Lipoxygenase inhibitors shift the yeast/mycelium dimorphism in Ceratocystis ulmi

Fungal Jasmonate as a Novel Morphogenetic Signal for Pathogenesis

A key question that has remained unanswered is how pathogenic fungi switch from vegetative growth to infection-related morphogenesis during a disease cycle. Here, we identify a fungal oxylipin analogous to the phytohormone jasmonic acid (JA), as the principal regulator of such a developmental switch to isotropic growth and pathogenicity in the rice-blast fungus Magnaporthe oryzae. Using specific inhibitors and mutant analyses, we determined the molecular function of intrinsic jasmonates during M. oryzae pathogenesis. Loss of 12-Oxo-phytodienoic Acid (OPDA) Reductase and/or consequent reduction of jasmonate biosynthesis, prolonged germ tube growth and caused delayed initiation and improper development of infection structures in M. oryzae, reminiscent of phenotypic defects upon impaired cyclic AMP (cAMP) signaling. Chemical- or genetic-complementation completely restored proper vegetative growth and appressoria in opr1Δ. Mass spectrometry-based quantification revealed increased OPDA accumulation and significantly decreased jasmonate levels in opr1Δ. Most interestingly, exogenous jasmonate restored proper appressorium formation in pth11Δ that lacks G protein/cAMP signaling; but failed to do so in the Mitogen-activated protein (MAP) kinase mutants. Epistasis analysis placed jasmonate upstream of the cAMP pathway in rice blast. Mechanistically, intrinsic jasmonate orchestrates timely cessation of the vegetative phase and induces pathogenic development via a complex regulatory interaction with the cAMP-PKA cascade and redox signaling in rice blast.

Volatile self-inhibitor of spore germination in pathogenic Mucorale Rhizopus arrhizus

Rhizopus arrhizus is a common pathogenic Mucoralean mold that exists as a saprophyte, and is disseminated through sporangiospores, which germinate to form mycelia under suitable environmental or infection settings. Such morphological transitions are often mediated by self-produced effector molecules in a density-dependent fashion. This study aimed to elucidate if a quorum-dependent, cell-density-driven phenomenon exists in R. arrhizus, and identify the molecule(s) involved. The germination of R. arrhizus was observed to be reliant on the seeding density, with nearly 71% and 47% germination in Sabouraud dextrose and glucose asparagine media respectively at 1 × 105-1 × 106 spores/mL, and only 10% and 1% germination respectively with 1 × 108 spores/mL. The late-growth-stage supernatant also hindered the spore germination and liquid-culture biomass in a dose-dependent way. These effects were being mediated by a volatile inhibitor present in the headspace and supernatant of R. arrhizus cultures, identified as 2-methyl-2-butene by gas chromatography and electron ionization-quadrupole mass spectrometry. The compound was present in a density-dependent manner and considerably impaired fungal germ-tube emergence and elongation during germination. Spore swelling remained unaffected. Multiple thin protrusions comprising of F-actin and microtubules were seen emanating from the treated cells, suggestive of filopodia-like and cytoneme-like extensions. The same compound was also detected in Rhizomucor pusillus.

Comparative RNAseq Analysis of the Insect-Pathogenic Fungus Metarhizium anisopliae Reveals Specific Transcriptome Signatures of Filamentous and Yeast-Like Development

The fungus Metarhizium anisopliae is a facultative insect pathogen used as biological control agent of several agricultural pests worldwide. It is a dimorphic fungus that is able to display two growth morphologies, a filamentous phase with formation of hyphae and a yeast-like phase with formation of single-celled blastospores. Blastospores play an important role for M. anisopliae pathogenicity during disease development. They are formed solely in the hemolymph of infected insects as a fungal strategy to quickly multiply and colonize the insect's body. Here, we use comparative genome-wide transcriptome analyses to determine changes in gene expression between the filamentous and blastospore growth phases in vitro to characterize physiological changes and metabolic signatures associated with M. anisopliae dimorphism. Our results show a clear molecular distinction between the blastospore and mycelial phases. In total 6.4% (n = 696) out of 10,981 predicted genes in M. anisopliae were differentially expressed between the two phases with a fold-change > 4. The main physiological processes associated with up-regulated gene content in the single-celled yeast-like blastospores during liquid fermentation were oxidative stress, amino acid metabolism (catabolism and anabolism), respiration processes, transmembrane transport and production of secondary metabolites. In contrast, the up-regulated gene content in hyphae were associated with increased growth, metabolism and cell wall re-organization, which underlines the specific functions and altered growth morphology of M. anisopliae blastospores and hyphae, respectively. Our study revealed significant transcriptomic differences between the metabolism of blastospores and hyphae. These findings illustrate important aspects of fungal morphogenesis in M. anisopliae and highlight the main metabolic activities of each propagule under in vitro growth conditions.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Diversity in yeast-mycelium dimorphism response of the Dutch elm disease pathogens: the inoculum size effect

