Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.

Net photosynthesis (Pn) is inhibited by moderate heat stress. To elucidate the mechanism of inhibition, we examined the effects of temperature on gas exchange and ribulose 1,5-bisphosphate carboxylase͞oxygenase (Rubisco) activation in cotton and tobacco leaves and compared the responses to those of the isolated enzymes. Depending on the CO 2 concentration, Pn decreased when temperatures exceeded 35-40°C. This response was inconsistent with the response predicted from the properties of fully activated Rubisco. Rubisco deactivated in leaves when temperature was increased and also in response to high CO 2 or low O2. The decrease in Rubisco activation occurred when leaf temperatures exceeded 35°C, whereas the activities of isolated activase and Rubisco were highest at 42°C and >50°C, respectively. In the absence of activase, isolated Rubisco deactivated under catalytic conditions and the rate of deactivation increased with temperature but not with CO 2. The ability of activase to maintain or promote Rubisco activation in vitro also decreased with temperature but was not affected by CO 2. Increasing the activase͞Rubisco ratio reduced Rubisco deactivation at higher temperatures. The results indicate that, as temperature increases, the rate of Rubisco deactivation exceeds the capacity of activase to promote activation. The decrease in Rubisco activation that occurred in leaves at high CO 2 was not caused by a faster rate of deactivation, but by reduced activase activity possibly in response to unfavorable ATP͞ADP ratios. When adjustments were made for changes in activation state, the kinetic properties of Rubisco predicted the response of Pn at high temperature and CO 2.C urrent models of global climate predict a gradual increase in the atmospheric concentrations of greenhouse gases over the next century and associated increases in global temperature (1). The resulting effects on net photosynthesis will have significant ecological and agricultural consequences. For temperate C3 plants, the rate of photosynthesis under current atmospheric conditions exhibits a broad temperature optimum, with the maximum rate occurring near the daytime temperature under which the plants are grown (2). The broad temperature optimum for photosynthesis has been attributed to differential changes in the solubility of CO 2 and O 2 with temperature (3, 4) and to specific changes in the kinetic properties of ribulose 1,5-bisphosphate carboxylase͞oxygenase (Rubisco) (4-6). Increasing the CO 2 concentration above ambient generally increases the temperature optimum for photosynthesis by several degrees (2, 4).Models of photosynthesis have been developed to explain the response of photosynthesis to environmental conditions (7,8). These models make a clear distinction between limitations attributable to Rubisco and those attributable to factors that limit the rate of ribulose-1,5-bisphosphate (RuBP) regeneration, but they usually do not include changes in the activation state of Rubisco among the factors that influence the rate of photosynthesis. Th...

Although the catalytic activity of Rubisco increases with temperature, the low affinity of the enzyme for CO(2) and its dual nature as an oxygenase limit the possible increase in net photosynthesis with temperature. For cotton, comparisons of measured rates of net photosynthesis with predicted rates that take into account limitations imposed by the kinetic properties of Rubisco indicate that direct inhibition of photosynthesis occurs at temperatures higher than about 30 degrees C. Inhibition of photosynthesis by moderate heat stress (i.e. 30-42 degrees C) is generally attributed to reduced rates of RuBP regeneration caused by disruption of electron transport activity, and specifically inactivation of the oxygen evolving enzymes of photosystem II. However, measurements of chlorophyll fluorescence and metabolite levels at air-levels of CO(2) indicate that electron transport activity is not limiting at temperatures that inhibit CO(2) fixation. Instead, recent evidence shows that inhibition of net photosynthesis correlates with a decrease in the activation state of Rubisco in both C(3) and C(4) plants and that this decrease in the amount of active Rubisco can fully account for the temperature response of net photosynthesis. Biochemically, the decrease in Rubisco activation can be attributed to: (1) more rapid de-activation of Rubisco caused by a faster rate of dead-end product formation; and (2) slower re-activation of Rubisco by activase. The net result is that as temperature increases activase becomes less effective in keeping Rubisco catalytically competent. In this opinionated review, we discuss how these processes limit photosynthetic performance under moderate heat stress.