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

Metabolic Engineering for Stress Tolerance
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absorbing Pfr-form. R FR ratio (R:FR) of light incident upon the plant is responsible for
setting up a dynamic equilibrium between the two forms (Smith and Holmes, 1977). In
open canopies, a high R:FR favors Pfr-form, which suppress elongation, growth and
flowering of the plant, resulting in a normal and healthy growth pattern (Whitelam and
Devlin, 1997). On the other hand, low R:FR reflected from adjacent foliage drives phytochrome
equilibrium in the direction of Pr, activating shade avoidance response (Smith,
1994; Delvin et. al., 2003). Changes in the R to FR ratio perceived by phytochromes
initiate a number of responses such as increased stem extension growth. Stem elongation
is blocked when phytochrome A (PhyA) is present at high levels. Therefore,
transgenic approaches can manipulate phytochrome genes to disable responses to R
to FR ratio so that a higher proportion of resources can be incorporated into harvestable
material of crops (Smith, 1992). Transgenic tobacco ectopically expressing oat PHYA
were indistinguishable from wild-type at the lowest plant density but became gradually
shorter as the plant density increased and led to increase in harvest index. This demonstrated
the suppression of shade avoidance response under high level of phyA expression
(McCormac et. al., 1992; Schmitt et al., 1995; Robson et. al.; 1996; Shlumkov et. al.,
2001). Phytochrome A overexpression also inhibited hypocotyl elongation in transgenic
Arabidopsis (Boylan and Quail, 1991, Thiele et. al., 1999). Expression of the Arabidopsis
PHYB (phytochrome B) gene can increase tuber yield by increasing photosynthesis
and delayed leaf senescence at high plant densities in field-grown transgenic potato
(Thiele et. al., 1999; Boccalandro et. al., 2003; Schittenhelm et. al., 2004).
Other strategies for increasing photosynthetic activity include modification
of key photosynthetic enzymes, and conversion of C 3
photosynthetic pathway into C 4
.
An effort has been made to transform Rubisco and the enzymes of the Calvin cycle in
tobacco (Miyagawa et. al., 2001; Whitney and Andrews, 2001). Transgenic rice plants
were also produced expressing pyruvate orthophosphate dikinase and NADP-malic
enzyme (Ku et. al., 1999). The maize gene encoding phosphoenolpyruvate carboxylase
(PEPC) has been transferred into several C 3
crops, including potato (Ishimaru et. al.,
1998) and rice (Matsuoka et. al., 1998; Ku et. al., 1999) in order to increase the overall
level of C fixation.
3.2.16. UV-B Stress
Destabilization of ozone layer is resulting in an increase in the ultraviolet-B (UV-B: 280–
320 nm) radiation reaching the earth’s surface. This has increased our attention about
potential effects of increased UV-B levels on plant growth and development. UV-B
radiation affects plants by causing damage to DNA directly or indirectly through formation
of free radicals (Kalbin et. al., 2001), photosystems, phytohormones, and membranes
by peroxidation of unsaturated fatty acids (Mackerness 2000). A number of
secondary metabolites such as flavonoids, tannins and lignins are increased at elevated
levels of UV-B radiation. These metabolites screen UV-B and protect the
cellular components against the UV-B damage (Hofmann et. al., 2000, Kondo and
Kawashima 2000, Ryan et al., 2002),