The physiologic role of alternative oxidase in Ustilago maydis

Comparative mitogenomic analysis of Sporisorium reilianum f. sp. zeae suggests recombination events during its evolutionary history

Introduction

Modern understanding of the concept of genetic diversity must include the study of both nuclear and organellar DNA, which differ greatly in terms of their structure, organization, gene content and distribution. This study comprises an analysis of the genetic diversity of the smut fungus Sporisorium reilianum f. sp. zeae from a mitochondrial perspective.

Methods

Whole-genome sequencing data was generated from biological samples of S. reilianum collected from different geographical regions. Multiple sequence alignment and gene synteny analysis were performed to further characterize genetic diversity in the context of mitogenomic polymorphisms.

Results

Mitochondria of strains collected in China contained unique sequences. The largest unique sequence stretch encompassed a portion of cox1, a mitochondrial gene encoding one of the subunits that make up complex IV of the mitochondrial electron transport chain. This unique sequence had high percent identity to the mitogenome of the related species Sporisorium scitamineum and Ustilago bromivora.

Discussion

The results of this study hint at potential horizontal gene transfer or mitochondrial genome recombination events during the evolutionary history of basidiomycetes. Additionally, the distinct polymorphic region detected in the Chinese mitogenome provides the ideal foundation to develop a diagnostic method to discern between mitotypes and enhance knowledge on the genetic diversity of this organism.

Uncovering the Mechanisms: The Role of Biotrophic Fungi in Activating or Suppressing Plant Defense Responses

This paper discusses the mechanisms by which fungi manipulate plant physiology and suppress plant defense responses by producing effectors that can target various host proteins. Effector-triggered immunity and effector-triggered susceptibility are pivotal elements in the complex molecular dialogue underlying plant-pathogen interactions. Pathogen-produced effector molecules possess the ability to mimic pathogen-associated molecular patterns or hinder the binding of pattern recognition receptors. Effectors can directly target nucleotide-binding domain, leucine-rich repeat receptors, or manipulate downstream signaling components to suppress plant defense. Interactions between these effectors and receptor-like kinases in host plants are critical in this process. Biotrophic fungi adeptly exploit the signaling networks of key plant hormones, including salicylic acid, jasmonic acid, abscisic acid, and ethylene, to establish a compatible interaction with their plant hosts. Overall, the paper highlights the importance of understanding the complex interplay between plant defense mechanisms and fungal effectors to develop effective strategies for plant disease management.

Deletion of the ATP20 gene in Ustilago maydis produces an unstable dimer of F1FO-ATP synthase associated with a decrease in mitochondrial ATP synthesis and a high H2O2 production

The F₁F_O-ATP synthase uses the energy stored in the electrochemical proton gradient to synthesize ATP. This complex is found in the inner mitochondrial membrane as a monomer and dimer. The dimer shows higher ATPase activity than the monomer and is essential for cristae folding. The monomer-monomer interface is constituted by subunits a, i/j, e, g, and k. The role of the subunit g in a strict respiratory organism is unknown. A gene knockout was generated in Ustilago maydis to study the role of subunit g on mitochondrial metabolism and cristae architecture. Deletion of the ATP20 gene, encoding the g subunit, did not affect cell growth or glucose consumption, but biomass production was lower in the mutant strain (gΔ strain). Ultrastructure observations showed that mitochondrial size and cristae shape were similar in wild-type and gΔ strains. The mitochondrial membrane potential in both strains had a similar magnitude, but oxygen consumption was higher in the WT strain. ATP synthesis was 20 % lower in the gΔ strain. Additionally, the mutant strain expressed the alternative oxidase in the early stages of growth (exponential phase), probably as a response to ROS stress. Dimer from mutant strain was unstable to digitonin solubilization, avoiding its isolation and kinetic characterization. The isolated monomeric state activated by n-dodecyl-β-D-maltopyranoside showed similar kinetic constants to the monomer from the WT strain. A decrease in mitochondrial ATP synthesis and the presence of the AOX during the exponential growth phase suggests that deletion of the g gene induces ROS stress.

