Phytophthorols-novel lipids produced byPhytophthora cactorum

Metabolic Diversity and Novelties in the Oomycetes

The eukaryotic microbes called oomycetes include many important saprophytes and pathogens, with the latter exhibiting necrotrophy, biotrophy, or obligate biotrophy. Understanding oomycete metabolism is fundamental to understanding these lifestyles. Genome mining and biochemical studies have shown that oomycetes, which belong to the kingdom Stramenopila, secrete suites of carbohydrate- and protein-degrading enzymes adapted to their environmental niches and produce unusual lipids and energy storage compounds. Despite having limited secondary metabolism, many oomycetes make chemicals for communicating within their species or with their hosts. Horizontal and endosymbiotic gene transfer events have diversified oomycete metabolism, resulting in biochemical pathways that often depart from standard textbook descriptions by amalgamating enzymes from multiple sources. Gene fusions and duplications have further shaped the composition and expression of the enzymes. Current research is helping us learn how oomycetes interact with host and environment, understand eukaryotic diversity and evolution, and identify targets for drugs and crop protection chemicals.

Stimulation of sexual reproduction in Phytophthora cactorum by phospholipids is not due to sterol contamination

Phytophthora cactorum did not form oospores on basal medium unless phosphatidylcholine (lecithin) or phosphatidylethanolamine (cephalin) was added. After removal of putative sterols by aminopropyl column chromatography, the activities of lecithin and cephalin were increased 47- and 2.8-fold, respectively, thus confirming the previous reports that sterols are not essential for sexual reproduction in this organism. Thin-layer chromatography (TLC) of the commercial lecithin revealed the presence of an unknown inhibitory substance which, when added to the purified lecithin, caused a 50% reduction of oospore formation. Commercial cephalin also showed a twofold increase in activity after removal of putative sterols and the existence of an unknown inhibitor when it was subjected to TLC. Addition of the inhibitor to the purified cephalin completely inhibited the growth of the test organism. One sample of lecithin tested was not stimulatory to oospore formation. However, after washing with deionized water or NaCl solution, it induced the production of 17300 and 24450 oospores (100 μg)-1, respectively. The ability of cephalin to induce oospore formation was increased 2⋅3-fold by washing with deionized water and 8⋅3-fold by washing with NaCl solution. Like sterols, the digitonin precipitable component (digitonide) of the non-phospholipid fraction of commercial lecithin or cephalin was stimulatory to oospore formation of P. cactorum but not Phytophthora parasitica. However, the non-digitonide component was not only more active than the digitonide component, but also stimulatory to P. parasitica. Gas chromatography and mass spectrometry (GC-MS) analysis of the digitonide component from lecithin failed to detect any putative sterol contaminant. The amount of the putative sterol contaminant in the digitonide component from cephalin was also below the detection limit of GC-MS. When 0.01-10 ng cholesterol was added to basal medium discs each containing 100 fig cephalin, the numbers of oospores produced by P. cactorum and P. parasitica were not significantly changed. It is concluded that, in the fungi tested, sterols did not play any significant role in the stimulation of sexual reproduction by highly purified phospholipids.

Effects of exogenous phospholipids on lipid composition and sporulation by three strains of Lagenidium giganteum

The California (LGCA) and Butte Sink (LGBS) strains of the sterol auxotrophic fungus Lagenidium giganteum (Oomycetes: Lagenidiales) enter the sexual cycle on media supplemented with sterols. A third isolate of this mosquito pathogen, the North Carolina strain (LGNC), requires sterols plus phospholipids to produce oospores in vitro. Enrichment of the polar and neutral lipid fractions of the LGCA and LGBS strains with unsaturated fatty acids promoted oospore induction, and increased oospore viability. With the exception of the LGCA strain, there was no consistent relationship between phospholipid supplementation in growth media and mycelial phospholipid content.

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.