Structure and function of the conidiospore pigments ofPenicillium cyclopium

Temperature, water activity and pH during conidia production affect the physiological state and germination time of Penicillium species

Conidial germination and mycelial growth are generally studied with conidia produced under optimal conditions to increase conidial yield. Nonetheless, the physiological state of such conidia most likely differs from those involved in spoilage of naturally contaminated food. The present study aimed at investigating the impact of temperature, pH and water activity (a_w) during production of conidia on the germination parameters and compatible solutes of conidia of Penicillium roqueforti and Penicillium expansum. Low temperature (5°C) and reduced a_w (0.900 a_w) during sporulation significantly reduced conidial germination times whereas the pH of the sporulation medium only had a slight effect at the tested values (2.5, 8.0). Conidia of P. roqueforti produced at 5°C germinated up to 45h earlier than those produced at 20°C. Conidia of P. roqueforti and P. expansum produced at 0.900 a_w germinated respectively up to 8h and 3h earlier than conidia produced at 0.980 a_w. Furthermore, trehalose and mannitol assessments suggested that earlier germination might be related to delayed conidial maturation even though no ultra-structural modifications were observed by transmission electron microscopy. Taken together, these results highlight the importance of considering environmental conditions during sporulation in mycological studies. The physiological state of fungal conidia should be taken into account to design challenge tests or predictive mycology studies. This knowledge may also be of interest to improve the germination capacity of fungal cultures commonly used in fermented foods.

The chromatin code of fungal secondary metabolite gene clusters

Secondary metabolite biosynthesis genes in fungi are usually physically linked and organized in large gene clusters. The physical linkage of genes involved in the same biosynthetic pathway minimizes the amount of regulatory steps necessary to regulate the biosynthetic machinery and thereby contributes to physiological economization. Regulation by chromatin accessibility is a proficient molecular mechanism to synchronize transcriptional activity of large genomic regions. Chromatin regulation largely depends on DNA and histone modifications and the histone code hypothesis proposes that a certain combination of modifications, such as acetylation, methylation or phosphorylation, is needed to perform a specific task. A number of reports from several laboratories recently demonstrated that fungal secondary metabolite (SM) biosynthesis clusters are controlled by chromatin-based mechanisms and histone acetyltransferases, deacetylases, methyltransferases, and proteins involved in heterochromatin formation were found to be involved. This led to the proposal that establishment of repressive chromatin domains over fungal SM clusters under primary metabolic conditions is a conserved mechanism that prevents SM production during the active growth phase. Consequently, transcriptional activation of SM clusters requires reprogramming of the chromatin landscape and replacement of repressive histone marks by activating marks. This review summarizes recent advances in our understanding of chromatin-based SM cluster regulation and highlights some of the open questions that remain to be answered before we can draw a more comprehensive picture.

Mutants and isolates of Metarhizium anisopliae are diverse in their relationships between conidial pigmentation and stress tolerance

Conidial pigmentation is involved in protection against heat and UV radiation in several fungal species. In this study, we compare the tolerance of 17 color mutants of wild-type ARSEF 23 plus 13 color mutants of wild-type ARSEF 2575 of Metarhizium anisopliae var. anisopliae to wet-heat and UV-B or simulated-solar radiation. The stress tolerance of each mutant was compared with that of its wild-type parent, and with the most thermo- and UV-tolerant wild-type Metarhizium we have tested to date, M. anisopliae var. acridum (ARSEF 324). The color of each isolate or mutant was identified with the PANTONE Color Standard book [Eiseman, L., Herbert, L., 1990. The PANTONE((R)) Book of Color: over 1000 color standards: color basics and guidelines for design, fashion, furnishing... and more. Harry N. Abrams, Inc., Publishers, New York]. In addition, the pigments of each mutant or wild-type were extracted and the UV absorbances of the extracts compared to the stress tolerance of those isolates; but no relationships were detected. Color mutants of ARSEF 23, in general, were less UV tolerant than their parent wild-type. With ARSEF 23 and its mutants, conidial pigmentation was important to conidial tolerance to UV-B and simulated-solar radiation; but color had less impact on ARSEF 2575 and its mutants. The ARSEF 2575 color mutants were less variable in UV tolerance than those of ARSEF 23, even though very similar colors occurred in the two groups of mutants. When color mutants of ARSEF 23 reverted to wild-type color they recovered wild-type levels of UV tolerance. Results of UV-B and UV-A exposures of wild-types ARSEF 23 and ARSEF 2575 conidia indicated that they are equally tolerant of UV-A, but differ in UV-B-response. For thermotolerance, several mutants were more heat tolerant than their wild-type parents. Accordingly, darker pigmentation of wild-type isolates was not important to protection against heat.