The Mitochondrial Alternative Oxidase in Ustilago maydis Is Not Involved in Response to Oxidative Stress Induced by Paraquat

It has been shown that the alternative oxidase in mitochondria of fungi and plants has important functions in the response against stress conditions, although their role in some organisms is still unknown. This is the case of Ustilago maydis. There is no evidence of the participation of the U. maydis Aox1 in stressful conditions such as desiccation, high or low temperature, and low pH, among others. Therefore, in this work, we studied the role of the U. maydis Aox1 in cells exposed to oxidative stress induced by methyl viologen (paraquat). To gain insights into the role of this enzyme, we took advantage of four strains: the FB2 wild-type, a strain without the alternative oxidase (FB2aox1Δ), other with the Aox1 fused to the Gfp under the control of the original promoter (FB2aox1-Gfp), and one expressing constitutively de Aox1-Gfp (FB2Potef:aox1-Gfp). Cells were incubated for various times in the presence of 1 mM paraquat and growth, replicative capacities, mitochondrial respiratory activity, Aox1 capacity, and the activities of several antioxidant enzymes (catalase, glutathione peroxidase, glutathione reductase, and superoxide dismutase) were assayed. The results show that (1) the response of U. maydis against oxidative stress was the same in the presence or absence of the Aox1; (2) the activities of the antioxidant enzymes remained constant despite the oxidative stress; and (3) there was a decrease in the GSH/GSSG ratio in U. maydis cells incubated with paraquat.

Cyanide resistant respiration and the alternative oxidase pathway: A journey from plants to mammals

In a large number of organisms covering all phyla, the mitochondrial respiratory chain harbors, in addition to the conventional elements, auxiliary proteins that confer adaptive metabolic plasticity. The alternative oxidase (AOX) represents one of the most studied auxiliary proteins, initially identified in plants. In contrast to the standard respiratory chain, the AOX mediates a thermogenic cyanide-resistant respiration; a phenomenon that has been of great interest for over 2 centuries in that energy is not conserved when electrons flow through it. Here we summarize centuries of studies starting from the early observations of thermogenicity in plants and the identification of cyanide resistant respiration, to the fascinating discovery of the AOX and its current applications in animals under normal and pathological conditions.

Identification and Functional Characterization of a Putative Alternative Oxidase (Aox) in Sporisorium reilianum f. sp. zeae

The mitochondrial electron transport chain consists of the classical protein complexes (I–IV) that facilitate the flow of electrons and coupled oxidative phosphorylation to produce metabolic energy. The canonical route of electron transport may diverge by the presence of alternative components to the electron transport chain. The following study comprises the bioinformatic identification and functional characterization of a putative alternative oxidase in the smut fungus Sporisorium reilianum f. sp. zeae. This alternative respiratory component has been previously identified in other eukaryotes and is essential for alternative respiration as a response to environmental and chemical stressors, as well as for developmental transitionaoxs during the life cycle of an organism. A growth inhibition assay, using specific mitochondrial inhibitors, functionally confirmed the presence of an antimycin-resistant/salicylhydroxamic acid (SHAM)-sensitive alternative oxidase in the respirasome of S. reilianum. Gene disruption experiments revealed that this enzyme is involved in the pathogenic stage of the fungus, with its absence effectively reducing overall disease incidence in infected maize plants. Furthermore, gene expression analysis revealed that alternative oxidase plays a prominent role in the teliospore developmental stage, in agreement with favoring alternative respiration during quiescent stages of an organism’s life cycle.

Effect of plant-based compounds on the antifungal and antiaflatoxigenic efficiency of strobilurins against Aspergillus flavus

Aflatoxins are a group of carcinogenic and mutagenic fungal secondary metabolites that have threatened human health and global food security. Aflatoxin contamination can be controlled by applying fungicides, such as strobilurins. Although these compounds have been effective, they may be risky to the environment due to their wide usage. In this study, plant-based compounds were tested to promote the performance of strobilurins (azoxystrobin, pyraclostrobin) against aflatoxigenic Aspergillus flavus; six natural compounds, namely baicalein, nobiletin, meso-dihydroguaiaretic acid, pinoresinol, syringaresinol, and celastrol, were found to exhibit synergistic antifungal effects with strobilurins with fractional inhibitory concentration index < 0.5. Among them, baicalein showed no inhibitory effects on A. flavus when applied alone, but strongly enhanced the in vitro and in situ antifungal and antiaflatoxigenic efficacy of strobilurins and transformed them from fungistatic to fungicidal agents. Therefore, baicalein may be used as an effective natural chemosensitizing agent to improve the performance of strobilurins against A. flavus. The findings of this study provide novel insights for the development of safer and more effective strategies for the control of aflatoxin contamination.

