Alternative Oxidase

The CO2 evolution of intact potato tubers (Solanum tuberosum, L., var. “Bintje”) was analyzed during a 10-day period of their warm (25 ± 2°C) or cold (5 ± 1°C) storage, to evaluate cold-stress effects on expression and activities of plant uncoupling mitochondrial protein (PUMP) and alternative oxidase (AOX). CO2 evolution rates were analyzed at 20°C, to reflect their possible capacities. The 20°C CO2 production declined from 13 to 8 mg kg−1 h−1 after 2 days of warm storage and then (after 3 to 7 days) decreased from 8 to 6.5 mg kg−1 h−1. In contrast, 20°C CO2 evolution did not change after the first day of cold storage, increased up to 14.5 mg kg−1 h−1 after 2 days, and decreased to about 12 mg kg−1 h−1 after 3 to 7 days of cold storage. Cold storage increased PUMP expression as detected by Western blots and led to elevated capacities of both PUMP (44%) and CN-resistant AOX (10 times), but not the cytochrome pathway. Since we found that cold storage led to about the same mitochondrial respiration of 40 nmol O2 min−1 mg−1 attributable to each of the respective proteins, we conclude that both AOX and PUMP equally contribute to adaptation of potato tubers to cold.

Alternative oxidases (Aox or Aod) are present in the mitochondria of plants, fungi and many types of yeast. These enzymes transfer electrons from the ubiquinol pool directly to oxygen without contributing to the proton transfer across the mitochondrial membrane. Alternative oxidases are involved in stress responses, programmed cell death and maintenance of the cellular redox balance. The alternative oxidase gene

Plant cells must react to a variety of adverse environmental conditions that they may experience on a regular basis. Part of this response centers around (1) ROS as damaging molecules and signaling molecules; (2) redox status, which can be influenced by ROS production; and (3) availability of metabolites. All of these are also likely to interface with changes in hormone levels [Desikan, R., Hancock, J., Neill, S., 2005. Reactive oxygen species as signalling molecules. In: Smirnoff, N. (ed.), Antioxidants and reactive oxygen species in plants. Blackwell Pub. Ltd., Oxford, pp. 169–196; Kwak, J.M., Nguyen, V., Schroeder, J.I., 2006. The role of reactive oxygen species in hormonal responses. Plant Physiol. 141, 323–329]. Each of these areas can be strongly influenced by changes in mitochondrial function. Such changes trigger altered nuclear gene expression by a poorly understood process of mitochondrial retrograde regulation (MRR), which is likely composed of several distinct signaling pathways. Much of what is known about plant MRR centers around the response to a dysfunctional mtETC and subsequent induction of genes encoding proteins involved in recovery of mitochondrial functions, such as AOX and alternative NAD(P)H dehydrogenases, and genes encoding enzymes aimed at regaining ROS level/redox homeostasis, such as glutathione transferases, catalases, ascorbate peroxidases and superoxide dismutases. However, as evidence of new and interesting targets of MRR emerge, this picture is likely to change and the complexity and importance of MRR in plant responses to stresses and the decision for cells to either recover or switch into programmed cell death mode is likely to become more apparent.

Mitochondrial precursor proteins synthesized in rabbit reticulocyte lysate (RRL) are readily imported into mitochondria, whereas the same precursors synthesized in wheat germ extract (WGE) fail to be imported. We have investigated factors that render import incompetence from WGE. A precursor that does not require addition of extramitochondrial ATP for import, the FAd ATP synthase subunit, is imported from WGE. Import

The contribution of xanthine dehydrogenase (XDH, EC 1.1.1.204) to fruit size was investigated using the normal and small-fruit variants of Persea americana Mill. cv. ‘Hass’. Inhibition of XDH by treatment of normal fruit, in the linear phase of growth (phase II), with allopurinol (Allo) arrested fruit growth. Adenine (Ade), a less effective inhibitor of this enzyme, also arrested fruit growth when applied in phase II and slowed fruit growth when applied in phase III. A time-course study on the activity of XDH in mesocarp tissue from normal and small fruit showed that maximum activity occurred late in phase II and that the peak in activity was absent in mesocarp of the small fruit. Feeding Ade to growing fruit in phase III caused a transient decline in fruit growth (measured as change in fruit length). Thereafter, growth resumed although fruit size was irreversibly affected. Treatment of fruit with Ade and Ade-containing cytokinins altered activity of another molybdenum enzyme, aldehyde oxidase (EC 1.2.3.1). Cytokinin oxidase was induced by cytokinin and auxin. Purine catabolism via hypoxanthine/xanthine was operative in normal fruit and in mesocarp from the small-fruit variant and as expected, Allo treatment caused accumulation of xanthine and adenine. In the absence of an increase in XDH during growth of the small-fruit phenotype, low levels of Ade were interpreted as resulting from respiration-enhanced adenylate depletion. Stress and/or pathogen induction of the alternative oxidase pathway is proposed as a possible cause.

the energy efficiency of respiration. In this paper we review theoretical and experi- mental studies about the structure and possible function of alternative oxidase. The evidence for specific gene expression dependent on the physiological, developmental and environmental conditions is also described. We underline the physiological role of alternative oxidase as a "survival" protein that allows plants to cope with the

The triple isotope composition (δ 17O and δ 18O) of dissolved O2 in the ocean and in ice cores was recently used to assess the primary productivity over broad spatial and temporal scales. However, assessment of the productivity with the aid of this method must rely on accurate measurements of the 17O/16O versus 18O/16O relationship in each of the main oxygen-producing and -consuming reactions. Data obtained here showed that cleavage of water in photosystem II did not fractionate oxygen isotopes; the δ 18O and δ 17O of the O2 evolved were essentially identical to those of the substrate water. The fractionation slopes for the oxygenase reaction of Rubisco and respiration were identical (0.518 ± 0.001) and that of glycolate oxidation was 0.503 ± 0.002. There was a considerable difference in the slopes of O2 photoreduction (the Mehler reaction) in the cyanobacterium Synechocystis sp. strain PCC 6803 (0.497 ± 0.004) and that of pea (Pisum sativum) thylakoids (0.526 ± 0.001). These values...