Proc. Natl. Acad. Sci. USA Vol. 96, pp. 8271–8276, July 1999 Plant Biology The alternative oxidase lowers mitochondrial reactive oxygen production in plant cells DENIS P. MAXWELL*, YONG WANG*, AND LEE MCINTOSH*†‡ *Department of Energy Plant Research Laboratory, and †Department of Biochemistry, Michigan State University, East Lansing, MI 48824 Communicated by Hans J. Kende, Michigan State University, East Lansing, MI, May 17, 1999 (received for review February 12, 1999) ABSTRACT Besides the cytochrome c pathway, plant Unlike animal mitochondria, those of plants possess a mitochondria have an alternative respiratory pathway that is bifurcated electron-transport chain. In addition to the cyto- comprised of a single homodimeric protein, alternative oxi- chrome respiratory pathway found in all eukaryotes, plants dase (AOX). Transgenic cultured tobacco cells with altered have a second, alternative pathway that diverges from the main levels of AOX were used to test the hypothesis that the respiratory chain at ubiquinone (4). Electron flow through the alternative pathway in plant mitochondria functions as a alternative pathway bypasses two of the three sites along the mechanism to decrease the formation of reactive oxygen cytochrome chain where electron transport is coupled to ATP species (ROS) produced during respiratory electron trans- synthesis. The alternative pathway is comprised of a single port. Using the ROS-sensitive probe 2؅,7؅-dichlorofluorescein protein, alternative oxidase (AOX; ref. 5), which is thought to diacetate, we found that antisense suppression of AOX re- exist in the inner mitochondrial membrane as a homodimer sulted in cells with a significantly higher level of ROS com- (6). In all species examined, AOX is encoded by a small family pared with wild-type cells, whereas the overexpression of AOX of nuclear genes whose members are differentially regulated in resulted in cells with lower ROS abundance. Laser-scanning a developmental and tissue-specific manner (see ref. 4 and confocal microscopy showed that the difference in ROS abun- references therein). Disruption of the cytochrome pathway dance among wild-type and AOX transgenic cells was caused leads to induction of AOX in many organisms. For example, by changes in mitochondrial-specific ROS formation. Mito- the addition of antimycin A to cultured tobacco cells resulted chondrial ROS production was exacerbated by the use of in the induction of both AOX mRNA (7) and protein (8). The antimycin A, which inhibited normal cytochrome electron overexpression in tobacco of Aox1, a nuclear gene encoding transport. In addition, cells overexpressing AOX were found AOX, led to a large increase in AOX protein and alternative to have consistently lower expression of genes encoding ROS- pathway capacity, whereas antisense inhibition of Aox1 effec- scavenging enzymes, including the superoxide dismutase tively reduced AOX protein to undetectable levels (9). Further genes SodA and SodB, as well as glutathione peroxidase. Also, work with these transgenic plants showed that changes in the the abundance of mRNAs encoding salicylic acid-binding level of AOX within the mitochondria did not have a signif- catalase and a pathogenesis-related protein were significantly icant effect on growth rate, except in the presence of antimycin higher in cells deficient in AOX. These results are evidence A (10). Under those conditions, cells overexpressing Aox1 grew that AOX plays a role in lowering mitochondrial ROS forma- significantly faster than wild type (WT), whereas cells with tion in plant cells. suppressed levels of AOX died. Although AOX is found in all plants investigated to date, as well as in some fungi and protists, its only confirmed function All organisms produce a range of reactive oxygen species Ϫ occurs in the thermogenic inflorescence of the Araceae (4). (ROS), including superoxide (⅐O ), the hydroxyl radical 2 Recently, however, it has been proposed (11, 12) that AOX (⅐OH), and hydrogen peroxide (H O ), during the course of 2 2 may serve a more general function in all plant species by normal metabolic processes. If not effectively and rapidly limiting mitochondrial ROS formation. An experimental basis removed from cells, ROS can damage a wide range of for this hypothesis is that conditions that induce AOX expres- macromolecules, possibly leading to cell death. Both enzy- sion, including chilling (13), pathogen attack (14), aging (15), matic and nonenzymatic mechanisms have evolved to protect and inhibition of the cytochrome pathway (8), also cause an cells from oxygen toxicity. These include antioxidants such increase in cellular ROS formation (16–18). Because stress- as ascorbate and glutathione, as well as ROS-scavenging induced physical changes in membrane