The Differential Growth Inhibition of Phytophthora spp. Caused by the Rare Sugar Tagatose Is Associated With Species-Specific Metabolic and Transcriptional Changes

Tagatose is a rare sugar with no negative impacts on human health and selective inhibitory effects on plant-associated microorganisms. Tagatose inhibited mycelial growth and negatively affected mitochondrial processes in Phytophthora infestans, but not in Phytophthora cinnamomi. The aim of this study was to elucidate metabolic changes and transcriptional reprogramming activated by P. infestans and P. cinnamomi in response to tagatose, in order to clarify the differential inhibitory mechanisms of tagatose and the species-specific reactions to this rare sugar. Phytophthora infestans and P. cinnamomi activated distinct metabolic and transcriptional changes in response to the rare sugar. Tagatose negatively affected mycelial growth, sugar content and amino acid content in P. infestans with a severe transcriptional reprogramming that included the downregulation of genes involved in transport, sugar metabolism, signal transduction, and growth-related process. Conversely, tagatose incubation upregulated genes related to transport, energy metabolism, sugar metabolism and oxidative stress in P. cinnamomi with no negative effects on mycelial growth, sugar content and amino acid content. Differential inhibitory effects of tagatose on Phytophthora spp. were associated with an attempted reaction of P. infestans, which was not sufficient to attenuate the negative impacts of the rare sugar and with an efficient response of P. cinnamomi with the reprogramming of multiple metabolic processes, such as genes related to glucose transport, pentose metabolism, tricarboxylic acid cycle, reactive oxygen species detoxification, mitochondrial and alternative respiration processes. Knowledge on the differential response of Phytophthora spp. to tagatose represent a step forward in the understanding functional roles of rare sugars.

Deletion of the natural inhibitory protein Inh1 in Ustilago maydis has no effect on the dimeric state of the F1FO-ATP synthase but increases the ATPase activity and reduces the stability

Transduction of electrochemical proton gradient into ATP synthesis is performed by F₁F_O-ATP synthase. The reverse reaction is prevented by the regulatory subunit Inh1. Knockout of the inh1 gene in the basidiomycete Ustilago maydis was generated in order to study the function of this protein in the mitochondrial metabolism and cristae architecture. Deletion of inh1 gen did not affect cell growth, glucose consumption, and biomass production. Ultrastructure and fluorescence analyzes showed that size, cristae shape, network, and distribution of mitochondria was similar to wild strain. Membrane potential, ATP synthesis, and oxygen consumption in wild type and mutant strains had similar values. Kinetic analysis of ATPase activity of complex V in permeabilized mitochondria showed similar values of V_max and K_M for both strains, and no effect of pH was observed. Interestingly, the dimeric state of complex V occurs in the mutant strain, indicating that this subunit is not essential for dimerization. ATPase activity of the isolated monomeric and dimeric forms of complex V indicated V_max values 4-times higher for the mutant strain than for the WT strain, suggesting that the absence of Inh1 subunit increased ATPase activity, and supporting a regulatory role for this protein; however, no effect of pH was observed. ATPase activity of WT oligomers was stimulated several times by dodecyl-maltoside (DDM), probably by removal of ADP from F₁ sector, while DDM induced an inactive form of the mutant oligomers.

Molecular characterization and gene expression modulation of the alternative oxidase in a scuticociliate parasite by hypoxia and mitochondrial respiration inhibitors

Philasterides dicentrarchi is a marine benthic microaerophilic scuticociliate and an opportunistic endoparasite that can infect and cause high mortalities in cultured turbot (Scophthalmus maximus). In addition to a cytochrome pathway (CP), the ciliate can use a cyanide-insensitive respiratory pathway, which indicates the existence of an alternative oxidase (AOX) in the mitochondrion. Although AOX activity has been described in P. dicentrarchi, based on functional assay results, genetic evidence of the presence of AOX in the ciliate has not previously been reported. In this study, we conducted genomic and transcriptomic analysis of the ciliate and identified the AOX gene and its corresponding mRNA. The AOX gene (size 1,106 bp) contains four exons and three introns that generate an open reading frame of 915 bp and a protein with a predicted molecular weight of 35.6 kDa. The amino acid (aa) sequence of the AOX includes an import signal peptide targeting the mitochondria and the protein is associated with the inner membrane of the mitochondria. Bioinformatic analysis predicted that the peptide is a homodimeric glycoprotein, although monomeric forms may also appear under native conditions, with EXXH motifs associated with the diiron active centers. The aa sequences of the AOX of different P. dicentrarchi isolates are highly conserved and phylogenetically closely related to AOXs of other ciliate species, especially scuticociliates. AOX expression increased significantly during infection in the host and after the addition of CP inhibitors. This confirms the important physiological roles of AOX in respiration under conditions of low levels of O₂ and in protecting against oxidative stress generated during infection in the host.