Dutch elm disease (DED) is caused by the dimorphic fungi Ophiostoma ulmi, Ophiostoma novo-ulmi, and Ophiostoma himal-ulmi. A cell population density-dependent phenomenon related to quorum sensing was previously shown to affect the reversible transition from yeast-like to mycelial growth in liquid shake cultures of O. novo-ulmi NRRL 6404. Since the response to external stimuli often varies among DED fungal strains, we evaluated the effect of inoculum size on 8 strains of the 3 species of DED agents by determining the proportion of yeast and mycelium produced at different spore inoculum concentrations in defined liquid shake medium. The results show that not all DED fungi strains respond similarly to inoculum size effect, since variations were observed among strains. It is thus possible that the different strains belonging to phylogenetically close species use different signalling molecules or molecular signalling pathways to regulate their growth mode via quorum-sensing mechanisms.

Control of yeast-mycelium dimorphism in vitro in Dutch elm disease fungi by manipulation of specific external stimuli

Dutch elm disease (DED) fungi exhibit yeast-mycelium dimorphism both in planta and in vitro. However, previously published data on the transition between these two growth forms in vitro were mostly obtained from a single strain. We examined the effect of six factors on yeast-mycelium dimorphism in vitro in ten strains of Ophiostoma ulmi, Ophiostoma novo-ulmi and Ophiostoma himal-ulmi. Nitrogen sources, calcium, and yeast extract, altogether with inhibitors of phosphodiesterase (caffeine) and dioxygenases (propyl gallate and salicylic acid) were tested in defined culture media. Morphological response to manipulation of several of these factors varied according to the strain of Ophiostoma being analysed. Responses ranged from no statistical differences in morphological transitions to stimulation or reversion of yeast-mycelium dimorphism with the treatments that were tested. These results suggest that different mechanisms and pathways operate in the control of the yeast-mycelium transition in DED pathogens. Oxylipins could be involved in the yeast-to-mycelium transition, since the addition of a dioxygenase inhibitor, salicylic acid, reduced mycelium production in all strains that were tested.

Aspergillus oxylipin signaling and quorum sensing pathways depend on g protein-coupled receptors

Oxylipins regulate Aspergillus development and mycotoxin production and are also involved in Aspergillus quorum sensing mechanisms. Despite extensive knowledge of how these oxylipins are synthesized and what processes they regulate, nothing is known about how these signals are detected and transmitted by the fungus. G protein-coupled receptors (GPCR) have been speculated to be involved as they are known oxylipin receptors in mammals, and many putative GPCRs have been identified in the Aspergilli. Here, we present evidence that oxylipins stimulate a burst in cAMP in A. nidulans, and that loss of an A. nidulans GPCR, gprD, prevents this cAMP accumulation. A. flavus undergoes an oxylipin-mediated developmental shift when grown at different densities, and this regulates spore, sclerotial and aflatoxin production. A. flavus encodes two putative GprD homologs, GprC and GprD, and we demonstrate here that they are required to transition to a high-density development state, as well as to respond to spent medium of a high-density culture. The finding of GPCRs that regulate production of survival structures (sclerotia), inoculum (spores) and aflatoxin holds promise for future development of anti-fungal therapeutics.

Quorum sensing activity in Ophiostoma ulmi: effects of fusel oils and branched chain amino acids on yeast-mycelial dimorphism

AIMS

  For Ophiostoma (Ceratocystis) ulmi, the ability to undergo morphological change is a crucial factor for its virulence. To gain an understanding of quorum-sensing activity in O. ulmi as it relates to yeast-mycelium dimorphism control, this study examines the effects of branched-chain amino acids as well as their fusel alcohols and fusel acids as quorum sensing molecules.

