Insertional mutagenesis of a fungal biocontrol agent led to discovery of a rare cellobiose lipid with antifungal activity

A unique effector secreted by Pseudozyma flocculosa mediates its biocontrol activity

BACKGROUND

Pseudozyma flocculosa is a highly efficient biocontrol agent (BCA) of powdery mildews whose mode of action remains elusive. It is known to secrete unique effectors during its interaction with powdery mildews but effectors have never been shown to be part of the arsenal of a BCA. Here, we characterize the role of the effector Pf2826 released by Pseudozyma flocculosa during its tripartite interaction with barley and the pathogen fungus Blumeria graminis f. sp. hordei.

RESULTS

We utilized CRISPR-Cas9-based genome editing and confirmed that secreted P. flocculosa effector Pf2826 is required for full biocontrol activity. We monitored the localization of the effector Pf2826 with C-terminal mCherry tag and found it localized around the haustoria and on powdery mildew spores. His-tagged Pf2826 recombinant protein was expressed, purified, and used as bait in a pull-down assay from total proteins extracted during the tripartite interaction. Potential interactors were identified by LC-MS/MS analysis after removing unspecific interactions found in the negative controls. A two-way yeast two-hybrid assay validated that Pf2826 interacted with barley pathogenesis-related (PR) proteins HvPR1a and chitinase and with an effector protein from powdery mildew.

CONCLUSIONS

In contrast to the usual modes of action of competition, parasitism, and antibiosis ascribed to BCAs, this study shows that effector pf2826 plays a vital role in the biocontrol activity of P. flocculosa by interacting with plant PR proteins and a powdery mildew effector, altering the host-pathogen interaction.

Fungal biosurfactants, from nature to biotechnological product: bioprospection, production and potential applications

Biosurfactants are in demand by the global market as natural commodities that can be added to commercial products or use in environmental applications. These biomolecules reduce the surface/interfacial tension between fluid phases and exhibit superior stability to chemical surfactants under different physico-chemical conditions. Biotechnological production of biosurfactants is still emerging. Fungi are promising producers of these molecules with unique chemical structures, such as sophorolipids, mannosylerythritol lipids, cellobiose lipids, xylolipids, polyol lipids and hydrophobins. In this review, we aimed to contextualize concepts related to fungal biosurfactant production and its application in industry and the environment. Concepts related to the thermodynamic and physico-chemical properties of biosurfactants are presented, which allows detailed analysis of their structural and application. Promising niches for isolating biosurfactant-producing fungi are presented, as well as screening methodologies are discussed. Finally, strategies related to process parameters and variables, simultaneous production, process optimization through statistical and genetic tools, downstream processing and some aspects of commercial products formulations are presented.

A reassessment of flocculosin-mediated biocontrol activity of Pseudozyma flocculosa through CRISPR/Cas9 gene editing

Pseudozyma flocculosa is an epiphytic yeast with powerful antagonistic activity against powdery mildews. This activity has been associated with the production of a rare antifungal glycolipid, flocculosin. In spite of the discovery of a specific gene cluster for flocculosin synthesis, attempts to ascribe a functional role to the molecule have been hampered by the inability to efficiently transform P. flocculosa. In this study, two different approaches, target gene replacement by homologous recombination (HR) and CRISPR-Cas9 based genome-editing, were utilized to decipher the role of flocculosin in the biocontrol activity of P.flocculosa. It was possible to alter the production of flocculosin through edition of fat1 by HR, but such mutants displayed abnormal phenotypes and the inability to produce sporidia. Sequencing analyses revealed that transformation by HR led to multiple insertions in the genome explaining the pleiotrophic effects of the approach. On the other hand, CRISPR-Cas9 transformation yielded one mutant that was altered specifically in the proper synthesis of flocculosin. Notwithstanding the loss of flocculosin production, such mutant was phenotypically similar to the wild-type, and when tested for its biocontrol activity against powdery mildew, displayed the same efficacy. These results offer strong evidence that flocculosin-mediated antibiosis is not responsible for the mode of action of P. flocculosa and highlight the potential of CRISPR-Cas9 for functional studies of otherwise difficult-to-transform fungi such as P. flocculosa.

