the cell wall of the root which
contains various ion transport channels. High affinity to the binding sites present in
the cell wall mediates the uptake of the metal ions through various transporters like
P-type ATPase (Bali et al. 2019; Khanna et al. 2019), iron-regulated transporters
(IRT), zinc-regulated transporters (ZRP) (Dubey et al. 2018), and channel proteins
(Chaney et al. 2007). Absorbed metals from the roots are then transported to the
aerial parts of the plant through the xylem by root pressure and transpiration pull
(Robinson et al. 2003). Effective loading of the metal ions requires radial symplastic
path along with xylem loading. Plant changes the chemistry of these ions by
catalyzing and chelation to nontoxic forms and finally ends in their vacuolar
compartmentalization for accumulating the heavy metals (McGrath and Zhao
2003). It was assumed by Verbruggen et al. (2009) in his study that metals accumulated in plants are bounded with amino acids, organic acids, proteins, etc. Binding of
intracellular metal with organic acids and their compartmentalization in vacuoles
was also studied by Rascio and Navari-Izzo (2011). Plant defense system like
antioxidative defense system—enzymatic and non-enzymatic, stress responsive proteins, various transporters have been well identified in playing protective role in
heavy metal stress conditions. Plant hormones such as brassinosteroids, jasmonic
acid, salicylic acid and abscisic acid play a key role in self-defense against stressful
environmental conditions (Sharma et al. 2017; Bali et al. 2019; Kohli et al. 2018).
Application of chelators in enhancing the process of phytoremediation has been
documented by many researchers and is considered as an effective approach in
increasing the metal uptake (Lasat 2002). In his study Wang et al. (2019a) mentioned
the significant role of chelators along with Amaranthus hypochondriacus L. in the
process of Cd remediation. Arsenov et al. (2020) suggested that application of citric
acid, a chelating agent along with Salix viminalis, plays an effective role in remediation of Cd-contaminated soil. Phytoextraction efficiency of Pb and Zn has been
enhanced by using EDTA along with Raphanus sativus L. and Brassica oleracea
L. (Chaturvedi et al. 2019). Similar results were observed by Liphadzi and Kirkham
(2006) with EDTA- on Pb-contaminated soil. Sharma et al. (2019) found that
exogenous application of ascorbic acid on copper-stressed Brassica juncea seedlings
enhanced various osmoprotectants like cysteine, proline, free amino acids, and metal
chelators like thiols.
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P. Sharma et al.
Various cellular and molecular mechanisms are involved in detoxification and
tolerance and maintaining the homeostasis of metal-tolerant plant species (Hall
2002; Bhargava et al. 2012). Metal uptake by plants depends upon their bioavailability in soil which varies due to their low solubility in water and strong interaction
with soil particles. Metal present in soil first binds with the cell wall of the root which
contains various ion transport channels. High affinity to the binding sites present in
the cell wall mediates the uptake of the metal ions through various transporters like
P-type ATPase (Bali et al. 2019; Khanna et al. 2019), iron-regulated transporters
(IRT), zinc-regulated transporters (ZRP) (Dubey et al. 2018), and channel proteins
(Chaney et al. 2007). Absorbed metals from the roots are then transported to the
aerial parts of the plant through the xylem by root pressure and transpiration pull
(Robinson et al. 2003). Effective loading of the metal ions requires radial symplastic
path along with xylem loading. Plant changes the chemistry of these ions by
catalyzing and chelation to nontoxic forms and finally ends in their vacuolar
compartmentalization for accumulating the heavy metals (McGrath and Zhao
2003). It was assumed by Verbruggen et al. (2009) in his study that metals accumulated in plants are bounded with amino acids, organic acids, proteins, etc. Binding of
intracellular metal with organic acids and their compartmentalization in vacuoles
was also studied by Rascio and Navari-Izzo (2011). Plant defense system like
antioxidative defense system—enzymatic and non-enzymatic, stress responsive proteins, various transporters have been well identified in playing protective role in
heavy metal stress conditions. Plant hormones such as brassinosteroids, jasmonic
acid, salicylic acid and abscisic acid play a key role in self-defense against stressful
environmental conditions (Sharma et al. 2017; Bali et al. 2019; Kohli et al. 2018).
Application of chelators in enhancing the process of phytoremediation has been
documented by many researchers and is considered as an effective approach in
increasing the metal uptake (Lasat 2002). In his study Wang et al. (2019a) mentioned
the significant role of chelators along with Amaranthus hypochondriacus L. in the
process of Cd remediation. Arsenov et al. (2020) suggested that application of citric
acid, a chelating agent along with Salix viminalis, plays an effective role in remediation of Cd-contaminated soil. Phytoextraction efficiency of Pb and Zn has been
enhanced by using EDTA along with Raphanus sativus L. and Brassica oleracea
L. (Chaturvedi et al. 2019). Similar results were observed by Liphadzi and Kirkham
(2006) with EDTA- on Pb-contaminated soil. Sharma et al. (2019) found that
exogenous application of ascorbic acid on copper-stressed Brassica juncea seedlings
enhanced various osmoprotectants like cysteine, proline, free amino acids, and metal
chelators like thiols.
232
P. Sharma et al.