Expression of α-Farnesene Synthase AFS1 and 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase HMG2 and HMG3 in Relation to α-Farnesene and Conjugated Trienols in `Granny Smith' Apples Heat or 1-MCP Treated to Prevent Superficial Scald

Abstract

Untreated control, 1-methylcyclopropene (1-MCP)-treated, and heated fruit of the superficial scald-susceptible `Granny Smith' cultivar of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] were compared with respect to scald incidence, internal ethylene concentration (IEC), α-farnesene metabolism, expression of the genes AFS1, which encodes α-farnesene synthase, the final, rate-limiting enzyme in the α-farnesene biosynthetic pathway, and HMG2 and HMG3, which encode isozymes of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the proposed rate-limiting enzyme in the mevalonate pathway of isoprenoid synthesis. The incidence of scald in untreated `Granny Smith' apples after 16 weeks at 0 °C plus 1 week at 20 °C was 100%; 1-MCP treatment prevented scald development, whereas heat treatment delayed and reduced scald development. 1-MCP also inhibited both α-farnesene and IEC, suggesting that ethylene induces transcription of key genes involved in α-farnesene biosynthesis. Heat treatment reduced levels of α-farnesene and and its oxidation products, conjugated trienols (CTols), but not to the extent of 1-MCP. Internal ethylene concentrations in heated apples did not differ from those in the controls. In both control and heated fruit, a sharp increase in AFS1 mRNA during the first 4 weeks of storage preceded an increase in α-farnesene and a subsequent increase in CTols. AFS1 transcript was absent from 1-MCP-treated apples for the first 10 weeks of storage, and even at 16 weeks was lower than in heated and untreated control fruit. Levels of the HMG2 and HMG3 transcripts varied during storage and among treatments, and were not correlated with the incidence of scald. HMG2 mRNA transcript accumulation was low at harvest and increased in abundance during storage in all treatments, with the greatest increase occurring in 1-MCP-treated fruit. In contrast, HMG3 transcript was constitutively present at all storage times, although it too was slightly more abundant in 1-MCP-treated fruit.