Secretion-Based Modes of Action of Biocontrol Agents with a Focus on Pseudozyma aphidis

Plant pathogens challenge our efforts to maximize crop production due to their ability to rapidly develop resistance to pesticides. Fungal biocontrol agents have become an important alternative to chemical fungicides, due to environmental concerns related to the latter. Here we review the complex modes of action of biocontrol agents in general and epiphytic yeasts belonging to the genus Pseudozyma specifically and P. aphidis in particular. Biocontrol agents act through multiple direct and indirect mechanisms, which are mainly based on their secretions. We discuss the direct modes of action, such as antibiosis, reactive oxygen species-producing, and cell wall-degrading enzyme secretions which can also play a role in mycoparasitism. In addition, we discuss indirect modes of action, such as hyperbiotrophy, induced resistance and growth promotion based on the secretion of effectors and elicitors from the biocontrol agent. Due to their unique characteristics, epiphytic yeasts hold great potential for use as biocontrol agents, which may be more environmentally friendly than conventional pesticides and provide a way to reduce our dependency on fungicides based on increasingly expensive fossil fuels. No less important, the complex mode of action of Pseudozyma-based biocontrol agents can also reduce the frequency of resistance developed by pathogens to these agents.

A fungal member of the Arabidopsis thaliana phyllosphere antagonizes Albugo laibachii via a GH25 lysozyme

Plants are not only challenged by pathogenic organisms but also colonized by commensal microbes. The network of interactions these microbes establish with their host and among each other is suggested to contribute to the immune responses of plants against pathogens. In wild Arabidopsis thaliana populations, the oomycete pathogen Albugo laibachii plays an influential role in structuring the leaf phyllosphere. We show that the epiphytic yeast Moesziomyces bullatus ex Albugo on Arabidopsis, a close relative of pathogenic smut fungi, is an antagonistic member of the A. thaliana phyllosphere, which reduces infection of A. thaliana by A. laibachii. Combination of transcriptomics, reverse genetics, and protein characterization identified a GH25 hydrolase with lysozyme activity as a major effector of this microbial antagonism. Our findings broaden the understanding of microbial interactions within the phyllosphere, provide insights into the evolution of epiphytic basidiomycete yeasts, and pave the way for novel biocontrol strategies.

Factors Affecting the Synthesis of Cellobiose Lipids by Sporisorium scitamineum

Cellobiose lipids (CL) are extracellular glycolipids that are produced by many microorganisms from the family Ustilaginaceae. The sugarcane smut fungus Sporisorium scitamineum has been long known as a producer of the glycolipids mannosylerythritol lipids (MEL) and was recently described to additionally secrete CL as a byproduct. In fact, we identified 11 homologous genes in S. scitamineum by in silico analysis sharing a high similarity to the CL biosynthesis gene cluster of Ustilago maydis. We here report the first systematic cultivation of S. scitamineum targeting the synthesis of CL with high product titers and its transfer to the bioreactor. In an initial screening we examined different fermentation media compositions, consisting of a mineral salts solution with vitamins and/or trace elements, three carbon sources (glucose, fructose, sucrose), three pH values (2.5, 4.0, 6.7) and three levels of C/N values (42.2, 83.8, 167.2 mol_C⋅mol_N ^-1) with urea as nitrogen source. A pH of 2.5 proved to result in the highest product titers. An increase of urea concentration from 0.6 to 1.2 g⋅L^-1 had a positive effect on biomass formation, however the glycolipid formation was favored at a C/N ratio of 83.8 mol_C⋅mol_N ^-1, using 0.6 g⋅L^-1 urea. Amongst the examined carbon sources, sucrose resulted in an increase in the secretion of cellobiose lipids, compared to glucose. Comparing different media compositions, vitamins were identified as not necessary for CL synthesis. We obtained a concentration of cellobiose lipids of 8.3 ± 1.0 g⋅L^-1 in shaking flasks. This increased to 17.6 g⋅L^-1 in the 1 L bioreactor with additional feeding of carbon source, with a final purity of 85-93%. As a side product, erythritol and mannosylerythritol lipids (MEL) were also synthesized. Via HPTLC coupled MALDI-TOF MS we were able to analyze the secreted CL structures. S. scitamineum produces a mixture of acylated low molecular weight D-glucolipids, linked to a 2,15,16-trihydroxy-hexadecanoic acid via their ω-hydroxyl group (CL-B). The produced cellobiose lipids precipitate as needle like crystals at an acidic pH value of 2.5.

