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the sugar trehalose imparts stress tolerance in plants

The paper below shows that enhancing plant production of a sugar ,
trehalose, improved response to a range of environmental stresses.
Trehalose has recently ben studied extensively for its ability to
improve cold and salt tolerance in plants. The genetic engineering is
unique in that much of the construction was based on rice regulatory
genes to control a pair of bacterial genes.
Some success has also been observed with addition of exogenous trehalose
but the engineered form is most stable.
Published online before print November 27, 2002
Proc. Natl. Acad. Sci. USA, 10.1073/pnas.252637799
Trehalose accumulation in rice plants confers high tolerance levels to
different abiotic stresses

Ajay K. Garg , Ju-Kon Kim , Thomas G. Owens , Anil P. Ranwala , Yang Do
Choi , Leon V. Kochian , and Ray J. Wu

Trehalose is a nonreducing disaccharide of glucose that functions as a
compatible solute in the stabilization of biological structures under
abiotic stress in bacteria, fungi, and invertebrates. With the notable
exception of the desiccation-tolerant "resurrection plants," trehalose
is not thought to accumulate to detectable levels in most plants. We
report here the regulated overexpression of Escherichia coli trehalose
biosynthetic genes (otsA and otsB) as a fusion gene for manipulating
abiotic stress tolerance in rice. The fusion gene has the advantages of
necessitating only a single transformation event and a higher net
catalytic efficiency for trehalose formation. The expression of the
transgene was under the control of either tissue-specific or
stress-dependent promoters. Compared with nontransgenic rice, several
independent transgenic lines exhibited sustained plant growth, less
photo-oxidative damage, and more favorable mineral balance under salt,
drought, and low-temperature stress conditions. Depending on growth
conditions, the transgenic rice plants accumulate trehalose at levels
3-10 times that of the nontransgenic controls. The observation that peak
trehalose levels remain well below 1 mg/g fresh weight indicates that
the primary effect of trehalose is not as a compatible solute. Rather,
increased trehalose accumulation correlates with higher soluble
carbohydrate levels and an elevated capacity for photosynthesis under
both stress and nonstress 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
stress-dependent overproduction of trehalose.