Effect of carbon dioxide on fruiting in Aspergillus nidulans

Determination of the reference genes for qRT-PCR normalization and expression levels of MAT genes under various conditions in Ulocladium

The genus Ulocladium is thought to be strictly asexual. One of the possible reasons for the lack of sexuality in Ulocladium species is the absence of the stimulus of environmental factors. Sexual reproduction in ascomycetes is controlled by a specific region in the genome referred to as mating-type locus (MAT) that consists of two dissimilar DNA sequences in the mating partners, termed MAT1-1 and MAT1-2 idiomorphs. To identify the response of MAT loci to environmental conditions, the mRNA transcription level of MAT1-1-1 and MAT1-2-1 genes was tested using qRT-PCR under different temperatures (−20 °C, −10 °C, 0 °C, 10 °C, 20 °C, 30 °C and 40 °C), culture medias (CM, OA, HAY, PCA, PDA and V8), photoperiods (24 h light, 24 h dark, 12 h light/12 h dark, 10 h light/14 h dark and 8 h light/16 h dark), and CO₂ concentrations (0.03%, 0.5%, 1%, 5%, 10%, 15% and 20%). For obtaining reliable results from qRT-PCR, the most stable internal control gene and optimal number of reference genes for normalization were determined under different treatments. The results showed that there is no universal internal control gene that is expressed at a constant level under different experimental treatments. In comparison to various incubation conditions, the relative expression levels of both MAT genes were significantly increased when fungal mycelia were grown on HAY culture media at 0–10 °C with a light/dark cycle, indicating that temperature, culture media, and light might be the key environmental factors for regulating the sexuality in Ulocladium. Moreover, MAT1-1-1 and MAT1-2-1 genes showed similar expression patterns under different treatments, suggesting that the two MAT genes might play an equally important role in the sexual evolutionary process.

Analysis of volatile compounds emitted by filamentous fungi using solid-phase microextraction-gas chromatography/mass spectrometry

Here, we describe a solid-phase microextraction-gas chromatography/mass spectrometry (SPME-GC/MS) analytical approach that identifies and analyzes volatile compounds in the headspace above a live fungal culture. This approach is a sensitive, solvent-free, robust technique; most importantly from a practical standpoint, this approach is noninvasive and requires minimal sample handling. Aliquots of liquid fungal cultures are placed into vials equipped with inert septa and equilibrated at a constant temperature, and headspace gases are sampled using an SPME fiber inserted through the septum into the headspace above the fungal culture for a standardized period of time. The outer polymer coating of a fused silica fiber absorbs volatiles from the headspace; the volatiles are then desorbed in the hot GC inlet and chromatographed in the usual manner. The separated compounds are subsequently identified by mass spectrometry. All steps in volatile profiling of a single sample from volatile sorption on a fiber to obtaining a list of volatiles can take as little as 15 min or can be extended to several hours if longer sorption is required for compounds present at very low levels and/or have low rates of diffusion.

Sexual development and cryptic sexuality in fungi: insights from Aspergillus species

Major insights into sexual development and cryptic sexuality within filamentous fungi have been gained from investigations using Aspergillus species. Here, an overview is first given into sexual morphogenesis in the aspergilli, describing the different types of sexual structures formed and how their production is influenced by a variety of environmental and nutritional factors. It is argued that the formation of cleistothecia and accessory tissues, such as Hülle cells and sclerotia, should be viewed as two independent but co-ordinated developmental pathways. Next, a comprehensive survey of over 75 genes associated with sexual reproduction in the aspergilli is presented, including genes relating to mating and the development of cleistothecia, sclerotia and ascospores. Most of these genes have been identified from studies involving the homothallic Aspergillus nidulans, but an increasing number of studies have now in addition characterized 'sex-related' genes from the heterothallic species Aspergillus fumigatus and Aspergillus flavus. A schematic developmental genetic network is proposed showing the inter-relatedness between these genes. Finally, the discovery of sexual reproduction in certain Aspergillus species that were formerly considered to be strictly asexual is reviewed, and the importance of these findings for cryptic sexuality in the aspergilli as a whole is discussed.

