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Bioremediation for Sustainable Environmental Cleanup

Figure 11.1. Uptake of lead from soil and its toxic effects within the plant.

IMPAIRED PHOTOSYNTHESIS

PHYTOHORMONAL IMBALANCE

DISTURBED NUTRIENT UPTAKE

DISTRESSED RESPIRATION

NUCLEAR DAMAGE

Decreased chlorophyll content

Altered chloroplast structure

Inefficient Hill reaction & Calvin cycle

Downregulation of TCA cycle enzymes

Improper electron and proton transport

Over-production of ABA & Ethylene

Low levels of Cytokinin & GAs

Decreased leaf & stomatal area

Less water uptake

Hinderance to uptake of essential ions-

Ca²⁺, Mg²⁺, Fe²⁺, Zn²⁺, Cu²⁺, NO3

Replication errors

Chromosome bridges & stickiness

Chromosomal aberrations- poly & aneuploidy

WATER POTENTIAL

Pb in Soil

Pb effects

within plant

and Calvin cycle

and Ethylene

and GAs

and stomatal area

and stickiness

and aneuploidy

Pb in PLANT SYSTEM

Detoxification Strategies

THICKENING OF CELL WALL

ADSORPTION OF METAL ON

CALLOSE THICKENINGS

EXTRACELLUAR SEQUESTRATION

VACOULAR SEQUESTRATION

PHYTOCHELATIN & METTALOTHIONEIN

SYNTHESIS

UPREGULATION OF SOD, POD,

CAT, GR, GST, GPOX.

INCREASED LEVELS OF GSH,

AsA & TOCOPHEROLS.

CONSTITUTIVE

INDUCIBLE

ADS ACTIVATION

Figure 11.2. Lead detoxification mechanisms within a plant system.

ASA AND TOCOPHEROLS.

PHYTOCHELATIN AND

METTALOTHIONEIN SYNTHESIS

11.3.1 Constitutive Mechanisms

Such mechanisms act as the first level of plant defense against mild Pb stress. This method of Pb

detoxification involves the adsorption of Pb on the cellular compartments, mainly on thickened cell

walls. Entry of Pb stimulates the synthesis of polysaccharides, callose and pectin, which causes

widening of the cell wall and hence restricts entry of Pb into the cell (Rucińska-Sobkowiak et al.

2013).

It also provides new sites for the Pb adsorption and enhances its sequestration extracellularly.

11.3.2 Inducible Mechanism

These mechanisms consume a large amount of energy and are usually activated under conditions of

high Pb toxicity. It involves the task of different transporters and ATPases for the efflux of symplastic

Pb into vacuoles. Along with this, the synthesis of Pb binding ligands such as phytochelatins (PCs),

metallothioneins (MTs) is also induced. PCs and MTs aid in Pb detoxification by its sequestration