Alternative Oxidase: A Potential Target for Controlling Aflatoxin Contamination and Propagation of Aspergillus flavus

Aflatoxins are among the most hazardous natural cereal contaminants. These mycotoxins are produced by Aspergillus spp. as polyketide secondary metabolites. Aflatoxigenic fungi including A. flavus express the alternative oxidase (AOX), which introduces a branch in the cytochrome-based electron transfer chain by coupling ubiquinol oxidation directly with the reduction of O₂ to H₂O. AOX is closely associated with fungal pathogenesis, morphogenesis, stress signaling, and drug resistance and, as recently reported, affects the production of mycotoxins such as sterigmatocystin, the penultimate intermediate in aflatoxin B₁ biosynthesis. Thus, AOX might be considered a target for controlling the propagation of and aflatoxin contamination by A. flavus. Hence, this review summarizes the current understanding of fungal AOX and the alternative respiration pathway and the development and potential applications of AOX inhibitors. This review indicates that AOX inhibitors, either alone or in combination with current antifungal agents, are potentially applicable for developing novel, effective antifungal strategies. However, considering the conservation of AOX in fungal and plant cells, a deeper understanding of fungal alternative respiration and fungal AOX structure is needed, along with effective fungal-specific AOX inhibitors.

Alternative Oxidase Is Involved in the Pathogenicity, Development, and Oxygen Stress Response of Botrytis cinerea

Alternative oxidase (AOX) is a ubiquinol terminal oxidase that is involved in fungal mitochondrial oxidative phosphorylation. In this study, we analyzed the roles of AOX in Botrytis cinerea by generating BcAOX deletion mutants. The mutants exhibited defects in mycelial growth, sporulation, spore germination, and virulence. Furthermore, the sensitivity of the mutants to quinone outside inhibitor fungicides and oxidative stress were increased. All phenotypic variations could be restored in the complemented strain. In summary, these results showed that BcAOX is involved in the regulation for vegetative development, adaptation to environmental stress, and virulence of B. cinerea.

Alternative oxidase inhibitors: Mitochondrion-targeting as a strategy for new drugs against pathogenic parasites and fungi

The alternative oxidase (AOX) is a ubiquitous terminal oxidase of plants and many fungi, catalyzing the four-electron reduction of oxygen to water alongside the cytochrome-based electron transfer chain. Unlike the classical electron transfer chain, however, the activity of AOX does not generate adenosine triphosphate but has functions such as thermogenesis and stress response. As it lacks a mammalian counterpart, it has been investigated intensely in pathogenic fungi. However, it is in African trypanosomes, which lack cytochrome-based respiration in their infective stages, that trypanosome alternative oxidase (TAO) plays the central and essential role in their energy metabolism. TAO was validated as a drug target decades ago and among the first inhibitors to be identified was salicylhydroxamic acid (SHAM), which produced the expected trypanocidal effects, especially when potentiated by coadministration with glycerol to inhibit anaerobic energy metabolism as well. However, the efficacy of this combination was too low to be of practical clinical use. The antibiotic ascofuranone (AF) proved a much stronger TAO inhibitor and was able to cure Trypanosoma vivax infections in mice without glycerol and at much lower doses, providing an important proof of concept milestone. Systematic efforts to improve the SHAM and AF scaffolds, aided with the elucidation of the TAO crystal structure, provided detailed structure-activity relationship information and reinvigorated the drug discovery effort. Recently, the coupling of mitochondrion-targeting lipophilic cations to TAO inhibitors has dramatically improved drug targeting and trypanocidal activity while retaining target protein potency. These developments appear to have finally signposted the way to preclinical development of TAO inhibitors.