Pseudozyma aphidis activates reactive oxygen species production, programmed cell death and morphological alterations in the necrotrophic fungus Botrytis cinerea

Many types of yeast have been studied in the last few years as potential biocontrol agents against different phytopathogenic fungi. Their ability to control plant diseases is mainly through combined modes of action. Among them, antibiosis, competition for nutrients and niches, induction of systemic resistance in plants and mycoparasitism have been the most studied. In previous work, we have established that the epiphytic yeast Pseudozyma aphidis inhibits Botrytis cinerea through induced resistance and antibiosis. Here, we demonstrate that P. aphidis adheres to B. cinerea hyphae and competes with them for nutrients. We further show that the secreted antifungal compounds activate the production of reactive oxygen species and programmed cell death in B. cinerea mycelium. Finally, P. aphidis and its secreted compounds negatively affect B. cinerea hyphae, leading to morphological alterations, including hyphal curliness, vacuolization and branching, which presumably affects the colonization ability and infectivity of B. cinerea. This study demonstrates additional modes of action for P. aphidis and its antifungal compounds against the plant pathogen B. cinerea.

Simple glycolipids of microbes: Chemistry, biological activity and metabolic engineering

Glycosylated lipids (GLs) are added-value lipid derivatives of great potential. Besides their interesting surface activities that qualify many of them to act as excellent ecological detergents, they have diverse biological activities with promising biomedical and cosmeceutical applications. Glycolipids, especially those of microbial origin, have interesting antimicrobial, anticancer, antiparasitic as well as immunomodulatory activities. Nonetheless, GLs are hardly accessing the market because of their high cost of production. We believe that experience of metabolic engineering (ME) of microbial lipids for biofuel production can now be harnessed towards a successful synthesis of microbial GLs for biomedical and other applications. This review presents chemical groups of bacterial and fungal GLs, their biological activities, their general biosynthetic pathways and an insight on ME strategies for their production.

Effectors involved in fungal-fungal interaction lead to a rare phenomenon of hyperbiotrophy in the tritrophic system biocontrol agent-powdery mildew-plant

Tritrophic interactions involving a biocontrol agent, a pathogen and a plant have been analyzed predominantly from the perspective of the biocontrol agent. We have conducted the first comprehensive transcriptomic analysis of all three organisms in an effort to understand the elusive properties of Pseudozyma flocculosa in the context of its biocontrol activity against Blumeria graminis f.sp. hordei as it parasitizes Hordeum vulgare. After inoculation of P. flocculosa, the tripartite interaction was monitored over time and samples collected for scanning electron microscopy and RNA sequencing. Based on our observations, P. flocculosa indirectly parasitizes barley, albeit transiently, by diverting nutrients extracted by B. graminis from barley leaves through a process involving unique effectors. This brings novel evidence that such molecules can also influence fungal-fungal interactions. Their release is synchronized with a higher expression of powdery mildew haustorial effectors, a sharp decline in the photosynthetic machinery of barley and a developmental peak in P. flocculosa. The interaction culminates with a collapse of B. graminis haustoria, thereby stopping P. flocculosa growth, as barley plants show higher metabolic activity. To conclude, our study has uncovered a complex and intricate phenomenon, described here as hyperbiotrophy, only achievable through the conjugated action of the three protagonists.

Glycolipid biosurfactants: main properties and potential applications in agriculture and food industry

Glycolipids, consisting of a carbohydrate moiety linked to fatty acids, are microbial surface active compounds produced by various microorganisms. They are characterized by high structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface, respectively. Rhamnolipids, trehalolipids, mannosylerythritol lipids and cellobiose lipids are among the most popular glycolipids. They have received much practical attention as biopesticides for controlling plant diseases and protecting stored products. As a result of their antifungal activity towards phytopathogenic fungi and larvicidal and mosquitocidal potencies, glycolipid biosurfactants permit the preservation of plants and plant crops from pest invasion. Also, as a result of their emulsifying and antibacterial activities, glycolipids have great potential as food additives and food preservatives. Furthermore, the valorization of food byproducts via the production of glycolipid biosurfactant has received much attention because it permits the bioconversion of byproducts on valuable compounds and decreases the cost of production. Generally, the use of glycolipids in many fields requires their retention from fermentation media. Accordingly, different strategies have been developed to extract and purify glycolipids. © 2016 Society of Chemical Industry.