METHODS AND RESULTS

  In a defined medium containing glucose, proline and salts, O. ulmi grew as yeasts when the culture was inoculated with a high density of spores (2 × 10(7)  CFU ml(-1) ) and as mycelia when inoculated with a low spore density (4 × 10(5)  CFU ml(-1) ). The cultures displaying yeast morphology secreted a quorum-sensing factor that shifted the morphology from mycelia to yeast. This quorum-sensing molecule was lipophilic and extractable by organic solvents from the spent medium. Using GC/MS analysis, it was determined that the major compound in the extract was 2-methyl-1-butanol. A similar effect was observed when the branched-chain amino acids (fusel alcohol precursors) were used as the nitrogen source. E, E-farnesol had no effect on the morphology of O. ulmi.

CONCLUSIONS

  Addition of the branched-chain amino acids or one of the compounds detected in the spent medium, 2-methyl-1-butanol or 4-hydroxyphenylacetic acid, or methylvaleric acid, decreased germ tube formation by more than 50%, thus demonstrating a quorum sensing molecule behaviour in O. ulmi cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

  This study presents advances in the investigation of dimorphism in O. ulmi, complementing the existing scientific basis, for studying, understanding and controlling this phenomenon.

Oxylipins in fungi

In nearly every living organism, metabolites derived from lipid peroxidation, the so-called oxylipins, are involved in regulating developmental processes as well as environmental responses. Among these bioactive lipids, the mammalian and plant oxylipins are the best characterized, and much information about their physiological role and biosynthetic pathways has accumulated during recent years. Although the occurrence of oxylipins and enzymes involved in their biosynthesis has been studied for nearly three decades, knowledge about fungal oxylipins is still scarce as compared with the situation in plants and mammals. However, the research performed so far has shown that the structural diversity of oxylipins produced by fungi is high and, furthermore, that the enzymes involved in oxylipin metabolism are diverse and often exhibit unusual catalytic activities. The aim of this review is to present a synopsis of the oxylipins identified so far in fungi and the enzymes involved in their biosynthesis.

Distinct roles for VeA and LaeA in development and pathogenesis of Aspergillus flavus

Aspergillus flavus, a mycotoxigenic filamentous fungus, colonizes several important agricultural crops, such as maize and peanuts. Two proteins, VeA and LaeA, known to form a nuclear complex in Aspergillus nidulans have been found to positively regulate developmental processes in several Aspergillus species. Here, an examination of near-isogenic A. flavus mutants differing in copy number of veA and laeA alleles (0, 1, or at least 2 each) revealed critical roles for VeA and LaeA in A. flavus development and seed colonization. In contrast to the wild type, both null mutants were unable to metabolize host cell lipid reserves and were inhibited by oleic acid in growth assays. The copy number of LaeA but not VeA appeared critical for a density-dependent sclerotial-to-conidial shift, since the multicopy laeA (MClaeA) strain produced relatively constant sclerotial numbers with increasing population size rather than showing the decrease in sclerotia seen in both the wild-type and MCveA strains. The MCveA-laeA strain yielded an intermediate phenotype. This study revealed unique roles of VeA and LaeA in seed pathogenesis and fungal biology, distinct from their cooperative regulatory functions in aflatoxin and sclerotial development.

Induction of morphological changes in Ustilago maydis cells by octyl gallate

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.

Morphological transitions governed by density dependence and lipoxygenase activity in Aspergillus flavus

Aspergillus flavus differentiates to produce asexual dispersing spores (conidia) or overwintering survival structures called sclerotia. Results described here show that these two processes are oppositely regulated by density-dependent mechanisms and that increasing the cell density (from 10(1) to 10(7) cells/plate) results in the lowest numbers of sclerotial and the highest numbers of conidial. Extract from spent medium of low-cell-density cultures induced a high-sclerotium-number phenotype, whereas high-cell-density extract increased conidiation. Density-dependent development is also modified by changes in lipid availability. Exogenous linoleic acid increased sclerotial production at intermediate cell densities (10(4) and 10(5) cells/plate), whereas oleic and linolenic acids inhibited sclerotium formation. Deletion of Aflox encoding a lipoxygenase (LOX) greatly diminished density-dependent development of both sclerotia and conidia, resulting in an overall increase in the number of sclerotia and a decrease in the number of conidia at high cell densities (>10(5) cells/plate). Aflox mutants showed decreased linoleic acid LOX activity. Taken together, these results suggest that there is a quorum-sensing mechanism in which a factor(s) produced in dense cultures, perhaps a LOX-derived metabolite, activates conidium formation, while a factor(s) produced in low-density cultures stimulates sclerotium formation.