Signalling pathways connecting mycotoxin production and sporulation

SUMMARY Mycotoxin contamination of food and feed presents a serious food safety issue on a global scale, causing tremendous yield and economic losses. These toxins, produced largely by members of the genera Aspergillus and Fusarium, represent a subset of the impressive array of secondary metabolites produced by filamentous fungi. Some secondary metabolites are associated temporally and functionally with sporulation. In Aspergillus and Fusarium, sporulation and mycotoxin production are both regulated by G protein signalling pathways. G protein signalling pathways commonly regulate fungal development, stress response and expression of virulence traits. In addition, fungal development is influenced by external factors. Among these are lipids, and in particular, oxylipin signals, which may be derived from either the fungus or infected seeds. Regardless of origin, oxylipins have the potential to elicit profound changes in both sporulation and mycotoxin production in the fungus. Signal transduction via G protein signalling pathways represents one mechanism by which oxylipin signals might elicit these changes. Therefore, in this review we integrate discussion of oxylipin signals and of G protein signalling cascades as regulators of fungal development.

Ketosynthase domain probes identify two subclasses of fungal polyketide synthase genes

Analysis of fungal polyketide synthase gene sequences suggested that these might be divided into two subclasses, designated WA-type and MSAS-type. Two pairs of degenerate PCR primers (LC1 and LC2c, LC3 and LC5c) were designed for the amplification of ketosynthase domain fragments from fungal PKS genes in each of these subclasses. Both primer pairs were shown to amplify one or more PCR products from the genomes of a range of ascomycetous Deuteromycetes and Southern blot analysis confirmed that the products obtained with each pair of primers emanated from distinct genomic loci. PCR products obtained from Penicillium patulum and Aspergillus parasiticus with the LC1/2c primer pair and from Phoma sp. C2932 with both primer pairs were cloned and sequenced; the deduced protein sequences were highly homologous to the ketosynthase domains of other fungal PKS genes. Genes from which LC1/2c fragments were amplified (WA-type) were shown by a phylogenetic analysis to be closely related to fungal PKS genes involved in pigment and aflatoxin biosynthetic pathways, whereas the gene from which the LC3/5c fragment was amplified (MSAS-type) was shown to be closely related to genes encoding 6-methylsalicylic acid synthase (MSAS). The phylogenetic tree strongly supported the division of fungal PKS genes into two subclasses. The LC-series primers may be useful molecular tools to facilitate the cloning of novel fungal polyketide synthase genes.

Nuclear inheritance of the biosynthesis of cyclopenin and cyclopenol in Penicillium cyclopium

Balanced heterokarions were grown from Penicillium cyclopium aux-glu 1, a glutamic acid auxotroph producing benzodiazepine alkaloids of the cyclopenin-cyclopenol group, and P. viridicatum aux-met 1, a methionine auxotroph forming these alkaloids in traces only. In contrast to the hyphae of the parent strains, the hyphae of the heterokarions were dark orange-brown and grew well on media without the auxotrophic factors. In surface cultures they synthesized the benzodiazepine alkaloids cyclopenin and cyclopenol in amounts similar to those formed by the hyphae of P. cyclopium aux-glu 1. From the monokariotic conidiospores of the heterokarions homokariotic daughter strains were obtained which were similar to the parent strains in every respect. Hence no exchange of features of cyclopenin-cyclopenol biosynthesis took place between the parent strains at the stage of the heterokarion. This result indicates that the formation of cyclopenin and cyclopenol in P. cyclopium aux-glu 1 and the nearly complete lack of biosynthesis of these compounds in P. viridicatum aux-met 1 is encoded within the nucleus and is not influenced by plasmic genetic material.

Phenol oxidase activity and pigment synthesis in conidiospores of Penicillium cyclopium

Phenol oxidase (PO) of Penicillium cyclopium belongs to the laccases (E.C. 1.14.18.1). By PAA-gel electrophoresis three forms of the enzyme were detected. Most of the PO in emerged sporulating cultures accumulates in the cell wall of the conidiospores. However, a high percentage is not bound to the wall constituents and can be easily removed with aqueous solutions. This soluble fraction was 570-fold enriched by (NH4)2SO4-precipitation, followed by chromatography on sephadex G 200 and on hydroxylapatite-cellulose, The necessity of PO for conidiospore pigmentation was shown by means of suspended spores whose rates of pigment accumulation resemble those of conidia maturing in situ. Pigmentation is stopped if PO is inhibited by diethyldithiocarbamate. This corresponds to the fact that in strains with heavily pigmented conidiospores PO activity is much higher than in strains with weakly pigmented or unpigmented spores. Also the time course of PO accumulation in the latter strains is different from that of the former. Furthermore, in mutant res-eth 1 which has unpigmented conidiospores, PO isoenzyme no. 3 could not be detected.