Effects of ethephon and norbornadiene on sterol and glycoalkaloid biosynthesis in potato tuber discs

Abstract

The accumulation of glycoalkaloids that normally takes place in aerobically incubated potato (Solanum tuberosum L.) tuber discs has been found to be inhibited by the ethylene‐releasing substance ethephon. Using ethephon and the ethylene action inhibitor norborna‐2,5‐diene, the effect of ethylene on the synthesis of sterols and glycoalkaloids, which partly share their biosynthetic pathway, was investigated.Control discs showed incorporation of (2‐14C)mevalonic acid into free sterols, steryl esters, steryl glycosides and acylated steryl glycosides at 24 h, thereafter the radioactivity decreased in free sterols and steryl esters concomitant with the appearance of radioactivity in glycoalkaloids. Discs with ethephon additions contained more radioactivity in all sterol classes at all time‐points, but no glycoalkaloids were formed.The enzyme S‐adenosyl‐l‐methionine:sterol C24 methyltransferase (SMT, EC 2. 1. 1. 41), located at one presumed branching point in the sterol and glycoalkaloid pathway, was characterized and found to exhibit similar characteristics as in other plants, but a lower specific activity. The activity of SMT increased in ageing tuber discs and this increase was further stimulated by ethephon, but inhibited by norborna‐2,5‐diene. The activity of the glycoalkaloid‐specific enzyme UDP‐glucose:solanidine glucosyltransferase (EC 2. 4. 1) also increased after slicing, but here ethephon additions counteracted the induction. The activity of the sterol‐specific UDP‐glucose:sterol glucosyltransferase (EC 2. 4. 1) was unaffected by either tuber slicing or ethephon additions.The results indicate that ethylene stimulates sterol synthesis in wounded potato discs, and that the wound‐induction of SMT is regulated by ethylene.