Formation of triterpenoids throughout Olea europaea fruit ontogeny

Drupes were handpicked from olive (Olea europaea L.) trees, cv chemlali, at 13 distinct stages of fruit development, referred to as weeks after flowering (WAF), and analyzed for their free and esterified sterols and triterpenoids content. These two classes of compounds are synthesized via the acetate/mevalonate pathway and share common precursors up to oxidosqualene (OS). Cyclization of OS in either cycloartenol or beta-amyrin constitutes a branch point between primary (sterol pathway) and secondary (triterpenoid pathway) metabolisms. At the onset of fruit development, i.e., between 12 and 18 WAF, drupes were found to contain high amounts of alpha- and beta-amyrins as well as more-oxygenated compounds such as triterpenic diols (erythrodiol and uvaol) and acids (oleanolic, ursolic and maslinic acids). Concomitantly, sterol precursors were barely detectable. From 21 WAF, when the olive fruit reached its final size and began to turn from green to purple, alpha- and beta-amyrins were no longer present, while 4,4-dimethyl- and 4alpha-methylsterols started to be formed, indicating a redirection of the carbon flux from the triterpenoid pathway towards the sterol pathway. Between 21 and 30 WAF, sterol end products, mainly represented by sitosterol, progressively accumulated and triterpenic diols were replaced by triterpenic acids, essentially maslinic acid. Interestingly, the developing olive fruit was found to accumulate significant amounts of parkeol as an ester conjugate. Whatever the stage of development, triterpenoids represent the major triterpenic compounds of the olive fruit.