CP/MAS spectroscopy in the determination of the tautomeric forms of gossypol, its Schiff bases and hydrazones in the solid state

New Schiff bases and new hydrazones were synthesized and studied by (13)C and (15)N CP/MAS spectroscopy and by (1)H--(1)H COSY, (1)H--(13)C HMBC, (1)H--(13)C HSQC, (1)H--(15)N HMQC and (1)H--(15)N HSQC correlations. The CP/MAS investigation of gossypol has demonstrated that in the solid state it exists exclusively in the aldehyde-aldehyde tautomeric form. In contrast, CP/MAS studies of hydrazones and Schiff bases reveal that these compounds occur in the solid state in the N-imine-N-imine and enamine-enamine tautomeric forms, respectively. It is shown that the (13)C resonances of C-6, C-7 and C-11 carbon atoms are suitable for distinguishing between the tautomeric forms of aza-derivatives of gossypol in the solid state. Furthermore, we have proved that the (15)N CP/MAS spectra can be used to identify these tautomeric forms.

Dimorphism in fungal plant pathogens

Fungi are mostly sessile organisms, and thus have evolved ways to cope with environmental changes. Many fungi produce 'dormant' structures, which allow them to survive periods of unfavorable conditions. Another ingenious active approach to a changing environment has been adopted by the 'dimorphic fungi', which simply shift their thallic organization as a way to adapt and thrive in the new conditions. Dimorphism is extensively exploited by both plant and animal pathogenic fungi, where the encounter with the host prompts a shift in the mode of growth. In this review, we focus on the phenomenon of dimorphism among plant pathogenic fungi through discussion of several relatively well-studied exemplar species.

Role of oxylipins and other lipid mediators in fungal pathogenesis

Recently there has been a focused interest in the production of bioactive lipid metabolites from eukaryotic microbes, and in the roles that these molecules play in development and pathological processes. These metabolites have long been known in mammals to be potent modulators of various physiological processes, such as the regulation of inflammation. This area of research has been of particular interest in fungi, where oxylipin production has been correlated with pathogenicity. The aim of this review is to discuss recent findings that show how oxylipins and other lipid mediators affect fungal development, quorum sensing and effecter molecule production, which all amount to a global control by oxylipins of fungal pathogenesis.

Oxylipins as developmental and host-fungal communication signals

Pathogenic microbes and their hosts have acquired complex signalling mechanisms to appraise themselves of the environmental milieu in the ongoing battle for survival. Several recent studies have implicated oxylipins as a novel class of host-microbe signalling molecules. Oxylipins represent a vast and diverse family of secondary metabolites that originate from the oxidation or further conversion of polyunsaturated fatty acids. Among the microbial oxylipins, the fungal oxylipins are best characterized and function as hormone-like signals that modulate the timing and balance between asexual and sexual spore development in addition to toxin production. Coupled with other studies that implicate a role for fungal oxylipins in pathogenesis by Aspergillus and Candida spp., these results suggest that host and microbial oxylipins might interfere with the metabolism, perception or signalling processes of each other.

Three putative oxylipin biosynthetic genes integrate sexual and asexual development in Aspergillus nidulans

Oxylipins called psi factors have been shown to alter the ratio of asexual to sexual sporulation in the filamentous fungusAspergillus nidulans. Analysis of theA. nidulansgenome has led to the identification of three fatty acid oxygenases (PpoA, PpoB and PpoC) predicted to produce psi factors. Here, it is reported that deletion ofppoB(ΔppoB) reduced production of the oleic-acid-derived oxylipin psiBβand increased the ratio of asexual to sexual spore development. Generation of the triple mutant ΔppoAΔppoBΔppoCresulted in a strain deficient in producing oleic- and linoleic-acid-derived 8′-hydroxy psi factor and caused increased and mis-scheduled activation of sexual development. Changes in asexual to sexual spore development were positively correlated to alterations in the expression ofbrlAandveA, respectively. PpoB and/or its products antagonistically mediate the expression levels ofppoAandppoC, thus revealing regulatory feedback loops among these three genes. Phylogenetic analyses showed thatppogenes are present in both saprophytic and pathogenic Ascomycetes and Basidiomycetes, suggesting a conserved role for Ppo enzymes in the life cycle of fungi.

