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

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B. Rathinasabapathi and R. Kaur
Rengel, 1999). Specific role of these enzymes or small molecules are better understood
by manipulating expression of the genes encoding antioxidant enzymes or by engineering
synthetic pathways to antioxidants (Allen et. al., 1997). Since many of both enzymatic
and non-enzymatic antioxidant systems are understood in plants, it must be
possible to improve oxidative stress tolerance in crops via metabolic engineering. Some
examples for such metabolic engineering attempts are listed in Table 5.
Transgenic tobacco, lucerne, potato and cotton overexpressing SOD in the
chloroplasts showed higher tolerance to oxidative stress (Bowler et. al., 1991, Gupta et.
al., 1993a, b, McKersie et. al., 1993, 1996, Perl et. al., 1993, Foyer et. al., 1994, Slooten et.
al., 1995). Similar results were observed with overproduction of SOD in the mitochondria
of lucerne (McKersie et. al., 1997) and in the cytosol of potato (Perl et. al., 1993).
Transgenic tobacco engineered to overexpress MnSOD in chloroplast provided protection
from Mn deficiency mediated oxidative stress (Yu et. al., 1999).
The tripeptide glutathione, the major cellular antioxidant, detoxifies the excess
hydrogen peroxide generated during oxidative stress. Glutathione metabolism has
therefore been manipulated to improve plant’s oxidative stress tolerance (Noctor et. al.,
1998). Transgenic plants overexpressing glutathione reductase had an increase in their
reduced:oxidized glutathione ratio with enhanced tolerance to oxidative stress (Noctor
et. al., 1998). Glutathione peroxidase is responsible for removal of unsaturated fatty acid
hydroperoxides generated in cellular membranes during oxidative stress. Transgenic
tobacco plants overexpressing Chlamydomonas glutathione peroxidase in the chloroplast
or the cytosol, exhibited higher membrane integrity by increasing tolerance against
oxidative stress (Yoshimura et. al., 2004).
3.2.9. Heat and Cold Stress
Temperate and subtropical plants are extremely vulnerable to high temperatures especially
during early tillering, flower initiation, anthesis and grain filling stages, leading to
considerable decrease in crop productivity. All tropical crops are exposed to high temperatures
and can benefit by increasing their high temperature stress tolerance further
(also see Chapter 4). However, this trait is particularly valuable in some of the world’s
most important food crops naturally adapted to cold weather. For example, improving
the heat stress tolerance in winter cereals and potato can extend these crops to areas
currently not suitable for them and increase their yield potential in some of the currently
grown locations (Veilleux et. al., 1997; Maestri et. al., 2002). Despite our vast knowledge
on how organisms respond to high temperature stress, little progress has been made in
breeding crops for high temperature stress tolerance.
3.2.10. Heat Shock Proteins
In response to high temperature, all organisms including plants produce a set of proteins
known as heat shock proteins (HSPs). Heat shock proteins (HSPs) have been