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PLANT PHYSIOLOGY , Vol 103, Issue 2 629-635, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Use of Transgenic Plants with Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Antisense DNA to Evaluate the Rate Limitation of Photosynthesis under Water Stress
D. Gunasekera and G. A. Berkowitz
Plant Science Department, Cook College, Rutgers-The State University of New Jersey, New Brunswick, New Jersey 08903
The biochemical lesion that causes impaired chloroplast metabolism (and,
hence, photosynthetic capacity) in plants exposed to water deficits is
still a subject of controversy. In this study we used tobacco (Nicotiana
tabacum L.) transformed with "antisense" ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whether Rubisco
or some other enzymic step in the photosynthetic carbon reduction cycle
pathway rate limits photosynthesis at low leaf water potential ([psi]w).
These transformants, along with the wild-type material, provided a novel
model system allowing for an evaluation of photosynthetic response to water
stress in near-isogenic plants with widely varying levels of functional
Rubisco. It was determined that impaired chloroplast metabolism (rather
than decreased leaf conductance to CO2) was the major cause of
photosynthetic inhibition as leaf [psi]w declined. Significantly, the
extent of photosynthetic inhibition at low [psi]w was identical in
wild-type and transformed plants. Decreasing Rubisco activity by 68% did
not sensitize photosynthetic capacity to water stress. It was hypothesized
that, if water stress effects on Rubisco caused photosynthetic inhibition
under stress, an increase in the steady-state level of the substrate for
this enzyme, ribulose 1,5-bisphosphate (RuBP), would be associated with
stress-induced photosynthetic inhibition. Steady-state levels of RuBP were
reduced as leaf [psi]w declined, even in transformed plants with low levels
of Rubisco. Based on the similarity in photosynthetic response to water
stress in wild-type and transformed plants, the reduction in RuBP as stress
developed, and studies that demonstrated that ATP supply did not rate limit
photosynthesis under stress, we concluded that stress effects on an enzymic
step involved in RuBP regeneration caused impaired chloroplast metabolism
and photosynthetic inhibition in plants exposed to water deficits.
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