Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 185

high, toxic doses. Some heavy metal ions play an important role in many meta-
bolic processes, making them essential for metabolism, growth and development
in trace element quantities. Heavy metals are only able to exert a stimulatory or
inhibitory effect on plants if they are present in a form available to the plants.
Availability is influenced by a range of abiotic and biotic factors. The abiotic
factors include the ionogenicity of the toxic metal, its solubility in water and its
ability to form complexes, and the pH and redox potential of the soil. The biotic
parameters include the protons and organic acids (e.g. citric acid, amino acids)
exuded into the rhizosphere by plant roots, the symbiosis of higher plants with
mycorrhizal fungi, and the quantity of humic acids and humin present in the soil as
the result of organic matter decomposition. Problems only arise when the cells
encounter a high concentration of heavy metal ions, which cause cell damage. The
heavy metals contaminating the soil, largely due to various anthropogenic activ-
ities, constitute one of the major environmental contaminants that restrict plant
productivity. Their non- biodegradability results in their prolonged persistence in
the environment, which is coupled with the tendency for bio-enrichment through
food chains (Sharma and Dietz 2009 ).
The effects of heavy metals on plant species have been well studied (Sanita di
Toppi and Gabbrielli 1999 ; Yadav 2010 ). Plants may be sensitive or tolerant,
based on their ability or inability to adapt to heavy metal ions. In plants sensitive to
heavy metals, cell damage, or in severe cases cell death, may be caused by a
number of mechanisms. Certain heavy metal ions can readily be exchanged for the
essential metal ions in the active centres of enzymes (e.g. Mg 2+ ,Zn 2+ ,Cu 2+, Co 2+ ),
thus inhibiting the regulatory role of the enzymes. Furthermore, heavy metal ions
may form free radicals, leading to the lipid peroxidation of cell membranes and
causing them to lose their ion permeability. The results have been found to vary as
a function of the type of metal ion, the concentration, the duration of treatment and
the plant organ investigated. The evaluation of the results is further complicated by
the fact that many symptoms are not specific to the responses induced by heavy
metal stress. In ananalogous manner to general stress theory, mechanisms which
result in heavy metal stress tolerance can be divided into two groups: avoidance
strategies and tolerance strategies (Hall 2002 ). Avoidance strategies involve the
limitation of heavy metal uptake, thus excluding them from the tissues, while
plants capable of tolerating heavy metals synthesise amino acids, proteins and
peptides that bind heavy metals which are accumulated, stored and immobilised in
these forms. Some of the most important compounds involved in heavy metal
tolerance are phytochelatins (PCs). These enzymatically synthesized Cys-rich
peptides are able to create chelates with heavy metals, thus decreasing their
damaging effects (Grill et al. 1989 ). PCs form a family of structures in which the c-
Glu-Cys dipeptide is repeated a number of times, followed by a terminal Gly: [(c-
Glu-Cys) n -Gly], where n is generally in the range of 2-5. As their glutamic acid
content is present in a c-bond, these peptides are capable of forming thiolate bonds
with heavy metal ions. They then transport the bound metal ions from the cyto-
plasm into the vacuoles, where they can be stored in non-toxic form, bound to
organic acid ligands.