Inhibition of Ethylene Action and Biosynthesis Differentially Affect α-Farnesene Metabolism and Superficial Scald Development in Greensleeves Apples

Ethylene plays a key role in the development of superficial scald in cold-stored apple fruit. The effects of transgenic suppression of ethylene biosynthesis and inhibition of ethylene action by 1-methylcyclopropene (1-MCP) on scald incidence and severity were evaluated in 'Greensleeves' apples stored in air at 0 °C. Three apple lines were used: untransformed (GS); line 68G, suppressed for 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase; and line 130Y, suppressed for ACC synthase. Untreated GS fruit exhibited similar climacteric patterns of ethylene production and respiration during 16 weeks of storage. Respiration rates in transgenic 68G and 130Y fruit equaled those in GS fruit, whereas peak ethylene evolution was reduced by about 90%. 1-MCP treatment prior to storage suppressed ethylene production in all fruit through 16 weeks, after which production in GS fruit recovered. Peel tissue transcript levels of α-farnesene synthase gene MdAFS1 were on average > 50% lower in 68G and 130Y compared with GS fruit, but effects on accumulation of α-farnesene and its conjugated trienol (CTol) oxidation products were relatively slight. 1-MCP suppressed MdAFS1 expression and α-farnesene production through 16 weeks; by 23 weeks MdAFS1 transcript had increased markedly and was proportional to α-farnesene and CTols levels in all fruit. Scald symptoms, evaluated after 7 days shelf life at 20 °C, were evident on > 10% of all fruit stored for 12 weeks. Among the untreated fruit, scald severity was much greater in GS than in 68G and 130Y, but scald incidence was high in all three lines at 16 weeks and close to 100% after 16–23 weeks. In all fruit treated with 1-MCP, scald was substantially reduced but not eliminated. It is concluded that up-regulation of MdAFS1 is induced by very low levels of ethylene, low to moderate expression of MdAFS1 is adequate for maximal α-farnesene production, and at least light scald symptoms can arise despite much reduced production of ethylene and accumulation of α-farnesene and CTols.