Fungicides as tools in studying postsqualene sterol synthesis in plants

A variety of fungicides have been found to interfere with plant sterol biosynthesis. They were shown to block the biosynthetic reactions in the conversion of cycloartenol to phytosterols. The first class of inhibitors comprises morpholine fungicides which inhibit cycloeucalenol isomerization, Δ8-Δ7-isomerization, Δ14-reduction, and Δ7 -reduction of sterol precursors. These observations are consistent with a carbonium ion mechanism for these reactions; morpholines probably act as reaction intermediate analogs. Azole fungicides are the second class of inhibitors which were shown to block the 14α-demethylation step in plants, and data gave evidence that P450-obtusifoliol-14α-demethylase is a herbicidal target. In this review we show that these two classes of chemicals are useful tools for (i) investigating the phytosterol biosynthesis and the function of sterols in plants, (ii) probing the catalytic mechanism of the target enzymes, and (iii) for the search of plant sterol mutants.