those which are toxic. These low molecular weight thiols are categorized as Biological thiols. Glutathione (GSH) and cysteine are known biological thiols. GSH works
as a substrate for phytochelatin synthesis and which help detoxifying heavy metals
like Cd and Ni (Freeman et al. 2004). Once PCs production in stimulated in response
to heavy metal stress they form complexes with toxic metal ions in the cytosol and
consequently get subsequently transported them into the vacuole (Salt and Rauser
1995). Another key element in this defence mechanism is the production of
metallothioneins (MTs) metal complex at the intra- and intercellular level. This is
followed by heavy metal removal from sensitive sites or legand metal sequestration
into vacuoles. Thus, first by chelation of metal ions and then by sequestration in
vacuoles, plants remain protected from deleterious effect of metals.
Though heavy metal accumulation in aquatic plants causes several morphological, biochemical, and ultrastructural abnormalities, chloroplast destruction, changes
in nitrogen metabolism inactivation of enzymes and alteration in normal physiological and metabolic pathways; the plants develop a very potential mechanism to
combat with toxicity induced by heavy metals. In response to heavy metal stress,
plants are furnished with a repository mechanism to impede heavy metal toxicity.
Also, in response to heavy metal stress, determining plants’ biochemical
responses play a key role in revealing mechanisms that have role in adaptation,
tolerance and expression of sensitivity towards the stress. Generation of Reactive
Oxygen Species (ROS) is the primary response of plants including Aquatic inhabitants due to the auto oxidation of heavy metal which is redox-active. This ROS
cause damage to the biomolecules. But as an adaptive strategy, plants develop a set
of defence mechanisms that deal with oxidative stress. This mechanism is complex
mechanism also known as ROS scavenging mechanisms. They occur at cellular as
well as molecular level. These mechanisms not only slow down the process of
oxidation of biomolecules and reduce damage to cell oxidation yet increase plant
resistance towards heavy metals (Sharma and Dubey 2005). One of the prime
constitutions of plant defence system includes antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), glutathione reductase
(GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase
(DHAR) and low-molecular weight quenchers (cysteine, ascorbic acid, thiols, proline (Sharma and Dubey 2005). α-tocopherol, carotenoids, phenolic and nitrogen
compounds (Singh et al. 2010). These enzymes have protective roles against oxidative damage (Nayek et al. 2010).
Heavy metal stress triggers the activity of these antioxidative enzymes which
decrease the level of H 2 O 2 that helps minimising the damage to cell membranes.
Also, MDA which is produced by lipid peroxidation under the condition of distress
also get enhanced. This increased MDA content play a key role that Pbs to adaptation and thus plants are able to overcome the stress finally Pbs to the survival.
In addition to this, Carboxylic acids and amino acids, such as citric, malic, and
histidine (His), are potential ligands produced by the plants and thus play a vital role
in detoxification and tolerance towards metal ions (Rauser 1999; Hall 2002) Heat
shock proteins (HSPs) that normally show enhanced expression in antiphon to the
growth of number of organisms at temperatures above the ideal temperature required
324
P. Parikh and K. Unadkat
as a substrate for phytochelatin synthesis and which help detoxifying heavy metals
like Cd and Ni (Freeman et al. 2004). Once PCs production in stimulated in response
to heavy metal stress they form complexes with toxic metal ions in the cytosol and
consequently get subsequently transported them into the vacuole (Salt and Rauser
1995). Another key element in this defence mechanism is the production of
metallothioneins (MTs) metal complex at the intra- and intercellular level. This is
followed by heavy metal removal from sensitive sites or legand metal sequestration
into vacuoles. Thus, first by chelation of metal ions and then by sequestration in
vacuoles, plants remain protected from deleterious effect of metals.
Though heavy metal accumulation in aquatic plants causes several morphological, biochemical, and ultrastructural abnormalities, chloroplast destruction, changes
in nitrogen metabolism inactivation of enzymes and alteration in normal physiological and metabolic pathways; the plants develop a very potential mechanism to
combat with toxicity induced by heavy metals. In response to heavy metal stress,
plants are furnished with a repository mechanism to impede heavy metal toxicity.
Also, in response to heavy metal stress, determining plants’ biochemical
responses play a key role in revealing mechanisms that have role in adaptation,
tolerance and expression of sensitivity towards the stress. Generation of Reactive
Oxygen Species (ROS) is the primary response of plants including Aquatic inhabitants due to the auto oxidation of heavy metal which is redox-active. This ROS
cause damage to the biomolecules. But as an adaptive strategy, plants develop a set
of defence mechanisms that deal with oxidative stress. This mechanism is complex
mechanism also known as ROS scavenging mechanisms. They occur at cellular as
well as molecular level. These mechanisms not only slow down the process of
oxidation of biomolecules and reduce damage to cell oxidation yet increase plant
resistance towards heavy metals (Sharma and Dubey 2005). One of the prime
constitutions of plant defence system includes antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), glutathione reductase
(GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase
(DHAR) and low-molecular weight quenchers (cysteine, ascorbic acid, thiols, proline (Sharma and Dubey 2005). α-tocopherol, carotenoids, phenolic and nitrogen
compounds (Singh et al. 2010). These enzymes have protective roles against oxidative damage (Nayek et al. 2010).
Heavy metal stress triggers the activity of these antioxidative enzymes which
decrease the level of H 2 O 2 that helps minimising the damage to cell membranes.
Also, MDA which is produced by lipid peroxidation under the condition of distress
also get enhanced. This increased MDA content play a key role that Pbs to adaptation and thus plants are able to overcome the stress finally Pbs to the survival.
In addition to this, Carboxylic acids and amino acids, such as citric, malic, and
histidine (His), are potential ligands produced by the plants and thus play a vital role
in detoxification and tolerance towards metal ions (Rauser 1999; Hall 2002) Heat
shock proteins (HSPs) that normally show enhanced expression in antiphon to the
growth of number of organisms at temperatures above the ideal temperature required
324
P. Parikh and K. Unadkat