Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2000
Publication Date: N/A
Interpretive Summary: It is well-documented that the global environment is changing with regard to carbon dioxide level and temperature, both of which have increased over the past century. These environmental changes have significant effects on the rate of photosynthesis, and will therefore impact plant growth and development in both ecological and agricultural settings. In this paper we ereport a new biochemical mechanism that explains how photosynthesis responds to high temperature and C02. Both high temperature and C02 cause the inactivation, or turning off, of an enzyme called Rubisco. This enzyme is responsible for the conversion of atmospheric C02 into plant dry matter. The inactivation of this enzyme occurs because of a limitation imposed by another enzyme, called Rubisco activase, that functions to maintain Rubsico in an active and functional form. We have shown that the mechanism in which high temperature and high C02 turn off Rubisco differs, but the end result is the same. Both high temperature and high C02 influence photosynthesis in a manner that is not predicted by models of photosynthesis that are commonly used by plant biologists. We show that temperature and C02 effects on photosynthesis can be explained simply by the properties of one enzyme, Rubisco. This work will be of general interest to biologists concerned with effects of global change on photosynthesis.
Technical Abstract: Net photosynthesis (Pn) is inhibited by moderate heat stress in many plant species. To elucidate the mechanism of inhibition, we examined the effects of temperature on gas-exchange and Rubisco activation in cotton and tobacco leaves and compared the responses to those of the isolated enzymes. Depending on CO2 concentration, Pn decreased when temperatures exceeded 35 to 40 deg C. This response was inconsistent with the response predicted from the properties of Rubisco. Rubisco deactivated in leaves when temperature was increased, and also in response to high C02 or low 02. The decrease in Rubisco activation occurred when leaf temperatures exceeded 35 deg C, whereas the activities of isolate activase and Rubisco were highest at 42 and >50 deg C, respectively. In the absence of activase, isolated Rubisco deactivated under catalytic conditions and the rate of deactivation increased with temperature but not with C02. The ability of activase to maintain or promote Rubisco activation in vitro also decreased with temperature but was not affected by C02. Increasing the activase/Rubisco ratio decreased the rate of 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 C02 was not caused by a faster rate of deactivation, but by reduced activase activity possibility in response to unfavorable ATP/ADP ratios. When adjustments were made for changes in activation, the kinetic properties of Rubisco predicted the response of Pn at high temperature and C02.