ECDYSTEROID BIOSYNTHESIS DURING THE ONTOGENY OF SPINACH LEAVES

During the growth and development of spinach (Spinacia oleracea), phytoecdysteroid biosynthesis is observed later in the ontogeny of cotyledons, first and second leaves, whereas early in leaf ontogeny no biosynthesis of phytoecdysteroid is observed. An excised leaf assay for incorporation of various substrates [2-C-14]mevalonic acid (MVA), [4-C-14]cholesterol, [2-C-14]acetate and [22,23-H-3]alpha-ecdysone into phytoecdysteroids was developed. All four radiolabelled substrates can be incorporated into the ecdysteroids of spinach (20-hydroxyecdysone and polypodine B). The [2-C-14]MVA is apparently incorporated into the C27-sterol lathosterol (5alpha-cholest-7-en-3beta-ol) prior to radiolabel incorporation into spinach ecdysteroids. Leaves early in their ontogeny fail to produce radiolabelled lathosterol or ecdysteroids from [2-C-14]MVA in spite of an active C29-sterol biosynthesis from [2-C-14]MVA. Conversion of [22,23-H-3]alpha-ecdysone into radiolabelled spinach ecdysteroids parallels the incorporation of [2-C-14]MVA into these ecdysteroids with leaf ontogeny. Lathosterol was structurally identified as a component of mature spinach leaves accounting for approximately 4% of the total sterol composition. The spinach excised leaf assay provides a model system for the study of ecdysteroid biosynthesis in plants.