Evidence for metabolic and functional discrimination of sterols by Phytophthora cactorum

Abstract

When fed 10 ppm of one of the following sterols: cholesterol (cholest-5-en-3l-ol), wingsterol (21-isopentylcholesterol), desmosterol [cholesta-5,24(25)-dien-3l-ol], 24-methylenecholesterol [ergosta-5,24(28)-dien-3,-ol], or fucosterol [stigmasta-5,24(28)-dien-3.&ol], the pathogenic fungus Phytophthora cactorum, which is naturally unable to epoxidize squalene, accumulated each of the test compounds to similar levels. Fucosterol, the only sterol metabolized, was reduced to yield 24-ethylcholesterol. All the sterols tested induced the formation of sex structures. Fertilization and subsequent maturation of oospores capable of germination occurred only with the naturally occurring sterols. Wingsterol treatments resulted in aborted oospores. None of the sterols tested was inhibitory to growth, measured as changes in the 21-day mycelial dry weight. The results are consistent with the view that the accumulated sterol functions to regulate the life cycle of P. cactorum. However, the metabolism and kinds of recognition of the sterol molecule, in terms of uptake and effects on growth and induction of the various sexual events, contrast sharply with what is known for other oomycetous fungi such as Achlya and Saprolegnia. This implies that the evolutionary histories of the Oomycetes may be different.The occurrence, biosynthesis, and metabolism of sterols has been studied in numerous oomycetous fungi [orders Saprolegniales (1-3), Leptomitales (1), Lagenidiales (4), and Peronosporales (5-10)]. The physiological roles of sterols and other polycyclic isopentenoids in effecting growth and reproduction have also been the subject of much investigation (11)(12)(13)(14).Phytophthora cactorum (order Peronosporales, family Pythiaceae) is a soil-borne pathogen of higher plants-e.g., fruit trees (15)-which, unlike many of the other oomycetous fungi, lacks sterol synthetic capabilities (6)(7)(8)(9)(10). This deficiency has resulted in a heterotrophic requirement for naturally occurring host sterols to stimulate vegetative growth and an auxotrophic requirement for these sterols to produce functional oospores-i. e., oospores capable of germination. Although P. cactorum displays an enzymic preference to metabolize certain nuclear B-ring unsaturated (16), but not saturated (17), sterols to A5 sterols and a strict sterol requirement for oospore production (18,19), the selectivity for sterols in terms of initial uptake (20), accumulation into logarithmic-phase cultures (17), and derivatization to esters (13, 21) and glycosides (13,21), and their effects on growth response (17,22) (HPLC) and other uses were purchased from MCB Labs. TLC plates were from Analtech (adsorption TLC) and Whatman (reversed-phase TLC). Aluminum oxide (neutral) was from ICN.Culture and Growth Conditions. P. cactorum, strain 51-22, was provided by R. Berman (University of California, Berkeley). The fungus was maintained on supplemented agar plates (27, 28) or cultured on a nonbuffered, liquid (sterol-free) synthetic medium (17). Four types of experimental cultu...

Stigmasterol and Cholesterol Regulate the Expression of Elicitin Genes in Phytophthora sojae

“…These results are significant and show that P. sojae (1) is exquisitely sensitive to the presence of sterols, responding to them at very low concentrations, and (2) apparently can discriminate between sterols with small structural differences. Our results are different from other sterol studies with Phytophthora (Wood and Gottlieb 1978;Nes and Stafford 1983;Marshall et al 2001) because we utilized sterol supplement concentrations that are approximately 250 or 2,500 times lower than the lowest concentration of sterol supplements used in other studies to examine a physiological response. The ability of P. sojae to respond to sterols at nanomolar concentrations indicates that it is efficient in utilizing sterols, even at low concentrations in the environment.…”

“…Members of Phytophthora all lack sterol biosynthetic pathways and acquire sterols exogenously from their environment to support their growth and sexual reproduction (Hendrix 1975;Elliot and Knights 1981;Nes and Stafford 1983;Ponchet et al 1999;Marshall et al 2001). It is speculated that these organisms acquire sterols from the environment via extracellular proteins belonging to class-I of elicitins .…”

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

“…Previous studies with P. cactorum have established that the level of sterols necessary for optimum oosporogenesis is ca. 10 mg/liter (15,16,49) in variations of a defined medium developed by Elliot (13). The low levels of sterols in the inoculum used in these investigations did not approach 10 mg/liter, and it can be argued that these trace levels had little or no role in inducing the observed levels of oosporogenesis; however, assumptions that all species or all isolates of one species maintained and grown on different media will have comparable sterol requirements (37) are questionable.…”

Strategies of attack and defense in plant–oomycete interactions, accentuated for Phytophthora parasitica Dastur (syn. P. Nicotianae Breda de Haan)

“…Firstly, oomycete cell walls are primarily composed of b-1,3-and b-1,6 glucanes, and of cellulose (a b-1,4-glucane), whereas chitin, an essential component of fungal cell walls, is only marginally important (Bartinicki-Garcia and Wang, 1983). Secondly, oomycetes are unable to synthesize sterols, because they lack the squalene epoxydase and the 14a-demethylase enzymes required to convert sterol precursors (Wood and Gottlieb, 1978;Nes and Stafford, 1983;Tyler et al, 2006). As most of the traditionally used fungicides target chitin and sterol synthesis, they are inefficient against oomycetes (Table 1).…”