Glycolipid biosurfactants: Potential related biomedical and biotechnological applications

Glycolipids, consisting of a carbohydrate moiety linked to fatty acids, are microbial surface active compounds produced by various microorganisms. They are characterized by highly structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface respectively. Rhamnolipids, trehalolipids, mannosylerythritol-lipids and cellobiose lipids are among the most popular glycolipids. Moreover, their ability to form pores and destabilize biological membrane permits their use in biomedicine as antibacterial, antifungal and hemolytic agents. Their antiviral and antitumor effects enable their use in pharmaceutic as therapeutic agents. Also, glycolipids can inhibit the bioadhesion of pathogenic bacteria enabling their use as anti-adhesive agents and for disruption of biofilm formation and can be used in cosmetic industry. Moreover, they have great potential application in industry as detergents, wetting agents and for flotation. Furthermore, glycolipids can act at the surface and can modulate enzyme activity permitting the enhancement or the inhibition of the activity of certain enzymes.

Biosurfactant gene clusters in eukaryotes: regulation and biotechnological potential

Biosurfactants (BSs) are a class of secondary metabolites representing a wide variety of structures that can be produced from renewable feedstock by a wide variety of micro-organisms. They have (potential) applications in the medical world, personal care sector, mining processes, food industry, cosmetics, crop protection, pharmaceuticals, bio-remediation, household detergents, paper and pulp industry, textiles, paint industries, etc. Especially glycolipid BSs like sophorolipids (SLs), rhamnolipids (RLs), mannosylerythritol lipids (MELs) and cellobioselipids (CBLs) have been described to provide significant opportunities to (partially) replace chemical surfactants. The major two factors currently limiting the penetration of BSs into the market are firstly the limited structural variety and secondly the rather high production price linked with the productivity. One of the keys to resolve the above mentioned bottlenecks can be found in the genetic engineering of natural producers. This could not only result in more efficient (economical) recombinant producers, but also in a diversification of the spectrum of available BSs as such resolving both limiting factors at once. Unraveling the genetics behind the biosynthesis of these interesting biological compounds is indispensable for the tinkering, fine tuning and rearrangement of these biological pathways with the aim of obtaining higher yields and a more extensive structural variety. Therefore, this review focuses on recent developments in the investigation of the biosynthesis, genetics and regulation of some important members of the family of the eukaryotic glycolipid BSs (MELs, CBLs and SLs). Moreover, recent biotechnological achievements and the industrial potential of engineered strains are discussed.

Identification of the gene PaEMT1 for biosynthesis of mannosylerythritol lipids in the basidiomycetous yeast Pseudozyma antarctica

The yeast Pseudozyma antarctica produces a large amount of glycolipid biosurfactants known as mannosylerythritol lipids (MELs), which show not only excellent surface-active properties but also versatile biochemical actions. To investigate the biosynthesis of MELs in the yeast, we recently reported expressed sequence tag (EST) analysis and estimated genes expressing under MEL production conditions. Among the genes, a contiguous sequence of 938 bp, PA_004, showed high sequence identity to the gene emt1, encoding an erythritol/mannose transferase of Ustilago maydis, which is essential for MEL biosynthesis. The predicted translation product of the extended PA_004 containing the two introns and a stop codon was aligned with Emt1 of U. maydis. The predicted amino acid sequence shared high identity (72%) with Emt1 of U. maydis, although the amino-terminal was incomplete. To identify the gene as PaEMT1 encoding an erythritol/mannose transferase of P. antarctica, the gene-disrupted strain was developed by the method for targeted gene disruption, using hygromycin B resistance as the selection marker. The obtained ΔPaEMT1 strain failed to produce MELs, while its growth was the same as that of the parental strain. The additional mannosylerythritol into culture allowed ΔPaEMT1 strain to form MELs regardless of the carbon source supplied, indicating a defect of the erythritol/mannose transferase activity. Furthermore, we found that MEL formation is associated with the morphology and low-temperature tolerance of the yeast.