The mitochondrial alternative oxidase Aox1 is needed to cope with respiratory stress but dispensable for pathogenic development in Ustilago maydis

The mitochondrial alternative oxidase is an important enzyme that allows respiratory activity and the functioning of the Krebs cycle upon disturbance of the respiration chain. It works as a security valve in transferring excessive electrons to oxygen, thereby preventing potential damage by the generation of harmful radicals. A clear biological function, besides the stress response, has so far convincingly only been shown for plants that use the alternative oxidase to generate heat to distribute volatiles. In fungi it was described that the alternative oxidase is needed for pathogenicity. Here, we investigate expression and function of the alternative oxidase at different stages of the life cycle of the corn pathogen Ustilago maydis (Aox1). Interestingly, expression of Aox1 is specifically induced during the stationary phase suggesting a role at high cell density when nutrients become limiting. Studying deletion strains as well as overexpressing strains revealed that Aox1 is dispensable for normal growth, for cell morphology, for response to temperature stress as well as for filamentous growth and plant pathogenicity. However, during conditions eliciting respiratory stress yeast-like growth as well as hyphal growth is strongly affected. We conclude that Aox1 is dispensable for the normal biology of the fungus but specifically needed to cope with respiratory stress.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

Involvement of an alternative oxidase in the regulation of hyphal growth and microsclerotial formation in Nomuraea rileyi CQNr01

Mitochondria of Nomuraea rileyi contain an alternative oxidase (Aox), which reduces oxygen to water by accepting electrons directly from ubiquinol. Furthermore, through a transcriptional analysis, we found that an alternative oxidase (Nraox) was up-regulated during microsclerotial formation. To study the function of NrAox, Nraox was cloned from N. rileyi CQNr01. The full-length cDNA was 1266 bp with an open reading frame of 1068 bp encoding 355 amino acids. A phylogenetic analysis revealed that the NrAox of N. rileyi was closely related to Metarhizium acridum Aox. The relative expression level of the Nraox was up-regulated during microsclerotial (MS) initiation. A salicylhydroxamic acid, a specific alternative oxidase inhibitor, application to the culture media severely decreased MS yields, changed the hyphae morphology and slowed the H2O2 removal. Nraox silencing caused mycelial deformations, reduced the MS yields by 97.3 % and increased MS size compared with those of the control. MS virulence was decreased to 26.2 % after Nraox was silenced. However, the Nraox-silenced strain was sensitive to environmental stress, and the growth rate was reduced under stress conditions. The results obtained suggested that Nraox is required for MS differentiation by regulating the intracellular H2O2 concentration and hypha growth. Additionally, Nraox had a great impact on the virulence of N. rileyi.

Functional expression of the Acanthamoeba castellanii alternative oxidase in Escherichia coli; regulation of the activity and evidence for Acaox gene function

To evidence Acanthamoeba castellanii alternative oxidase (AcAOX) gene product function, we studied alterations in the levels of mRNA and protein and AcAOX activity during growth in amoeba batch culture. Moreover, heterologous expression of AcAOX in AOX-deficient Escherichia coli confirmed by the protein immunodetection and functional studies was performed. Despite the presence of native bo and bd quinol oxidases in E. coli membrane, from which the latter is known to be cyanide-resistant, functional expression of AcAOX in E. coli conferred cyanide-resistant benzohydroxamate-sensitive respiration on the bacteria. Moreover, AcAOX activity in transformed bacteria was stimulated by GMP and inhibited by ATP, indicating that AcAOX is regulated by mutual exclusion of purine nucleotides, which was previously demonstrated in the mitochondria of A. castellanii.

Effects of chitosan and oligochitosan on development and mitochondrial function of Rhizopus stolonifer

The antifungal activities of chitosan and oligochitosan have been used to control postharvest decay of the fruits. The effect of chitosan and oligochitosan on mycelium growth, spore germination, and mitochondrial function of Rhizopus stolonifer was evaluated in order to establish a connection between fungus development and the main organelle in charge to provide energy to the cell. The mycelium growth of R. stolonifer was significantly reduced on minimum media amended with chitosan or oligochitosan. The highest antifungal indexes were obtained on media containing chitosan or oligochitosan at 2.0 mg ml(-1). Microscopic observation showed that chitosan and oligochitosan affected the spore germination and hyphae morphology. Both polymers increased oxygen consumption of R. stolonifer. Respiratory activity was restored with NADH in permeabilized treated and untreated cells, and was inhibited with rotenone and flavones. Complex III and IV were inhibited by antimycin A and cyanide, respectively, in treated and untreated cells. Chitosan and oligochitosan increased NADH dehydrogenase activity in isolated mitochondria. However, there were not changes in the cytochrome c oxidase and ATPase activities by effect of these polymers. These results suggest that both chitosan and oligochitosan affect the development of R. stolonifer and might be implicated in the mitochondrial dysfunction.

