Reassessment of the role of phospholipids in sexual reproduction by sterol-auxotrophic fungi

Several genera of oomycete fungi which are incapable of de novo sterol synthesis do not require these compounds for vegetative growth. The requirement for an exogenous source of sterols for sexual reproduction by several members of the Pythiaceae has been questioned by reports of apparent induction and maturation of oospores on defined media supplemented with phospholipids in the absence of sterols. A more detailed examination of this phenomenon suggested that trace levels of sterols in the inoculum of some pythiaceous fungi act synergistically with phospholipid medium supplements containing unsaturated fatty acid moieties to induce oosporogenesis. Phospholipid analysis of one species, Pythium ultimum, suggested that only the fatty acid portion of the exogenous phospholipid is taken up by the fungus. Enrichment of the phospholipid fraction of total cell lipid of P. ultimum with unsaturated fatty acids promoted oospore induction, and enhanced levels of unsaturated fatty acids in the neutral lipid fraction increased oospore viability. For some pythiaceous fungi, the levels of sterols required for the maturation of oospores with appropriate phospholipid medium supplementation suggest that these compounds are necessary only for the sparking and critical domain roles previously described in other fungi.

Fungitoxicity of m-fluorophenylalanine-containing peptides towards Pythium ultimum

The tripeptide L-m-fluorophenylalanyl-L-alanyl-L-alanine was much more fungitoxic towards Pythium ultimum than the dipeptide L-m-fluorophenylalanyl-L-alanine or m-fluorophenylalanine. The fungitoxicity of the tripeptide was reduced by L-alanyl peptides and phenylalanine, but not by other amino acids. In contrast, the fungitoxicity of m-fluorophenylalanine was unaffected by peptides, and was antagonized by several amino acids. These results suggest the effective delivery of m-fluorophenylalanine into the cell by a tripeptide carrier.

Multiple antibiotics produced by Pseudomonas fluorescens HV37a and their differential regulation by glucose

Pseudomonas fluorescens HV37a inhibited growth of the fungus Pythium ultimum on potato dextrose agar (PDA). An antibiotic activity produced under these conditions was fractionated and partially characterized. Extracts prepared from the PDA on which HV37a was grown revealed a single peak of antibiotic activity on thin-layer chromatograms. Similar extracts were prepared from mutants of HV37a. Their analysis indicated that the antibiotic observed in thin-layer chromatograms was responsible for fungal inhibition observed on PDA. The production of the PDA antibiotic required the presence of glucose, whereas two other antibiotic activities were produced only on potato agar without added glucose. Two mutants (denoted AfuIa and AfuIb) previously characterized as deficient in fungal inhibition on PDA showed altered regulation of the production of all three antibiotics in response to glucose. These mutants were also deficient in glucose dehydrogenase. Mutants isolated as deficient in glucose dehydrogenase were also deficient in fungal inhibition and were grouped into two classes on the basis of complementation analysis with an AfuI cosmid. Glucose regulation of antibiotic biosynthesis therefore involves at least two components and requires glucose dehydrogenase.

Characterization of an antibiotic produced by a strain of Pseudomonas fluorescens inhibitory to Gaeumannomyces graminis var. tritici and Pythium spp

The production, isolation, and characterization of an antibiotic substance from cultures of Pseudomonas fluorescens 2-79 (NRRL B-15132) is described. P. fluorescens 2-79 originally was isolated from the roots of wheat and is suppressive to the wheat root disease take-all caused by Gaeumannomyces graminis var. tritici. The antibiotic was isolated from potato glucose broth cultures of strain 2-79 by solvent extraction. It was purified by silica gel column chromatography and was a greenish yellow, needle-shaped crystal with a melting point of 242 degrees C (decomposition). It was soluble in methylene chloride, chloroform, acetone, 2 N sodium hydroxide, and 2 N hydrochloric acid and was insoluble in water, methanol, ethyl acetate, tetrahydrofuran, diethyl ether, carbon tetrachloride, hexane, and petroleum ether. On the basis of UV, infrared, 1H-nuclear magnetic resonance, 13C-nuclear magnetic resonance, mass spectral analysis, and elemental analysis, the structure of the antibiotic is proposed to be a dimer of phenazine carboxylic acid. Lithium aluminum hydride reduction of the antibiotic yielded hydroxymethyl phenazine as a major product which retained most of the biological characteristics of the parent molecule. There were no toxic symptoms when mice received this antibiotic by oral doses up to 464 mg/kg. The antibiotic showed excellent activity against several species of fungi, including the wheat pathogens Gaeumannomyces graminis var. tritici, Rhizoctonia solani, and Pythium aristosporum; and it may have a role in suppression of take-all in vivo by strain 2-79.

Effect of coumarin on growth and metabolism of Pythium

Coumarin concentrations that inhibited growth of Pythium sp. rapidly decreased the rate of incorporation of [14C]glucose into the mycelium. Coumarin also reduced drastically the carbohydrate and protein content of the cytoplasm and, to a lesser extent, the amino acid content and cell wall fraction. Enzymes related to the metabolism of cell wall polysaccharides were not affected during early exposure to the inhibitor. The possible mechanism of coumarin action via inhibition of glucose uptake is discussed in the light of the present findings.

Effect of inhibitors of sterol biosynthesis on growth and sexuality of Pythium and Zygorhynchus

Seven commercial inhibitors of sterol biosynthesis were tested for their effect upon vegatative growth and sexuality in Pythium acanthicum and Zygorhynchus moelleri. Five compounds (SKF 525, Chlorpropamide, Tolbutamide, Benzmalacene, AY 9944) inhibited both growth and sexuality of both fungi and two (SKF 7732 and SKF 7997) were ineffective. Pythium was more sensitive to the activity of the inhibitors than was Zygorhynchus. None of the compounds tested inhibited sexuality without similarly reducing vegetative growth.