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

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B. Rathinasabapathi and R. Kaur
Figure 2 . Structures of some osmoprotectants found in stress-tolerant organisms
mannitol’s utilization (Williamson et al., 1995). In the first demonstration of engineering
for mannitol overproduction in plants, the gene for M6PR was not used. Instead, a
bacterial gene for the reversible mannitol-1-phosphate dehydrogenase (mtlD), which
catalyzes the conversion of fructose-6-phosphate to mannitol-1-phosphate was employed.
This led to mannitol synthesis in transgenic tobacco, resulting in an
osmotolerance phenotype against salinity (Tarczynski et. al., 1993; Karakas et. al., 1997).
In independent experiments, transgenic tobacco with increased mannitol in
the chloroplasts was shown to exhibit increased resistance to methyl viologen-induced
oxidative stress (Shen et. al., 1997). Transgenic wheat expressing the bacterial mtlD
gene also exhibited tolerance to water stress and salinity (Abebe et. al., 2003), despite
the lack of osmotically significant quantities of mannitol in these transgenic plants.
This suggested that mannitol’s effects are from protective mechanisms other than osmotic
adjustment or via mannitol’s osmoprotective role in the meristematic regions
(Abebe et. al., 2003).
Arabidopsis, a non-mannitol producer, was transformed with the celery leaf
M6PR gene under the control of the CaMV 35S promoter. Transgenic plants accumulated
from 0.5 to 6 ìmoles g -1 fresh weight mannitol. Salt tolerance in transgenic plants
was demonstrated in soil irrigated with 300 mM NaCl in the nutrient solution (Zhifang
and Loescher, 2003). Pathways to other sugar alcohols have also been successfully
installed in transgenic plants and stress tolerance phenotypes have been observed
(Sheveleva et. al., 1997).