ABSTRACTTHE LIPOXYGENASE PATHWAY IN APPLE PEELByCarolina ContrerasMany aroma volatiles in fresh apple are produced autonomously, however, several important odor-active volatiles are also produced via processes engaged by cellular disruption brought about by cutting or mastication. Six-carbon (C6-) volatiles, including the aldehydes cis-3-hexenal, trans-2-hexenal, and hexanal, as well as their corresponding alcohols, are produced from action of the lipoxygenase (LOX) pathway on substrates released by tissue disruption. This study provides evidence that disruption-dependent aldehyde biosynthesis and aroma perception are dependent upon changes or availability of lipid substrates and/or related to LOX gene expression. Three different approaches of the lipoxygenase pathway were investigated in `Jonagold' apple: the products (volatiles), the substrates (lipid content) and the genes involved in the metabolic route. Sensory tests revealed that panelists were able to distinguish between non-ripening and normally ripening fruit more than two weeks before the onset of the ethylene and respiratory climacterics in control fruit, but their ability to discriminate was not related to changes in LOX-related volatiles. LOX-derived aldehydes cis-3-hexenal and hexanal were not detectable for whole fruit, consistent with its dependence on cellular disruption. For disrupted fruit, cis-3-hexenal declined markedly during ripening, coincident with autocatalytic ethylene formation, even as trans-2-hexenal and hexanal emissions increased. Changes in these aldehydes occurred two weeks after perceived differences in aroma. Analysis of free fatty acids and the polar lipids of ripening apple fruit revealed that, of the free fatty acids, 18:1 (oleic) and 18:2 (linoleic) increased several-fold during ripening, but 18:3 (linolenic) content was exceedingly low and was unchanged during ripening. For polar lipids, the 18:1 and 18:2 fatty acid content increased modestly, while the 18:3 content declined sharply as ripening progressed. The increase in 18:2 free fatty acids is mirrored by the increase in emissions of hexanal and hexyl esters during ripening, however, there does not appear to be a similar relationship between 18:3 free fatty acids and cis-3-hexenal emissions. Rather, the decline in cis-3-hexenal more closely follows the pattern in the 18:3-content of polar lipids. Twenty-two LOX gene sequences were retrieved and assayed from the apple genome, and of these, only 6 LOXs were highly expressed in a ripening-dependent manner in tissue samples from apple peel. Expression analysis by qRT-PCR revealed that 4 genes were down-regulated and 2 were up-regulated as ripening progressed. Confocal microscopy analysis confirmed that 2 of the down-regulated genes were chloroplast-targeted and the other 4 were targeted to plasma membrane. The pattern of formation of cis-3-hexenal may be related to the decline in 18:3 polar lipids and gene expression. The pattern of formation of trans-2-hexenal and all other C6 esters cannot be explained by changes in LOX gene expression but may be related to increases in polar lipids and free fatty acids. The data suggest that much of the control of C6 ester formation is at the level of fatty acid biosynthesis, rather than the degradation of a pre-existing pool.
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