Oomycetes, sterols

With certain Oomycetes, "sterol-like molecules, eg. tetracyclic and pentacyclic triterpenoids (Fig. 2), may produce the same physiological end response as sterols (2). The growth response to these polycyclic isopentenoids is intimately associated with their 3-dimen-sional geometry, molecular features and subcellular quantity. In order to discuss sterol function for What we assume is an evolutionary determinant of developmental change, a brief overview of sterol occurrence and biosynthesis is given first. 

Wood SG, Gottheb D (1978) Evidence from mycelial studies for differences in the sterol biosynthetic pathway of Rhizoctonia solani and Phytophthora cinnamomi. Biochem J 170 343 Nes WD et al (1986) A comparison of cycloartenol and lanosterol biosynthesis and metabolism by the Oomycetes. Expeiientia 42 556... 

Mikes V (1997) The fungal ehcitor cryptogein is a sterol carrier protein. FEES Lett 416 190 Ponchet M et al (1999) Are ehcitins cryptograms in plant-Oomycete communications Cell Mol Life Sci 56 1020... 

This primary mode of action of tomatine, that involves the formation of complexes with membrane sterols is similar to that described for polyene antibiotics [2, 4], and results in pore formation and loss of membrane integrity. This mode of action is supported by the reduced activity of tomatine on sterol-free bacteria and Oomycete fungi such as Pythium and Phytophthora [15, 28], and the strongly reduced toxicity of hydrolysis products of the glycoalkaloid which fail to bind sterols [57]. 

Among the fungi, the class Oomycetes is by number the smallest of the five classes, comprising some 70 genera with 500 species. However, in many respects, they are a unique and important class 1) their cell walls differ from those of all other fungi, inasmuch as they contain cellulose instead of chitin 2) they do not synthesize sterols 3) their life cycle is diploid (in contrast to all other fungi) and 4) they form motile spores (zoospores). 

Nes, D.W. Biosynthesis and requirement for sterols in die growth and reproduction of Oomycetes. In Ecology and Metabolism of Plant Lipids. Fuller, G, Nes, W.D. ed., American Chemical Society, Washington, DC. 1987 pp. 304-328. 

Berg LR, Patterson GW Phylogenetic implications of sterol biosynthesis in Oomycetes. ExpMycol 1986 10 175-183. 

McCorKindale, et al., were the first to structurally and quantitatively examine the sterol composition of a variety of Oomycetes (8). From their study it was evident that the Oomycetes produced a sterol profile dissimilar from the more-advanced fungi. 

This trace sterol may also permit other conq>ounds, eg. cycloartenol (6.31). to act in a synergistic or sparking (32) capacity allowing for growth stimulation and oogonia induction (cf. Sterol Function Section). Concerning the cholesterol identified in other Oomycetes... 

The steroid biosynthetic pathway of Oomycetes may be operationally divided into 4 stages (1) acetate conversion to squalene-oxide (2) polycyclization of squalene-oxlde to a tetracyclic product (3) metabolism of the tetracycle to a series of amphipathic neutral sterols having a free 3S-0H, planar nucleus and intact side chain of 8 to 10 C-atoms and (4) side chain hydroxylation and reduction followed by additional nuclear metabolism of specifically the 24-alkylidenesterol-fucosterol to polar steroids (Fig.3). 

PhYtophthora and related "sterol-less Oomycetes possess an enzymic defect in Stage I. They synthesize squalene (9.19) but fail to produce squalene- oxide (9.10.18). VIhile some fungi cyclize squalene-oxide to pentacyclic triterpenoids it remains unknown Whether the pythiaceous fungi similarily perform this kind of cyclization. 

S. ferax and related sterol-producing Oomycetes synthesize lanosterol rather than cycloartenol (10) in Stage II. The mechanism of squalene-oxide cyclization to lanosterol involves a nonconcerted process in which a terminal intermediate,... 

There are suggestions in the literature, however, that Oomycetes (13.36) may synthesize cycloartenol, and metabolize it to a 4-desmethylsterol, such as fucosterol if the fungus cannot synthesize or metabolize the cyclopropyl sterol, then it may use the conq>ound unaltered as a unique structural membrane insert ( ) due to the butterfly or bent shape of the cyclopropyl sterol ( 2). Our original interest in cycloartenol utilization by Oomycetes was phylogenetic rather than functional. The utilization of a cycloartenol-versus a lanosterol-basad pathway is known to... 

S. ferax alkylates the A -bond is by a 1, 2-hydride shift analogous to yeast (47.) and 6. fu.iikuroi (Le and lies unpublished observations). since the second alkylation can be con letely blocked by a carbonium ion high energy intennediate enzyme blocker (48), i.e., 25-azacholesterol, in addition to the loss of tritium at C-25 following iodine isomerization, it is assumed the first and second alkylations proceed through a similar mechanism. Presumably the other Oomycetes which synthesize these same sterols biosynthesize them in an analogous manner. 

Gaulin, E., Bottin, A., and Dumas, B. (2010). Sterol bios5mthesis in oomycete pathogens. Plant Signal. Behav. 5, 258-260. 

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