Inhibition of the sterol pathway in leek seedlings impairs phosphatidylserine and glucosylceramide synthesis but triggers an accumulation of triacylglycerols.

Like most higher plants, leek seedlings (Allium porrum L.) contain a mixture of Δ5-sterols in which sitosterol largely predominates. As previously reported (Plant Physiol., 117 (1998) 931), these compounds, which are synthesized at the endoplasmic reticulum level, were shown to be actively transported to the plasma membrane via a membrane-mediated process, together with phosphatidylserine (PS). In the present work, leek seedlings were allowed to germinate for 7 days in the presence of fenpropimorph, a sterol biosynthesis inhibitor. Such a treatment was found to trigger an almost complete replacement of the usual sterols by 9β,19-cyclopropylsterols (mainly cycloeucalenol and 29-norcycloartenol). Extensive lipid analyses and labeling experiments with sodium [14C]acetate were performed to examine potential changes in the content and the rate of synthesis of the other lipid molecular species. The results indicate that the inhibition of the sterol pathway was accompanied by a severe decrease in PS and glucosylceramide synthesis as well as by a redirection of fatty acids toward the storage triacylglycerol pathway. Triacyglycerols are shown to accumulate concomitantly with a significant increase in intracellular lipid droplets in both aerial parts and roots of leek seedlings. Taken together, the present data emphasize that a coordinated regulation of the biosynthetic pathways of sterols and some specific lipid molecular species could take place during plant membrane biogenesis.