Analysis of the inhibitory effect of 1-Methylcyclopropene on skin greasiness in postharvest apples by revealing the changes of wax constituents and gene expression

The surface waxes of some apple (Malus domestica Borkh.) cultivars are prone to become greasy when fruit over-ripen. Although 1-Methylcyclopropene (1-MCP) treatment is effective in suppressing the skin greasiness during storage in susceptible cultivars, its effect on the skins' wax constituents is unknown. In this study, 'Cripps Pink' apples were treated with 1-MCP and then stored at 20 degrees C. Changes in skin wax content, wax morphology and gene expression were recorded. We found the accumulation of fluid wax constituents, which contributed to skin greasiness, was delayed and reduced in 1-MCP treated fruit. The fluid wax constituents consisted mainly of linoleate and oleate esters of short-chain alcohols (C-3-C-5) and (E,E)-farnesol. Different transcript patterns of wax biosynthesis genes were observed in apples during storage. Several genes probably related to the biosynthesis and export of fluid wax constituents, including MdKASs, MdCACs, MdSAD6, MdFAD2, MdWSD1, MdLTPG1 and MdWBC11, showed significant increases in expression during fruit postharvest ripening. Moreover, their transcript levels were suppressed in 1-MCP treated fruit. Meanwhile, transcript levels of genes probably related to the biosynthesis of solid compounds (waxes with very long chains) did not show significant increases and were not strongly affected in 1-MCP treated fruit. This pattern fits with the changes in the amounts of the solid compounds present. The results suggest that MdKASs, MdCACs, MdSAD6, MdFAD2, MdWSD1, MdLTPG1 and MdWBC11 may be critical for the development of skin greasiness in apple fruit.