PARENT SESSION
Posters P3C C3 and Rubisco. Abstracts (617-630)

Mechanism for inactivation of Rubisco under heat stress. Michael Salvucci^*,1, Steven Crafts-Brandner¹, ¹ USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ, USA

ABSTRACT- Photosynthesis is particularly sensitive to direct inhibition by heat stress. This inhibition is closely related to inactivation of Rubisco. Recent evidence from our laboratory indicates that activase is unable to maintain Rubisco in an activated state under moderate heat stress, in part, because of its exceptional sensitivity to thermal denaturation. To develop a more complete understanding of the mechanism for inactivation of Rubisco under moderate heat stress, we examined various aspects of the process both in vivo and in vitro. Measurements of the activation state of Rubisco in cotton and tobacco leaves showed that Rubisco inactivates within 5 s of imposing a heat stress. The extent of inactivation under steady-state conditions correlated precisely with the degree of photosynthetic inhibition, even in plants from very different thermal environments. In vitro measurements with purified Rubisco and activase showed that Rubisco deactivated when temperatures exceed the rather low temperature optimum for activase ATPase activity. Trapping experiments with the reaction intermediate analog, 2-carboxyarabinitol-1,5-bisphosphate, demonstrated that inactivation of Rubisco did not involve loss of the activating CO₂. Instead, measurements of xylulose-1,5-bisphosphate synthesis by Rubisco indicated that the rate of catalytic misfire increased with temperature. 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. That the net effect of elevated temperatures is similar in leaves and in vitro with the isolated enzymes 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.

KEY WORDS: activase, catalytic misfire, Rubisco, heat stress