Ecological basis of the interaction between Pseudozyma flocculosa and powdery mildew fungi

In this work, we sought to understand how glycolipid production and the availability of nutrients could explain the ecology of Pseudozyma flocculosa and its biocontrol activity. For this purpose, we compared the development of P. flocculosa to that of a close relative, the plant pathogen Ustilago maydis, under different environmental conditions. This approach was further supported by measuring the expression of cyp1, a pivotal gene in the synthesis of unique antifungal cellobiose lipids of both fungi. On healthy cucumber and tomato plants, the expression of cyp1 remained unchanged over time in P. flocculosa and was undetected in U. maydis. At the same time, green fluorescent protein (GFP) strains of both fungi showed only limited green fluorescence on control leaves. On powdery mildew-infected cucumber leaves, P. flocculosa induced a complete collapse of the pathogen colonies, but glycolipid production, as studied by cyp1 expression, was still comparable to that of controls. In complete contrast, cyp1 was upregulated nine times when P. flocculosa was applied to Botrytis cinerea-infected leaves, but the biocontrol fungus did not develop very well on the pathogen. Analysis of the possible nutrients that could stimulate the growth of P. flocculosa on powdery mildew structures revealed that the complex Zn/Mn played a key role in the interaction. Other related fungi such as U. maydis do not appear to have the same nutritional requirements and hence lack the ability to colonize powdery mildews. Whether production of antifungal glycolipids contributes to the release of nutrients from powdery mildew colonies is unclear, but the specificity of the biocontrol activity of P. flocculosa toward Erysiphales does appear to be more complex than simple antibiosis.

Identification of genes potentially involved in the biocontrol activity of Pseudozyma flocculosa

Flocculosin is an antifungal cellobiose lipid linked to the biocontrol activity of Pseudozyma flocculosa and whose structure is very similar to that of ustilagic acid produced by Ustilago maydis. In this work, homologs of the U. maydis cyp1 gene, involved in the biosynthesis of ustilagic acid, were isolated and sequenced from P. flocculosa and P. fusiformata, the latter species being also known to produce ustilagic acid. Interestingly, no homologs were found in four other closely related Pseudozyma spp. from which no evidence of ustilagic acid production has ever been obtained, thus supporting the specificity of cyp1 with ustilagic acid synthesis. In addition, a homolog of the U. maydis uat1 gene involved in the acetylation of the molecule and located next to the cyp1 gene was partially sequenced from P. flocculosa. All three newly sequenced genes showed strong sequence similarity to their counterparts in U. maydis. Cyp1 expression was monitored in conditions that were either conducive or repressive to flocculosin production. Expression increased markedly (>100x) when P. flocculosa was inoculated in a growth medium conducive to flocculosin production but was rapidly downregulated in a repressive medium (in vitro) or on powdery mildew-infected cucumber leaves (in vivo). This suggests that the molecule was preferentially synthesized early in the process of searching for a growth substrate. This study provides the first identification of genes involved in the production of flocculosin, a molecule potentially associated with the biocontrol properties of P. flocculosa.

Production of antifungal cellobiose lipids by Trichosporon porosum

The yeast Trichosporon porosum suppresses growth of ascomycetes and basidiomycetes belonging to 52 genera. It is due to secretion of a thermostable fungicidal agent. The suppression was maximal at pH 3.5-4.0. Fungicidal preparation obtained from the culture broth was shown to be a mixture of cellobiosides of dihydrodecane acid with different degree of acetylation of cellobiose residue. The preparation caused the death of Candida albicans and Filobasidiella neoformans cells in the concentrations of 0.2 and 0.03 mM, respectively.

In vitro antibacterial activity and antifungal mode of action of flocculosin, a membrane-active cellobiose lipid

AIMS

To investigate the in vitro antibacterial activity and antifungal mode of action of flocculosin, a cellobiose lipid produced by Pseudozyma flocculosa.

METHODS AND RESULTS

When tested against clinical bacterial isolates, the compound was particularly active against Gram-positive bacteria and its effect was not mitigated against isolates known as resistant to other antibiotics. The antifungal activity of flocculosin was found to be rapid and concentration-dependent. At lethal concentrations against Candida albicans, flocculosin caused a rapid leakage of intracellular potassium and inhibited acidification of the medium by plasma membrane ATPases suggesting a physical rather than a biochemical effect. TEM observations of cells exposed 6 h to flocculosin revealed disrupted membranes and disorganized mitochondria.