Ophiostoma mitovirus 3a, ascorbic acid, glutathione, and photoperiod affect the development of stromata and apothecia by Sclerotinia homoeocarpa

Hypovirulence in Sclerotinia homoeocarpa is associated with infection by Ophiostoma mitovirus 3a (OMV3a). OMV3a is also present in asymptomatic isolates, with growth and virulence comparable to that of virus-free isolates. Hypovirulent isolates have impaired mitochondrial function resulting in increased activity of the alternative oxidase pathway, which is implicated in the reduction of reactive oxygen species in other fungi. In this study, hypovirulent, asymptomatic, and virus-free isolates were grown on potato dextrose agar amended with ascorbic acid or glutathione and were incubated under various photoperiods to determine the importance of reactive oxygen species, light, and OMV3a infection for differentiation of stromata and apothecia by S. homoeocarpa. Hypovirulent isolates did not form stromata or apothecia. Glutathione and darkness reduced stromata size and apothecia production by virulent and asymptomatic isolates. Apothecia formed under several different photoperiods, and ascorbic acid increased apothecia production. Ascospores were not detected in these apothecia. The results suggest that hypovirulence, light, and the superoxide radical are important factors in the formation of stromata and apothecia by S. homoeocarpa isolates. This is the first report of sterile apothecia production by North American isolates of S. homoeocarpa and provides a starting point for attempts to produce fertile apothecia.

Induction of morphological changes in Ustilago maydis cells by octyl gallate

The effects of octyl gallate on Ustilago maydis yeast cells were analysed in relation to its capacity to oxidize compounds (pro-oxidant actions). All phenolic compounds tested inhibited the alternative oxidase (AOX). However, only octyl gallate induced a morphological change in yeast cells and collapsed the mitochondrial membrane potential. In contrast to octyl gallate, propyl gallate and nordihydroguaiaretic acid caused only a negligible cell change and the membrane potential was not affected. Our findings show that structurally related phenolic compounds do not necessarily exert similar actions on target cells. Preincubation of U. maydis cells with trolox inhibited the change to pseudohyphal growth produced by octyl gallate. These results suggest that in addition to the inhibitory action of octyl gallate on the AOX, this compound induces a switch from yeast to a mycelium, probably through the formation of lipid peroxides.

Regulation of Acanthamoeba castellanii alternative oxidase activity by mutual exclusion of purine nucleotides; ATP's inhibitory effect

The effects of different adenine and guanine nucleotides on the cyanide-resistant respiration (i.e. alternative oxidase (AcAOX) activity) of mitochondria from the amoeba A. castellanii mitochondria were studied. We found that guanine nucleotides activate AcAOX to a greater degree than adenine nucleotides, and that nucleoside monophosphates were more efficient activators than nucleoside di- or triphosphates. The extent of the nucleotides' influence on AcAOX was dependent on the medium's pH and was more pronounced at pH 6.8, which is optimal for AcAOX activity. In contrast to other purine nucleosides, we demonstrate, for the first time, that ATP has an inhibitory effect on AcAOX activity. Since we also observed the inhibition by ATP in the mitochondria of another protozoon, such as Dictyostelium discoideum, and the yeast, Candida maltosa, it may be a regulatory feature common to all purine nucleotide-modulated non-plant AOXs. The physiological importance of this discovery is discussed. Kinetic data show that the binding of GMP (a positive allosteric effector) and the binding of ATP (a negative allosteric effector) to AcAOX are mutually exclusive. ATP's inhibition of the enzyme can be overcome by sufficiently high concentrations of GMP, and conversely, GMP's stimulation can be overcome by sufficiently high concentrations of ATP. However, an approximately three times lower concentration of GMP compared to ATP gives a half maximal effect on AcAOX activity. This is indicative of a higher binding affinity for the positive effector at the same or, at least overlapping, nucleotide-binding sites on AcAOX. These results suggest that AcAOX activity in A. castellanii mitochondria might be controlled by the relative intracellular concentrations of purine nucleotides.

