Biosynthesis of α-farnesene and its relation to superficial scald development in 'delicious' apples

Abstract

α-Farnesene is an acyclic sesquiterpene hydrocarbon that is a constituent of the volatile components and the surface wax of apples (Malus xdomestica Borkh.). Although oxidation products of α-farnesene have been implicated in the development of superficial scald in apples) the relation between α-farnesene biosynthesis and scald development is not well understood. In vivo labeling studies using isolated tissue segments showed that α-farnesene is derived from trans, trans[1,2-14C or 1-3H]-farnesyl pyrophosphate (FPP) mostly in the skin rather than cortex tissue. Among other labeled products, farnesol was >100-fold higher compared to α-farnesene. However, HPLC analysis of hexane-extractable components from apple skin revealed farnesol is not a predominant natural constituent of apple skin tissue. In addition, trans, trans[1-3H]-farnesol was not converted to α-farnesene by apple skin tissue. Our results indicate that biosynthesis of α-farnesene in apple tissue occurs through the isoprenoid pathway, and the conversion of FPP to α-farnesene is catalyzed by a single sesquiterpene synthase enzyme, trans, trans-α-farnesene synthase, rather than via farnesol as an intermediate. A comparison of α-farnesene biosynthesis between scald-developing and scald-free regions of the same apple showed that incorporation of radiolabel into α-farnesene from trans, trans-[1-3H]-FPP was nearly 3-fold lower in scald-developing skin tissue than in scald-free skin tissue.