components may lead enzymes such as superoxide dismutase (SOD), catalase, and to a restriction in cytochrome pathway respiration (19) and peroxidase (1). thus increase ROS formation, the presence of a second quinol Mitochondria are a major source of ROS in eukaryotic cells. oxidase may help to prevent overreduction of upstream elec- In humans, aberrant mitochondrial ROS formation has been tron-transport components. In so doing, alternative pathway associated with conditions such as Parkinson’s disease, amyo- respiration would also continue to reduce oxygen to water and trophic lateral sclerosis, and aging (2). Superoxide, which is thus keep the intracellular concentration of this potential toxin rapidly converted to H2O2 through the action of SOD, is low. produced during respiration primarily by the autooxidation of This study sought to test the hypothesis that AOX may serve reduced mitochondrial electron-transport components (3). to keep mitochondrial ROS formation low. Our goal was to One of these components, a major site of superoxide produc- measure ROS formation in intact cells rather than isolated tion along the respiratory chain, is the ubiquinone pool (3). The formation of superoxide is exacerbated by compounds, Ј Ј such as antimycin A, that block electron transport downstream Abbreviations: DCF, dichlorofluorescein; H2DCF, 2 ,7 -DCF; H2DCF-DA, 2Ј,7Јdichlorofluorescein diacetate; ROS, reactive oxygen of the ubiquinone pool (3). species; SOD, superoxide dismutase; WT, wild type; GPX, glutathione peroxidase; SA-Cat, salicylic acid-binding catalase; RT-PCR, reverse The publication costs of this article were defrayed in part by page charge transcription–PCR; BHA, butylated hydroxyanisole; S11, containing Aox1 in sense orientation; AS8, containing Aox1 in antisense orien- payment. This article must therefore be hereby marked ‘‘advertisement’’ in tation. accordance with 18 U.S.C. §1734 solely to indicate this fact. ‡To whom reprint requests should be addressed. e-mail: mcintos1@ PNAS is available online at www.pnas.org. pilot.msu.edu. 8271 Downloaded by guest on September 29, 2021 8272 Plant Biology: Maxwell et al. Proc. Natl. Acad. Sci. USA 96 (1999) mitochondria to gain a more biologically accurate assessment elF4A (27) was used to confirm the equal loading of RNA. of mitochondrial ROS formation as it occurs in vivo. To meet Blots were quantified by densitometry. our goals, we used heterotrophically grown tobacco cells that Total Protein Determination. Total cellular protein was overexpress or underexpress Aox1. Although these cells lack isolated as described (28), and protein concentrations were developed chloroplasts, which themselves represent a major determined by using a BCA protein assay kit (Pierce). source of ROS within plants, our system has the experimental Chemicals. Antimycin A, butylated hydroxyanisole (BHA), advantage of enabling one to estimate mitochondrial ROS flavone, and menadione were obtained from Sigma. Each was formation more easily and arguably more accurately within dissolved in an appropriate solvent at a concentration 1,000- plant cells through the use of fluorescent probes and confocal fold greater than the final concentration. Mitotracker Red and microscopy. H2DCF-DA were obtained from Molecular Probes. MATERIALS AND METHODS RESULTS Plant Material and Growth Conditions. Cultured tobacco Detection of Intracellular ROS in Intact Tobacco Cells. The cells (Nicotiana tabacum cv. Petit Havana SR1) containing chemical probe H2DCF-DA has been used extensively as a Aox1 in either sense (S11) or antisense (AS8) orientation have noninvasive, in vivo measure of intracellular ROS (20). been characterized (9, 10). All experiments were conducted H2DCF-DA was used here to determine whether changes in with exponentially growing cells 3–4 days after subculture. the intracellular level of ROS correlated with the abundance Detection of Reactive Oxygen Species. Spectrofluorometry. of AOX in mitochondria. As shown in Fig. 1, the level of DCF Intracellular production of ROS was measured by using 2Ј,7Ј- fluorescence was approximately five times greater in cells with dichlorofluorescein diacetate (H2DCF-DA; Molecular suppressed AOX levels (AS8) compared with WT cells, Probes). This nonpolar compound is converted to the mem- whereas the level of DCF fluorescence in cells overexpressing brane-impermeant polar derivative H2DCF by esterases when AOX (S11) was 57% lower than that of WT cells. it is taken up by the cell. H2DCF is nonfluorescent but is Inhibition of Mitochondrial Electron Transport Increases rapidly oxidized to the highly fluorescent DCF by intracellular ROS Production in Vivo. It has been shown in isolated animal H2O2 and other peroxides (20). Stocks of H2DCF-DA (5 mM) mitochondria that chemical