Functional properties of the Ustilago maydis alternative oxidase under oxidative stress conditions

The mitochondrial respiratory chain of Ustilago maydis contains two terminal oxidases, the cytochrome c oxidase (COX) and the alternative oxidase (AOX). To understand the biochemical events that control AOX activity, we studied the regulation and function of AOX under oxidative stress. The activity of this enzyme was increased by both pyruvate (K(05)=2.6 mM) and purine nucleotides (AMP, K(05)=600 microM) in mitochondria using succinate as respiratory substrate. When U.maydis cells were grown in the presence of antimycin A, the amount of AOX in mitochondria was markedly increased and its selectivity towards AMP and pyruvate changed, suggesting that post-translational events may play a role in the regulation of AOX activity under stress conditions. Addition of antimycin A to isolated mitochondria induced the inactivation of AOX, the formation of lipid peroxides and the loss of glutathione from mitochondria. The two last processes are probably related with the time dependent inactivation of AOX, in agreement with the inhibition of the enzyme by tert-butyl hydroperoxide. Our results suggest that the in vivo operation of AOX in U. maydis depends on the mitochondrial antioxidant machinery, including the glutathione linked systems.

A genome survey of Moniliophthora perniciosa gives new insights into Witches' Broom Disease of cacao

Background

The basidiomycete fungus Moniliophthora perniciosa is the causal agent of Witches' Broom Disease (WBD) in cacao (Theobroma cacao). It is a hemibiotrophic pathogen that colonizes the apoplast of cacao's meristematic tissues as a biotrophic pathogen, switching to a saprotrophic lifestyle during later stages of infection. M. perniciosa, together with the related species M. roreri, are pathogens of aerial parts of the plant, an uncommon characteristic in the order Agaricales. A genome survey (1.9× coverage) of M. perniciosa was analyzed to evaluate the overall gene content of this phytopathogen.

Results

Genes encoding proteins involved in retrotransposition, reactive oxygen species (ROS) resistance, drug efflux transport and cell wall degradation were identified. The great number of genes encoding cytochrome P450 monooxygenases (1.15% of gene models) indicates that M. perniciosa has a great potential for detoxification, production of toxins and hormones; which may confer a high adaptive ability to the fungus. We have also discovered new genes encoding putative secreted polypeptides rich in cysteine, as well as genes related to methylotrophy and plant hormone biosynthesis (gibberellin and auxin). Analysis of gene families indicated that M. perniciosa have similar amounts of carboxylesterases and repertoires of plant cell wall degrading enzymes as other hemibiotrophic fungi. In addition, an approach for normalization of gene family data using incomplete genome data was developed and applied in M. perniciosa genome survey.

Conclusion

This genome survey gives an overview of the M. perniciosa genome, and reveals that a significant portion is involved in stress adaptation and plant necrosis, two necessary characteristics for a hemibiotrophic fungus to fulfill its infection cycle. Our analysis provides new evidence revealing potential adaptive traits that may play major roles in the mechanisms of pathogenicity in the M. perniciosa/cacao pathosystem.

Alternative oxidase: an inter-kingdom perspective on the function and regulation of this broadly distributed 'cyanide-resistant' terminal oxidase

Alternative oxidase (AOX) is a terminal quinol oxidase located in the respiratory electron transport chain that catalyses the oxidation of quinol and the reduction of oxygen to water. However, unlike the cytochrome c oxidase respiratory pathway, the AOX pathway moves fewer protons across the inner mitochondrial membrane to generate a proton motive force that can be used to synthesise ATP. The energy passed to AOX is dissipated as heat. This appears to be very wasteful from an energetic perspective and it is likely that AOX fulfils some physiological function(s) that makes up for its apparent energetic shortcomings. An examination of the known taxonomic distribution of AOX and the specific organisms in which AOX has been studied has been used to explore themes pertaining to AOX function and regulation. A comparative approach was used to examine AOX function as it relates to the biochemical function of the enzyme as a quinol oxidase and associated topics, such as enzyme structure, catalysis and transcriptional expression and post-translational regulation. Hypotheses that have been put forward about the physiological function(s) of AOX were explored in light of some recent discoveries made with regard to species that contain AOX. Fruitful areas of research for the AOX community in the future have been highlighted.