Characterization of a Split Respiratory Pathway in the Wheat “Take-all” Fungus, Gaeumannomyces graminis var.tritici

Abstract

This article describes the first detailed analysis of mitochondrial electron transfer and oxidative phosphorylation in the pathogenic filamentous fungus, Gaeumannomyces graminis var. tritici. While oxygen consumption was cyanide insensitive, inhibition occurred following treatment with complex III inhibitors and the alternative oxidase inhibitor, salicylhydroxamic acid (SHAM). Similarly, maintenance of a ⌬ across the mitochondrial inner membrane was unaffected by cyanide but sensitive to antimycin A and SHAM when succinate was added as the respiratory substrate. As a result, ATP synthesis through complex V was demonstrated to be sensitive to these two inhibitors but not to cyanide. Analysis of the cytochrome content of mitochondria indicated the presence of those cytochromes normally associated with electron transport in eukaryotic mitochondria together with a third, b-type heme, exhibiting a dithionite-reduced absorbance maxima at 560 nm and not associated with complex III. Antibodies raised to plant alternative oxidase detected the presence of both the monomeric and dimeric forms of this oxidase. Overall this study demonstrates that a novel respiratory chain utilizing the terminal oxidases, cytochrome c oxidase and alternative oxidase, are present and constitutively active in electron transfer in G. graminis tritici. These results are discussed in relation to current understanding of fungal electron transfer and to the possible contribution of alternative redox centers in ATP synthesis.Under aerobic conditions, respiration of carbon metabolites in animal, plant, and fungal cells occurs in a tightly regulated manner to produce carbon dioxide and water. Transfer of the electron pairs associated with the respiration of carbon metabolites is indirect and complex, involving the reduction of the coenzymes NADϩ and FAD at two sites within glycolysis and four sites within the citric acid cycle. The electrons (associated with NADH and FADH 2 ) are subsequently transferred, via at least four distinct sites, into the electron-transport chain, where a series of reduction and oxidation events occur in a sequential manner at approximately 10 different redox centers.In the classical scheme, these redox centers are composed of a series of cytochromes and iron-sulfur complexes. About half the energy generated is lost as heat. The remaining energy generated by the electron flow is utilized in the translocation of hydrogen ions from the mitochondrial matrix to the inter mitochondrial membrane space. The free energy is thus stored in the proton gradient (proton motive force) and is subsequently used to drive the synthesis of ATP.While in mammalian systems the components and sequence of events associated with electron transport appear tightly conserved, those of plants and fungi appear more complex and diverse in nature, often involving alternative redox centers and pathways. Significant research has been conducted in the area of plant respiration, leading to the characterization of these alternative systems, but research into fun...

Effect of azoxystrobin on activities of antioxidant enzymes and alternative oxidase in wheat head blight pathogens Fusarium graminearum and Microdochium nivale

“…But interestingly, M. nivale strains were highly sensitive to nPG on PDA. Similar high sensitivity to AOX inhibitors has been reported for Gaeumannomyces graminis and Tapesia acuformis (Joseph-Horne et al 1998. These results suggested that both complex III and AOX are necessary for fundamental respiration even under typical growth conditions.…”

“…Bars represent the standard deviation of the mean. Different letters above columns indicate significant difference at the 5% level according to Duncan's multiple range test the ATP level in G. graminis (Joseph-Horne et al 1998). Additionally, AOX protein was constitutively present in several fungi such as G. graminis, B. cinerea, Histoplasma capsulatum and Blastomyces dermatitidis (Joseph-Horne et al 1998;Johnson et al 2003;Tamura et al 1999).…”

“Respirasome”-like Supercomplexes in Green Leaf Mitochondria of Spinach

“…In plants, the mitochondrial membrane potential remains unchanged, as is expected because of the non-proton-pumping nature of the AOX (48). In contrast, in filamentous fungi and yeasts respiring simultaneously by complex IV and AOX, a rapid collapse of the mitochondrial membrane potential is observed (46,47,49,50). Interestingly, the AOX-dependent respiratory chain in the filamentous fungus P. anserina, in which the AOX is only induced by the specific deficiency of complex IV (51), as in some other fungi (e.g.…”

Alternative respiration of fungus Phycomyces blakesleeanus

“…In the rest of fungi the AOX expression and activity are very complex and highly diverse among different species. In some like Aspergillus niger (Kirimura et al 1987), Gaeumannomyces graminis (Joseph-Horne et al 1998) and Candida parapsilosis (Milani et al 2001) AOX is both constitutively present and active component of the electron transport chain, in others like Allomyces macrogynus (Heldt-Hansen et al 1983a) and Ustilago maydis (Juárez et al 2006) it becomes active only after inhibition of CSR, while in fungi like Neurospora crassa, (Tanton et al 2003) and Magnaporthe grisea, (Yukioka et al 1998) inhibition of CSR, or some other environmental stimuli, is necessary for induction of AOX expression.…”