15002:

Involvement of the alternative oxidase in phosphorus limitation.

Authors:	Gonzalez-Meler, Miquel, A.(A)Siedow, James, N.(A)
Affiliations:	(A): DCMB group/Botany Department, Duke University
Presenter:	Gonzalez-Meler, Miquel A., mmeler@acpub.duke.edu

Phosphorus deficiency can limit oxidative carbon metabolism and hence directly alter respiration. This includes the activation of pathways that bypass Pi-dependent reactions in glycolysis. Low phosphorus may also limit mitochondrial ATP synthesis and therefore electron transport through the cytochrome pathway, resulting in overreduction of the ubiquinone (UQ)-pool and possible activation of the alternative oxidase. The alternative oxidase transfers electrons from reduced UQ to oxygen, conserving no energy and, as such, is not subject to adenylate control. Post-translational regulatory features of the enzyme include activation by a-ketoacids, such as pyruvate, and a redox-sensitive sulfhydryl/disulfide system which, when oxidized, results in an inactive enzyme. We have used the differential fractionation of oxygen isotopes by the cytochrome and alternative oxidases during electron transfer in the absence of inhibitors to assess the effects of long-term and transient Pi deprivation on alternative oxidase activity. Results from leaves of Glyciridia sp. tree seedlings grown at either high (1m M) or low (0.5 µM) Pi suggest that the alternative pathway plays a role under Pi-limited conditions. Respiration in low-Pi grown plants increased 40% compared to high-Pi grown plants. Electron partitioning to the alternative pathway was 16% and 34% for plants grown at high and low Pi, respectively. Therefore, 80% of the increase seen in total leaf respiration in plants grown at low Pi was due to an enhancement of alternative pathway activity. Transient Pi limitation in spinach leaves was induced by incubation with mannose resulting in the sequestering of cellular Pi into non-metabolizable mannose-6-P and was compared to incubation with metabolizable glucose. Preliminary results indicate that electron partitioning to the alternative pathway increased after four hours of induced cellular phosphorus depletion. In contrast, incubation with glucose did not result in increased electron partitioning to the alternative pathway. These results, combined with the effects of low phosphorus on the alternative oxidase protein and reduction state, detected by immunoblotting, will be used to characterize the potential role of the alternative oxidase in plant responses to low phosphorus. USDA-NRI 94-37306-0352