2,3-Oxidosqualene cyclase and cycloartenol-s-adenosylmethionine methyltransferase activities in vivo in the cotyledon and axis tissues of germinating pea seeds.

Axis tissues, root and shoot, of germinating pea seedlings actively synthesize sterol from [2-14C]mevalonate during the first 3 days of germination. In addition to the intermediates of sterol synthesis, cycloartenol and 24-methylenecycloartanol, these tissues also form the triterpene beta-amyrin. The cyclase catalysing the formation of cycloartenol from oxidosqualene is about four times as active as that for beta-amyrin synthesis. 2. Sterol synthesis in the cotyledon is negligible, but cycloartenol and 24-methylenecycloartanol, as well as beta-amyrin, are synthesized there. Oxidosqualene cyclase activity in this tissue is 2.6 times as active for beta-amyrin synthesis as for cycloartenol synthesis. 3. Comparison of the relative amounts of 14C in cycloartenol and 24-methylenecycloartanol in the axis tissues and cotyledons of 3-day-old seedlings point to relatively active cycloartenol-S-adenosylmethionine methyltransferase systems in both axis tissues and a poorly active system in the cotyledon. 4. The role of beta-amyrin synthesis in the germinating pea seedling is discussed.