Differences in the Sterol Synthesizing Pathways of Sterol-Producing and Non-Sterol-Producing Fungi

GOTTLIEB, D., R. J. KNAUS, and S. G. WOOD. 1978. Differences in the Sterol synthesizing pathways of sterol-producing and non-sterol-producing fungi. Phytopathology 68: I 168-1169. The mycelia of several sterol-producing and non-sterolsqualene was detected in the non-sterol-producing fungi. The producing fungi were analyzed for the presence of squalene, block in the synthesis of ergosterol by the latter organisms is squalene epoxide, lanosterol, and ergosterol. The sterolpresumed to occur at the stage at which squalene is usually producing fungi contained all four compounds, whereas only oxidized to squalene epoxide. Additional key word: Pythiaceae. Most fungi produce sterols, primarily ergosterol; required: (i) to convert squalene to its epoxide, (ii) to however, one family, the Pythiaceae, is unique in that its cyclize the epoxide to lanosterol, and (iii) to catalyze at members do not make detectable amounts of such least some of the steps in the transformation of lanosterol compounds. The characteristics and general nature of to ergosterol. The first and key block in the system was at these fungi have been discussed in previous papers (2, 3). the epoxidase step. Whether or not the differences found These earlier studies involved one sterol-producing between these two species are representative of the species, Rhizoctonia solani, and a non-sterol-producing differences between sterol-producing and non-sterolspecies, Phytophthora cinnamomi, and revealed producing fungi in general needed elucidation and is the differences in the extent of the biosynthetic pathways of subject of this communication. the two fungi. Rhizoctonia solani had enzyme activities for all conversion steps of the pathway from acetate to MATERIALS AND METHODS ergosterol. In contrast, P. cinnamomi appeared to have only the enzymes that converted these substrates to The materials and methods used in the present study squalene and not further. The absent enzymes were those were the same as those used in our earlier studies (2). The TABLE 1. Sterol pathway components present in 3-day-old mycelia of several sterol-producing and non-sterol-producing fungi Squalene Fungus Squalene epoxide Lanosterol Ergosterol (Mg/g dry wt) (btg/g dry wt) (Ag/g dry wt) (mg/g dry wt) Sterol-producing: Rhizoctonia solani 7.8a 6.5 11.9 2.2 Aspergillusflavus 9.4 8.5 14.9 1.0 A. fumigatus 7.3 6.1 11.0 4.0 Penicillium atrovenetum 6.1 5.5 16.6 3.2 Non-sterol-producing: Phytophthora cinnamomi 6.9 _b _ _ P. cactorum (#39) 5.6 P. cactorum (IMI 21168) 4.0 Pythium graminicola 5.5 P. ultimum 4.2 (±)c _ 'Each value represents the average of multiple determinations. "The symbol = none detected; the limits of detection by gas chromatography were 2 ng of squalene, 5 ng of squalene epoxide or ergosterol, and 10 ng of lanosterol per injected sample; these values represent, respectively, 2 mg, 5 gg, and 10 pg of a I -g sample of dried mycelium. cIn the gas chromatographic scans of 27% of the extracts of this organism, a small peak with a retention time similar to that of squalene epoxide was evident; no further attempt was made to identify this peak. 00032-949X/78/000 207$03.00/0 Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.