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item Salvucci, Michael
item Crafts-Brandner, Steven

Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2004
Publication Date: 9/3/2004
Citation: Salvucci, M.E., Crafts-Brandner, S.J. 2004. Mechanism for deactivation of rubisco under moderate heat stress. Physiologia Plantarum 122(4) 513-519.

Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize sugars and other foodstuffs. Heat stress inhibits photosynthesis, reducing the overall yield of the plant. Previous research from this research group identified an enzyme called Rubisco activase as the component of photosynthesis that is most sensitive to inhibition by heat. Rubisco activase is a regulatory enzyme that controls the activity of the major carbon dioxide-fixing enzyme in plants. In this manuscript, we present new data that provides additional details to the biochemical mechanism for inhibition of photosynthesis by moderate heat stress. Specifically, we show how the process can be simulated in the test tube, thereby proving that reduced activase activity, coupled with faster loss of Rubisco activity together can account for the inhibition of photosynthesis under heat stress that occurs in leaves. This information eventually can be used to make changes that improve the activity and stability of Rubisco activase in order to improve the efficiency of photosynthesis under heat stress.

Technical Abstract: Photosynthesis is particularly sensitive to direct inhibition by heat stress. This inhibition is closely associated with inactivation of Rubisco. To develop a more complete understanding of the mechanism for inactivation of Rubisco under moderate heat stress, various aspects of the process were examined both in vivo and in vitro. Experiments with isolated Rubisco revealed that the rate of synthesis of the catalytic misfire product, xylulose-1,5-bisphosphate, increased with temperature. Activated Rubisco, produced by reaction with activase at a control temperature of 25°C or by incubation with high CO2, deactivated when the temperature of the reaction exceeded temperatures that were equivalent to the optimum for activase ATPase activity. Measurements of the activation state of Rubisco in cotton and tobacco leaves showed that Rubisco inactivates within 7 s of imposing a heat stress. Thus, elevated temperatures have a direct and opposite effect on the two processes that ultimately determine the activation state of Rubisco, decreasing activase activity but stimulating the catalytic misfire reaction that inactivates Rubisco. These data support a mechanism for inactivation of Rubisco at high temperature involving an inability of activase to overcome the inherently faster rates of Rubisco inactivation. That the net effect of elevated temperatures on Rubisco activation is similar both in vivo and under controlled conditions in vitro argues for a direct effect of temperature on the activation of Rubisco by activase and against the proposal that deactivation of Rubisco under moderate heat stress is a secondary consequence of perturbations in the thylakoid membrane.