Expression of Alpha-farnesene Synthase Gene AFS1 in Relation to Levels of Farnesene and Conjugated Trienols in Peel Tissue of Scald-susceptible `Law Rome' and Scald-resistant `Idared' Apple fruit

Abstract

Fruit of different apple cultivars vary widely in susceptibility to superficial scald. The genetic and biochemical factors involved in this variation are unknown. Conjugated trienol (CTol) oxidation products of alpha-farnesene have been linked with scald induction, and a high rate of farnesene synthesis in peel tissue of scald-prone apples early in storage is often associated with development of the disorder. Pre-storage treatment of apple fruit with 1-methylcyclopropene (1-MCP) inhibits the early burst of farnesene production and prevents scald, suggesting that ethylene induces transcription of genes involved in farnesene synthesis. We recently cloned a gene from apple peel tissue, AFS1, which encodes alpha-farnesene synthase, the last enzyme in the farnesene biosynthetic pathway. In this study, expression of AFS1 was compared in scaldsusceptible Law Rome (LR) and scald-resistant Idared (IR) apples at harvest and over 20 weeks of storage at 0.5 C. AFS1 transcript levels were closely correlated with accumulation of farnesene and CTols. In fruit of both cultivars, a sharp increase in AFS1 mRNA during the first 4 to 8 weeks of storage preceded a proportional rise in farnesene and a subsequent increase in CTols. However, maximum levels of AFS1 transcript, farnesene, and CTols were, respectively, 2.5-, 4-, and 33-fold greater in LR than in IR apples. Treatment of fruit with 1-MCP at harvest suppressed the increases in AFS1 transcript and farnesene early in storage, but AFS1 expression and farnesene synthesis recovered in LR fruit after 20 weeks. Scald incidence in LR apples after 20 weeks at 0.5 °C plus 1 week at 20 °C averaged 86%, whereas IR fruit had no scald. 1-MCP treatment reduced scald incidence in LR to <1%.