Willow volatiles influence growth, development, and secondary metabolism in Aspergillus parasiticus

Aflatoxin is a mycotoxin and the most potent naturally occurring carcinogen in many animals. Aflatoxin contamination of food and feed crops causes a significant global burden on human and animal health. However, available methods to eliminate aflatoxin from food and feed are not fully effective. Our goal is to discover novel, efficient, and practical methods to control aflatoxin contamination in crops during storage. In the present study, we tested the effect of volatiles produced by willow (Salix acutifolia and Salix babylonica) and maple (Acer saccharinum) bark on fungal growth, development, and aflatoxin production by the fungus Aspergillus parasiticus, one economically important aflatoxin producer. S. acutifolia bark volatiles nearly eliminated aflatoxin accumulation (>90% reduction) by A. parasiticus grown on a minimal agar medium. The decrease in aflatoxin accumulation correlated with a twofold reduction in ver-1 (encodes a middle aflatoxin pathway enzyme) transcript level. Expression data also indicate that one histone H4 acetyltransferase, MYST3, may play a role in epigenetic control of aflatoxin gene transcription in response to volatile exposure. Volatiles derived from wood bark samples also increased fungal growth up to 20% and/or enhanced conidiospore development. Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of bark samples identified sets of shared and unique volatile compounds that may mediate the observed regulatory effects on growth, development, and aflatoxin synthesis. This work provides an experimental basis for the use of willow industry by-products to control aflatoxin contamination in food and feed crops.

Ethylene modulates development and toxin biosynthesis in aspergillus possibly via an ethylene sensor-mediated signaling pathway

Ethylene, a biologically active natural compound, inhibited aflatoxin accumulation by Aspergillus parasiticus on a solid growth medium in a dose-dependent manner at concentrations of 0.1 to 150 ppm. The activity of the nor-1 promoter (an early aflatoxin gene) was reduced to nondetectable levels by similar quantities of ethylene, suggesting that the inhibitory effect on toxin synthesis occurred, at least in part, at the level of transcription. The inhibitory effect of ethylene on aflatoxin accumulation was also observed when A. parasiticus was grown on raw peanuts. Under similar growth conditions and doses, ethylene strongly inhibited development of asci and ascospores in Aspergillus nidulans, with no detectable effect on Hülle cell formation, conidiation, or sterigmatocystin accumulation. During early growth, A. parasiticus and A. nidulans produced ethylene with approximately twofold higher quantities measured in continuous light than in the dark. 1-Methylcyclopropene (an inhibitor of ethylene receptors in plants), light, CO2, temperature, and growth medium composition altered the effect of ethylene on A. nidulans and A. parasiticus. These observations are consistent with the existence of an ethylene sensor molecule that mediates the function of an ethylene-responsive signaling pathway(s) in Aspergillus.

Characterisation of carbon dioxide-inducible genes of the marine bacterium, Pseudomonas sp. S91

Characterisation of two genes in Pseudomonas sp. S91 that are responsive to carbon dioxide is reported. These were identified by random transposon mutagenesis leading to fusion of the Escherichia coli lacZ reporter gene to the genes of interest. Expression of the genes' promoters was quantified by measuring the reporter gene product, beta-galactosidase. beta-Galactosidase synthesis was induced when cells were exposed to 10% CO2 on solid media or during growth in aqueous phase when the culture density was greater than 1 at 610 nm, in either rich or minimal media. Induction of beta-galactosidase synthesis was not due to: increased alkalinity, onset of stationary phase, build up of soluble metabolites in the culture supernatant, or cell density-dependent signalling. The CO2-inducible gene fusions were not induced by other environmental conditions that are known to stimulate global regulators of environmental gene expression. Benzoic acid (2 mM) induced beta-galactosidase synthesis in one of the mutants indicating the Co2 response may involve the intracellular CO2 partial pressure/bicarbonate ion concentration/pH equilibrium.