Impact of Suppression of Ethylene Action or Biosynthesis on Flavor Metabolites in Apple (Malus domestica Borkh) Fruits

To understand the role of ethylene in regulating the overall flavor of apple fruits, ethylene production or action was reduced using transgenic apple trees suppressed for ACC-synthase or ACC-oxidase enzyme activity or by the addition of 1-methylcyclopropene (1-MCP), an ethylene action inhibitor. Flavor components were differentially regulated in response to the suppression of both ethylene biosynthesis and action. Headspace analysis of aroma production, an ethylene-associated event, showed a reduction in ester and alcohol production in the ethylene-suppressed lines and in the apples treated with 1 μL L-1 1-MCP for 20 h at 20 °C. However, no major differences were observed in the concentrations of aldehyde volatiles. Other flavor metabolites that showed an ethylene-dependent pattern were organic acids and sugars. Malic acid degradation was significantly reduced under ethylene-suppressed conditions, showing a recovery after the fruit was exposed to ethylene. Sucrose and fructose concentrations were influenced by suppression or enhancement of ethylene. Total phenolics as well as individual phenolic compounds showed an ethylene-dependent regulation only in response to the suppression of ethylene biosynthesis, but not when ethylene action was inhibited. Keywords: Aroma; fruit flavor complex; phenolic compounds; sugars; Malus domestica; organic acids; transgenic apple fruit