A novel polysaccharide secreted by pal/rim mutants of the phytopathogen fungus Ustilago maydis

GC-MS-Based Metabolomics for the Smut Fungus Ustilago maydis: A Comprehensive Method Optimization to Quantify Intracellular Metabolites

Ustilago maydis, a smut fungus, is an appealing model in fundamental research and an upcoming cell factory for industrial biotechnology. The genome of U. maydis has been sequenced and some synthesis pathways were biochemically described; however, the operation of the cellular metabolic network is not well-characterized. Thus, we conducted a comprehensive study to optimize the sample preparation procedure for metabolomics of U. maydis using GC-MS/MS. Due to the unique characteristics of U. maydis cell culture, two quenching solutions, different washing steps, eight extraction methods, and three derivatization conditions have been examined. The optimal method was then applied for stable isotope-assisted quantification of low molecular weight hydrophilic metabolites while U. maydis utilized different carbon sources including sucrose, glucose, and fructose. This study is the first report on a methodology for absolute quantification of intracellular metabolites in U. maydis central carbon metabolism such as sugars, sugar phosphates, organic acids, amino acids, and nucleotides. For biotechnological use, this method is crucial to exploit the full production potential of this fungus and can also be used to study other fungi of the family Ustilaginaceae.

Cell wall glucans of fungi. A review

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Glucans are the most abundant compounds in the fungal cell walls.

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The most common type of glucose bonding is 1 → 3, both alpha and beta.

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Microfibrillar glucans with chitin provide rigidity to the fungal wall.

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Fungal beta glucans act as PAMPS during infection of animals and plants.

Glucans are the most abundant polysaccharides in the cell walls of fungi, and their structures are highly variable. Accordingly, their glucose moieties may be joined through either or both alpha (α) or beta (β) linkages, they are either lineal or branched, and amorphous or microfibrillar. Alpha 1,3 glucans sensu strictu (pseudonigerans) are the most abundant alpha glucans present in the cell walls of fungi, being restricted to dikarya. They exist in the form of structural microfibrils that provide resistance to the cell wall. The structure of beta glucans is more complex. They are linear or branched, and contain mostly β 1,3 and β 1,6 linkages, existing in the form of microfibrils. Together with chitin they constitute the most important structural components of fungal cell walls. They are the most abundant components of the cell walls in members of all fungal phyla, with the exception of Microsporidia, where they are absent.

Taking into consideration the importance of glucans in the structure and physiology of the fungi, in the present review we describe the following aspects of these polysaccharides: i) types and distribution of fungal glucans, ii) their structure, iii) their roles, iv) the mechanism of synthesis of the most important ones, and v) the phylogentic relationships of the enzymes involved in their synthesis.

Transcriptomic analysis of the role of Rim101/PacC in the adaptation of Ustilago maydis to an alkaline environment

Alkaline pH triggers an adaptation mechanism in fungi that is mediated by Rim101/PacCp, a zinc finger transcription factor. To identify the genes under its control in Ustilago maydis, we performed microarray analyses, comparing gene expression in a wild-type strain versus a rim101/pacC mutation strain of the fungus. In this study we obtained evidence of the large number of genes regulated mostly directly, but also indirectly (probably through regulation of other transcription factors), by Rim101/PacCp, including proteins involved in a large number of physiological activities of the fungus. Our analyses suggest that the response to alkaline conditions under the control of the Pal/Rim pathway involves changes in the cell wall and plasma membrane through alterations in their lipid, protein and polysaccharide composition, changes in cell polarity, actin cytoskeleton organization, and budding patterns. Also as expected, adaptation involves regulation by Rim101/PacC of genes involved in meiotic functions, such as recombination and segregation, and expression of genes involved in ion and nutrient transport, as well as general vacuole functions.