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Ustilago maydis is a fungal phytopathogen that infects wild and commercial types of corn (Zea teocintle and Zea mays, respectively), and is related to other fungal pathogens of great economic significance. As a pest, a culinary delicacy or a model organism it is considered an interesting subject, and a large amount of information on diverse aspects of its biology is available [1][2][3]. Despite this, many issues are not understood, in particular its energy and intermediary metabolism.We have previously described the organization of the U. maydis mitochondrial respiratory chain [4]. Among our major results were the identification of alternative oxidase (AOX) and the external isoform of alternative NADH dehydrogenase in this organism. These enzymes are widespread among eukaryotes [5,6] and can be found in some bacterial species [7,8], suggesting a significant metabolic relevance. However, their participation in metabolism has been difficult to Alternative oxidase (AOX) is a ubiquitous respiratory enzyme found in plants, fungi, protists and some bacterial species. One of the major questions about this enzyme is related to its metabolic role(s) in cellular physiology, due to its capacity to bypass the proton-pumping cytochrome pathway, and as a consequence it has great energy-wasting potential. In this study, the physiological role and regulatory mechanisms of AOX in the fungal phytopathogen Ustilago maydis were studied. We found evidence for at least two metabolic functions for AOX in this organism, as a major part of the oxidative stress-handling machinery, a well-described issue, and as part of the mechanisms that increase the metabolic plasticity of the cell, a role that might be valuable for organisms exposed to variations in temperature, nutrient source and availability, and biotic or abiotic factors that limit the activity of the cytochrome pathway. Experiments under different culture conditions of ecological significance for this organism revealed that AOX activity is modified by the growth stage of the culture, amino acid availability and growth temperature. In addition, nucleotide content, stimulation of AOX by AMP and respiratory rates obtained after inhibition of the cytochrome pathway showed that fungal ⁄ protist AOX is activated under low-energy conditions, in contrast to plant AOX, which is activated under high-energy conditions. An estimation of the contribution of AOX to cell respiration was performed by comparing the steady-state concentration of adenine nucleotides, the mitochondrial membrane potential, and the respiratory rate.Abbreviations AOX, alternative oxidase; IAA, iodoacetate; JC-1, 5,5¢,6,6¢-tetrachloro-1,1¢,3,3¢-tetraethylbenzimidazolocarbocyanine iodide; KCN, potassium cyanide; nOg, n-octilgallate; PCP, pentachlorophenol; ROS, reactive oxygen species. understand because they are not directly associated with ATP synthesis. The experimental evidence shown in this study illustrates that the main metabolic roles of AOX in U. maydis are: (a) the prevention of reactive oxygen species (ROS) pr...

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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.