CONCLUSIONS

Data obtained in this study confirm that flocculosin acts by disrupting the membrane surface of sensitive micro-organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The elucidation of an antifungal mode of action of flocculosin can be exploited in furthering its antimicrobial potential against fungi and bacteria whose cell membranes are particularly sensitive to the action of the molecule.

Nutritional regulation and kinetics of flocculosin synthesis by Pseudozyma flocculosa

This study sought to identify the factors and conditions that affected production of the antifungal glycolipid flocculosin by the biocontrol agent Pseudozyma flocculosa. For this purpose, different parameters known or reported to influence glycolipid release in fungi were tested. Concentration of the start-up inoculum was found to play an important role in flocculosin production, as the optimal level increased productivity by as much as tenfold. Carbon availability and nitrogen source (i.e., organic vs inorganic) both had a direct influence on the metabolism of P. flocculosa, leading to flocculosin synthesis. In general, if conditions were conducive for production of the glycolipid, carbon availability appeared to be the only limiting factor. On the other hand, if yeast extract was supplied as nitrogen source, fungal biomass was immediately stimulated to the detriment of flocculosin synthesis. Unlike other reports of glycolipid release by yeast-like fungi, inorganic nitrogen starvation did not trigger production of flocculosin. The relationship between the factors influencing flocculosin production in vitro and the conditions affecting the release of the molecule by P. flocculosa in its natural habitat appears to be linked to the availability of a suitable and plentiful food source for the biocontrol agent.

Antagonistic effects of the endophytic fungus Meira geulakonigii on the citrus rust mite Phyllocoptruta oleivora

AIMS

The fungus Meira geulakonigii has been shown to reduce populations of citrus rust mite (CRM; Phyllocoptruta oleivora) on citrus leaves and fruits, in both the field and laboratory. However, attempts to isolate the fungus from leaves and fruits have been unsuccessful. The aims of this study were therefore to determine whether M. geulakonigii is a citrus endophyte, and to assess possible mechanisms involved in its mite-antagonist activity.

METHODS AND RESULTS

A quantitative real-time PCR and regular PCR approaches were developed to detect M. geulakonigii in both the field and laboratory. The fungus was detected throughout. Different methods revealed that M. geulakonigii is an endophyte, which colonizes both the peel of grapefruits. Applications of conidia protected the grapefruits against CRM, and fungal secretions extracted from growth media caused 100% CRM mortality.

CONCLUSIONS

Meira geulakonigii is a beneficial endophyte of grapefruits that colonizes the fruit's peel, and protects it from CRM.

SIGNIFICANCE AND IMPACT OF THE STUDY

Findings from this study demonstrate the endophytic nature of M. geulakonigii in its interaction with grapefruits. In addition, a molecular approach was developed to specifically detect the fungus inside the grapefruit peel. This approach can be used to assess the natural occurrence of M. geulakonigii in grapefruit.

A biosynthetic gene cluster for a secreted cellobiose lipid with antifungal activity from Ustilago maydis

The phytopathogenic basidiomycetous fungus Ustilago maydis secretes large amounts of the glycolipid biosurfactant ustilagic acid (UA). UA consists of 15,16-dihydroxypalmitic or 2,15,16-trihydroxypalmitic acid, which is O-glycosidically linked to cellobiose at its terminal hydroxyl group. In addition, the cellobiose moiety is acetylated and acylated with a short-chain hydroxy fatty acid. We have identified a 58 kb spanning gene cluster that contains 12 open reading frames coding for most, if not all, enzymes needed for UA biosynthesis. Using a combination of genetic and mass spectrometric analysis we were able to assign functional roles to three of the proteins encoded by the gene cluster. This allowed us to propose a biosynthesis route for UA. The Ahd1 protein belongs to the family of non-haem diiron reductases and is required for alpha-hydroxylation of palmitic acid. Two P450 monooxygenases, Cyp1 and Cyp2, catalyse terminal and subterminal hydroxylation of palmitic acid. We could demonstrate that infection of tomato leaves by the plant pathogenic fungus Botrytis cinerea is prevented by co-inoculation with wild-type U. maydis sporidia. U. maydis mutants defective in UA biosynthesis were unable to inhibit B. cinerea infection indicating that UA secretion is critical for antagonistic activity.