Multidrug pump inhibitors uncover remarkable activity of plant antimicrobials

Plant antimicrobials are not used as systemic antibiotics at present. The main reason for this is their low level of activity, especially against gram-negative bacteria. The reported MIC is often in the range of 100 to 1,000 micro g/ml, orders of magnitude higher than those of common broad-spectrum antibiotics from bacteria or fungi. Major plant pathogens belong to the gram-negative bacteria, which makes the low level of activity of plant antimicrobials against this group of microorganisms puzzling. Gram-negative bacteria have an effective permeability barrier, comprised of the outer membrane, which restricts the penetration of amphipathic compounds, and multidrug resistance pumps (MDRs), which extrude toxins across this barrier. It is possible that the apparent ineffectiveness of plant antimicrobials is largely due to the permeability barrier. We tested this hypothesis in the present study by applying a combination of MDR mutants and MDR inhibitors. A panel of plant antimicrobials was tested by using a set of bacteria representing the main groups of plant pathogens. The human pathogens Pseudomonas aeruginosa, Escherichia coli, and Salmonella enterica serovar Typhimurium were also tested. The results show that the activities of the majority of plant antimicrobials were considerably greater against the gram-positive bacteria Staphylococcus aureus and Bacillus megaterium and that disabling of the MDRs in gram-negative species leads to a striking increase in antimicrobial activity. Thus, the activity of rhein, the principal antimicrobial from rhubarb, was potentiated 100- to 2,000-fold (depending on the bacterial species) by disabling the MDRs. Comparable potentiation of activity was observed with plumbagin, resveratrol, gossypol, coumestrol, and berberine. Direct measurement of the uptake of berberine, a model plant antimicrobial, confirmed that disabling of the MDRs strongly increases the level of penetration of berberine into the cells of gram-negative bacteria. These results suggest that plants might have developed means of delivering their antimicrobials into bacterial cells. These findings also suggest that plant antimicrobials might be developed into effective, broad-spectrum antibiotics in combination with inhibitors of MDRs.

Chemotyping of yeast mutants using robotics

By now, the EUROFAN programme for the functional analysis of genes from the yeast genome has attained its cruising speed. Indeed, several hundreds of yeast mutants with no phenotype as tested by growth on standard media and no significant sequence similarity to proteins of known function are available through the efforts of various laboratories. Based on the methodology initiated during the pilot project on yeast chromosome III (Yeast 13, 1547-1562, 1997) we adapted it to High Throughput Screening (HTS), using robotics. The first 100 different gene deletions from EUROSCARF, constructed in an FY1679 strain background, were run against a collection of about 300 inhibitors. Many of these inhibitors have not been reported until now to interfere in vivo with growth of Saccharomyces cerevisiae. In the present paper we provide a list of novel growth conditions and a compilation of 49 yeast deletants (from chromosomes II, IV, VII, X, XIV, XV) corresponding to 58% of the analysed genes, with at least one clear and stringent phenotype. The majority of these deletants are sensitive to one or two compounds (monotropic phenotype) while a distinct subclass of deletants displays a hyper-pleiotropic phenotype with sensitivities to a dozen or more compounds. Therefore, chemotyping of unknown genes with a large spectrum of drugs opens new vistas for a more in-depth functional analysis and a more precise definition of molecular targets.

Metabolism of Linoleic Acid or Mevalonate and 6-Pentyl-α-Pyrone Biosynthesis by Trichoderma Species

The understanding of the biosynthetic pathway of 6-pentyl-α-pyrone in Trichoderma species was achieved by using labelled linoleic acid or mevalonate as a tracer. Incubation of growing cultures of Trichoderma harzianum and T. viride with [U- 14 C]linoleic acid or [5- 14 C]sodium mevalonate revealed that both fungal strains were able to incorporate these labelled compounds (50 and 15%, respectively). Most intracellular radioactivity was found in the neutral lipid fraction. At the initial time of incubation, the radioactivity from [ 14 C]linoleic acid was incorporated into 6-pentyl-α-pyrone more rapidly than that from [ 14 C]mevalonate. No radioactivity incorporation was detected in 6-pentyl-α-pyrone when fungal cultures were incubated with [1- 14 C]linoleic acid. These results suggested that β-oxidation of linoleic acid was a probable main step in the biosynthetic pathway of 6-pentyl-α-pyrone in Trichoderma species.