Sterol biosynthesis by the arbuscular mycorrhizal fungus Glomus intraradices.

Ri-T-DNA-transformed carrot roots were used for investigating sterol metabolism by the arbuscular mycorrhizal (AM) fungus Glomus intraradices under three distinct experimental conditions: (i) a symbiotic stage (fungus still attached to the host roots); (ii) a detached stage (fungus physically separated from the roots); and (iii) a germinating stage (germinating spores). In all three stages, G. intraradices was found to contain a mixture of 24-alkylated sterols, with 24-methyl and 24-ethyl cholesterol as the main compounds, but no ergosterol, the predominant sterol in most fungi. Feeding experiments with [1-14C]sodium acetate were performed to check the ability of the fungus to synthesize sterols. Whatever the experimental conditions, G. intraradices was able to actively take up exogenous acetate and to incorporate it into sterols and their precursors. Our data provide first evidence for de novo sterol synthesis by an AM fungus.