The use of killer sensitivity patterns for biotyping yeast strains: the state of the art, potentialities and limitations

In recent years molecular techniques have been the most useful tools for the unequivocal identification of undetermined strains at the species level. In many instances, however, a further discrimination at the strain level (biotyping) is required, such as during epidemiological investigations, in which the distribution of pathogenic microorganisms is studied, and for patent protection purposes. Although molecular methods are routinely used also for yeast biotyping, several nonmolecular techniques have been proposed. One of these, the determination of the killer sensitivity pattern (KSP) towards a panel of selected killer toxins has proven to be a good auxiliary method. Despite the plethora of studies published, the potential and limitations of the determination of KSPs have never been critically evaluated. In this review the use of this nonmolecular technique as a biotyping tool is discussed and compared with some currently used DNA-based procedures. In addition, methodological, mechanistic and ecological implications are evaluated.

GFP technology for the study of biocontrol agents in tritrophic interactions: A case study with Pseudozyma flocculosa

GFP technology was applied to the biocontrol agent (BCA) Pseudozyma flocculosa to study its development and interactions at the tritrophic level plant-powdery mildew-BCA. Transformation experiments with GFP led to the production of a strongly fluorescent strain, Act-4, that displayed biocontrol traits typical of P. flocculosa WT. Following inundative applications, growth of P. flocculosa Act-4 was closely and almost exclusively associated with the colonies of the pathogen regardless of the powdery mildew species or the host plant tested. Development of P. flocculosa Act-4 on control leaves alone was extremely limited 24 h after its application and was typical of the epiphytic growth characterizing this type of yeast-like fungus. Based on the strong correlation between the colonization pattern of the different powdery mildew species tested and the presence of P. flocculosa Act-4, as determined by its fluorescence, it seems that growth of the BCA is dependant on the presence of powdery mildews. These results demonstrate that the GFP technology can be used to study plant-pathogen-BCA interactions and fulfill a wide array of purposes ranging from fundamental observations of the biocontrol behavior of a BCA to very applied ones serving some of the requirements for the registration of BCA's such as defining their environmental fate.

The Pseudozyma flocculosa actin promoter allows the strong expression of a recombinant protein in the Pseudozyma species

Fungi belonging to the recently classified genus Pseudozyma possess some unique properties such as biocontrol activity, production of rare antimicrobial glycolipids and production of recombinant proteins. In this work, we report the first cloning of a promoter endogenous to the multi-faceted yeast-like Pseudozyma flocculosa, that of the actin gene. The promoter region lacked typical TATA or CAAT box but displayed three putative GC box and two CT-rich regions. As in other related basidiomycetes, only one copy of the actin gene was present in the genome of P. flocculosa. The activity of the actin promoter was compared to that of the HSP70 promoter from Ustilago maydis in two Pseudozyma species. In P. flocculosa, the actin promoter allowed the expression of a very high amount of GFP protein (27.8 mg g(-1) total protein) compared to those obtained with the HSP70 promoter in liquid culture. By contrast, the levels of GFP expression obtained in liquid culture were similar with the actin or the HSP70 promoter in Pseudozyma antarctica. A similar pattern of GFP expression was observed in solid culture. The cloning of this new promoter offers a unique genetic tool to further exploit and study the unusual properties of fungi from the Pseudozyma genus.

Use of molecular techniques to elucidate the mechanisms of action of fungal biocontrol agents: a review

Biological control of fungal plant pathogens appears as an attractive and realistic approach, and numerous microorganisms have been identified as biocontrol agents. There have been many efforts to understand the mechanisms of action of fungal biocontrol agents. Microbiological, microscopic, and biochemical techniques applied over many years have shed light on these mechanisms without fully demonstrating them. More recently, the development of molecular techniques has yielded innovative alternative tools for understanding and demonstrating the mechanisms underlying biocontrol properties. To date, more than 70 publications describe the use of molecular techniques for this purpose. They describe work exploiting targeted or non-targeted gene isolation, gene expression profiling, gene inactivation and/or overexpression, the study of regulatory factors. This work has shed considerable light on mechanisms underlying biocontrol properties. It has also fully demonstrated a number of targeted action mechanisms of some biocontrol agents. This review describes the techniques used in such studies, with their potential and limitations. It should provide a guide for researchers wanting to study the molecular basis of the biocontrol in diverse biocontrol agents.

