them to conclude that it is trehalose-6-P and not trehalose that
is modulating photosynthetic capacity (29).
Fig. 5 shows the light intensity dependence of PS II electron
transport rates, as determined by
measurements (21) for
nontransgenic rice and transgenic lines R80 and A05 measured
under control (nonstress) conditions. Although the differences
in photosynthesis are small at limiting light intensities, at light
saturation, the rates of photosynthesis in the transgenic plants
are 5–15% higher than in the NTCs. At light saturation, pho-
tosynthetic rate is limited by the capacity of the dark reactions,
in particular the Calvin cycle and triose phosphate utilization in
the cytoplasm (27). Together with the observed higher levels of
soluble carbohydrate under both stress and nonstress conditions
(Table 3), the elevated levels of light-saturated photosynthesis in
the transgenic plants supports the suggestion that in plants,
trehalose acts as a regulator of sugar sensing and, thus, the
expression of genes associated with carbon metabolism (29). The
presence of a higher capacity for photosynthesis before stress
provides a larger sink for the products of photosynthesis during
stress, thus limiting the extent of excess-light-induced photooxi-
dative damage and accounting, in part, for the more vigorous
growth of the transgenic lines during stress. Interestingly, the
higher efficiency of trehalose synthesis by the TPSP fusion gene
product (13) would suggest that trehalose, rather than trehalose-
6-P is leading the enhanced capacity for photosynthesis.
We have demonstrated that regulated overexpression of treha-
lose biosynthetic genes in rice has considerable potential for
improving abiotic stress tolerance and, at the same time, aug-
menting productivity under both stress and nonstress conditions.
This work showed successful conferment of tolerance to multiple
abiotic stresses by means of overexpression of trehalose biosyn-
thesis without the negative pleiotropic effects seen in previous
studies. The modest increase in trehalose levels in transgenic
lines, using either the tissue-specific or stress-dependent pro-
moters, resulted in a higher capacity for photosynthesis and a
concomitant decrease in the extent of photo-oxidative damage
during stress. In addition, trehalose must be interacting with
other physiological processes to account for changes in ion
uptake and partitioning during salt stress. Because other cereal
crops, like rice, are also sensitive to abiotic stresses, it is likely
that overexpression of trehalose biosynthetic genes in maize and
wheat may also confer high levels of abiotic stress tolerance.
We thank A. Jagendorf, M. Hanson, W. B. Miller, and T. L. Setter for
critical review of the manuscript. We also thank J. Lee, H. Manslank, and
A. S. Stolfi for technical assistance. This research was supported in part
by Rockefeller Foundation Grant RF 98001-606 (to R.J.W.), by a
postdoctoral fellowship (to A.K.G.) from the Rockefeller Foundation,
and by grants from the Ministry of Science and Technology of Korea
through the Crop Functional Genomics Center (to J.-K.K. and Y.D.C.).
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