Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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B. Rathinasabapathi and R. Kaur
expressing rice pyruvate decarboxylase had slightly more pyruvate decarboxylase activity
than untransformed plants (Elllis et. al., 2000).
Under O 2
insufficiency conditions, the plant growth regulator ethylene is produced
in the aerial plant parts. 1-Aminocyclopropane-1-carboxylic acid (ACC) is the
precursor of ethylene. The enzyme ACC synthase is responsible for increase in ethylene
synthesis in flooded plants. ACC is synthesized in the roots and converted to
ethylene due to the action of ACC oxidase in the aerial parts (Grichko and Glick, 2001a).
ACC oxidase requires oxygen for its activity. Transgenic tomato expressing antisense
ACC synthase or ACC oxidase showed lower ethylene levels under root submergence
(John, 1997; English et. al., 1995). Another enzyme ACC deaminase can draw away ACC
from the ethylene synthesis pathway. Therefore, transgenic tomato plants expressing
this catabolic enzyme displayed higher tolerance to flooding than non-transformed
plants (Grichko and Glick, 2001b).
3.2.2. Heavy Metal Stress
Plants grown under acidic soils high in Al and Mn, saline soils high in Na and soils
contaminated with As, Cu, Zn, Pb, Ni, and Cd due to mining, industrial effluents and
other human activities or natural causes, show restricted growth and productivity (also
see Chapter 7 and 8). Response of plants to heavy metal toxicity varies greatly, such as
immobilization, exclusion, chelation and compartmentalization of the metal ions, and
expression of general stress responses, like ethylene and stress proteins synthesis
(Cobbett, 2000).
Certain toxic heavy metals can be removed from the soil or water using specific
plants that can remove and concentrate the toxic metal in its harvestable parts. This
technology, termed phytoremediation, is an environmentally friendly and cost effective
method than engineering technologies for environmental remediation. Although naturally
occurring heavy metal resistant plant species can be used for phytoremediation,
characterization of genes involved in heavy metal resistance and hyperaccumulation,
has led to the possibility for engineering plants for more efficient phytoremediation
(Pilon-Smits and Pilon, 2002). Ideally a transgenic plant for phytoremediation will have
the following characteristics: a) the plants will have the ability to grow at a fast rate
under environments that need to be remediated, b) they will take up and concentrate the
heavy metal into their harvestable biomass, c) they will not spread as uncontrolled
weeds via vegetative means or pollen and d) they will pose little threat to animals and
humans. Some examples for metabolic engineering for heavy metal stress tolerance are
summarized in Table 4.