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

Metabolic Engineering for Stress Tolerance
265
trade-off between increased stress tolerance and unwanted physiological effects on
plant growth needs to be considered.
In a recent example, Garg et. al., (2002) expressed bacterial genes (otsA and
otsB) in rice using tissue-specific and stress-dependent promoters. Transgenic rice
plants were not stunted and accumulated trehalose 3-10 times that of the non-transgenic
controls and exhibited tolerance to salt, drought and low-temperature stress. Increased
trehalose accumulation correlates with higher soluble carbohydrate levels and an elevated
capacity for photosynthesis under both stress and non-stress conditions, consistent
with a suggested role in modulating sugar sensing and carbohydrate metabolism.
These findings demonstrate the feasibility of engineering rice for increased tolerance
of abiotic stress and enhanced productivity through tissue-specific or stressdependent
overproduction of trehalose (Garg et. al., 2002). This made the authors
suggest that stress inducible transgene expression is important in recovering trehalose-accumulating
transgenics without deleterious effects. However, in a similar study
where a bacterial gene encoding both trehalose-6-phosphate synthase and trehalose-
6-phosphate phosphatase activities was expressed in rice under the control of a maize
ubiquitin promoter (Jan et. al., 2002), stress tolerant transgenic rice with no growth
abonormalities, was recovered, suggesting that rice may be more tolerant to trehalose
pathway modification than dicots. It will be instructive therefore to engineer plants for
novel osmoprotectants with and without stress inducible promoters and to use both
monocots and dicots to evaluate the technology.
3.1.3. Fructans
Fructans are polyfructose molecules synthesized by many plants and bacteria mainly
as a storage carbohydrate (Hendry, 1993; Pilon-Smits et. al., 1995). Fructans play an
important role in root branching, thus helpful in increasing root surface area and water
uptake by the plant. Transgenic plants overexpressing gene sacB exhibited higher
capacity for osmotic adjustment. The additional gain in carbohydrate storage may
direct deeper rooting and enhanced water uptake (Sharp et. al., 1996; Pilon-Smits et. al.,
1995; Schellenbaum et. al., 1999).
3.1.4. Proline and Proline Betaines
Proline accumulation has been shown to be linked with drought and salinity stresses in
plants (Delauney and Verma, 1990) and is caused by activation of proline biosynthesis
and inhibition of proline degradation. Glutamate and ornithine are both precursors of
proline. Proline is synthesized from glutamate via two intermediates, glutamic-Qsemialdehyde
(GSA) and Ä- 1 pyrroline-5-carboxylate (P5C), catalyzed by two enzymes,
P5C synthase (P5CS) in the first step and P5C reductase (P5CR) in the final step. Overproduction
of proline in transgenic tobacco (10-18 folds) (Kavi Kishor et. al., 1995), rice
(Zhu et. al., 1998), wheat (Sawahel and Hassan 2002) and hybrid larch, Laria