Ustilagic acid secretion by Pseudozyma fusiformata strains

Eight strains of Pseudozyma fusiformata were examined for antifungal activity. All of them had the same spectrum of action and were active against many species of yeasts, yeast-like and filamentous fungi. They secreted glycolipids, which were purified from the culture liquid by column and thin-layer chromatography. According to nuclear magnetic resonance and mass-spectroscopy experiments all strains produced ustilagic acid, a cellobioside-containing 2,15,16-trihydroxypalmitic acid as aglycon, 3-hydroxycaproic acid and acetic acid as O-acylic substituents.

Genetic analysis of biosurfactant production in Ustilago maydis

The dimorphic basidiomycete Ustilago maydis produces large amounts of surface-active compounds under conditions of nitrogen starvation. These biosurfactants consist of derivatives of two classes of amphipathic glycolipids. Ustilagic acids are cellobiose lipids in which the disaccharide is O-glycosidically linked to 15,16-dihydroxyhexadecanoic acid. Ustilipids are mannosylerythritol lipids derived from acylated beta-d-mannopyranosyl-d-erythritol. Whereas the chemical structure of these biosurfactants has been determined, the genetic basis for their biosynthesis and regulation is largely unknown. Here we report the first identification of two genes, emt1 and cyp1, that are essential for the production of fungal extracellular glycolipids. emt1 is required for mannosylerythritol lipid production and codes for a protein with similarity to prokaryotic glycosyltransferases involved in the biosynthesis of macrolide antibiotics. We suggest that Emt1 catalyzes the synthesis of mannosyl-d-erythritol by transfer of GDP-mannose. Deletion of the gene cyp1 resulted in complete loss of ustilagic acid production. Cyp1 encodes a cytochrome P450 monooxygenase which is highly related to a family of plant fatty acid hydroxylases. Therefore we assume that Cyp1 is directly involved in the biosynthesis of the unusual 15,16-dihydroxyhexadecanoic acid. We could show that mannosylerythritol lipid production is responsible for hemolytic activity on blood agar, whereas ustilagic acid secretion is required for long-range pheromone recognition. The mutants described here allow for the first time a genetic analysis of glycolipid production in fungi.

Antifungal activity of flocculosin, a novel glycolipid isolated from Pseudozyma flocculosa

Flocculosin, a glycolipid isolated from the yeast-like fungus Pseudozyma flocculosa, was investigated for in vitro antifungal activity. The compound displayed antifungal properties against several pathogenic yeasts. Synergistic activity was observed between flocculosin and amphotericin B, and no significant cytotoxicity was demonstrated when tested against human cell lines.

Characterization of an antifungal glycolipid secreted by the yeast Sympodiomycopsis paphiopedili

An antifungal glycolipid was purified from the culture liquid of the ustilaginomycetous yeast Sympodiomycopsis paphiopedili by column and thin-layer chromatography. According to nuclear magnetic resonance and mass-spectroscopy experiments it was a cellobioside containing 2,15,16-trihydroxypalmitic acid as an aglycon. The minimal effective concentrations leading to ATP leakage and growth inhibition were 45 and 160 microg ml(-1) for Cryptococcus terreus and Candida albicans, respectively.

Using fungi and yeasts to manage vegetable crop diseases

Vegetable crops are grown worldwide as a source of nutrients and fiber in the human diet. Fungal plant pathogens can cause devastation in these crops under appropriate environmental conditions. Vegetable producers confronted with the challenges of managing fungal pathogens have the opportunity to use fungi and yeasts as biological control agents. Several commercially available products have shown significant disease reduction through various mechanisms to reduce pathogen development and disease. Production of hydrolytic enzymes and antibiotics, competition for plant nutrients and niche colonization, induction of plant host defense mechanisms, and interference with pathogenicity factors in the pathogen are the most important mechanisms. Biotechnological techniques are becoming increasingly valuable to elucidate the mechanisms of action of fungi and yeasts and provide genetic characterization and molecular markers to monitor the spread of these agents.