Developmental regulation of sterol biosynthesis inCucurbita maxima L.

Twenty-two sterols were identified by capillary gas chromatography and capillary gas chromatography/mass spectroscopy inCucurbita maxima grown under green-house conditions. Both whole plants and individual tissues (leaves, stems, roots, cotyledons, flowers) were analyzed at weekly intervals during the 12-week development of the plant. In whole plants, sterol accumulation parallels plant growth except for a period in the mid-life cycle where there is a reduction in the amount of sterol accumulated on a total sterol/plant and mg sterol/g dry wt basis. This reduction in the amount of sterol is coincident with the visual onset of flowering. During development, the percent contribution of each class of sterol (Δ^5_, Δ^7_, Δ^0_-sterols) remains relatively constant. However, the percent contribution of an individual sterol species varies depending on the tissue examined and the developmental period selected for analysis. While the young plant (<2 weeks) possesses elevated levels of sterols with the Δ²⁵⁽²⁷⁾-double bond, the trend was toward a reduction in the amounts of these sterols with development. Leaves and stems accumulate large quantities of 24ζ-ethyl-5α-cholesta-7,22-dien-3β-ol (7,22-stigmastadienol) and 24ζ-ethyl-5α-cholest-7-en-3β-ol (7-stigmastenol), while roots accumulate only 7,22-stigmastadienol as their principal sterol. Male flowers and roots were found to contain elevated levels of Δ^5_-sterols.