Abstract
Aroma-related volatiles, together with sugars and acids, play an important role in determining fruit flavor quality. Characteristic volatiles of peach fruit are mainly derived from fatty acids such as linoleic acid (18:2) and linolenic acid (18:3). In the present study, six genes encoding fatty acid desaturases (FAD) were cloned, including two ω-6 FAD genes (PpFAD2, PpFAD6) and four ω-3 FAD genes (PpFAD3-1, PpFAD3-2, PpFAD7 and PpFAD8). Heterologous expression of peach FADs in tobacco plants showed that PpFAD3-1, and PpFAD3-2 significantly reduced contents of 18:2, and accumulated significant higher levels of 18:3. In the case of volatiles, transgenic plants produced lower concentrations of hexanal and higher levels of (E)-2-hexenal. Consequently, the ratio of the (E)-2-hexenal and hexanal was about 5- and 3-fold higher than that of wild type (WT) in PpFAD3-1 and PpFAD3-2 transformants, respectively. No significant changes in volatile profiles were observed in transgenic plants overexpressing the four other peach FAD genes. Real-time quantitative polymerase chain reaction (qPCR) analysis showed that ripe fruit had high PpFAD3-1 and low PpFAD3-2 transcript levels. In contrast, high PpFAD3-2 and low PpFAD3-1 transcript levels were observed in young fruit. These results indicate a temporal regulation of these two ω-3 FADs during development and ripening, influencing peach fruit volatile formation.
Highlights
Peach fruit were sampled at different development and ripening stages followed by analysis of volatile concentrations in skin
Together with the previous study, our results indicate that the reduction of fatty acids rather than oversupply may have a greater effect on production of the derived volatiles [19]
Six fatty acid desaturases (FAD) genes were cloned from peach, and heterologous expression in tobacco plants resulted in modifications in fatty acid composition, indicating that these genes are involved in desaturation of fatty acids
Summary
Among more than 100 volatiles that have been identified from peach fruit [1], the compounds derived from fatty acids make a major contribution to sensory quality [2]. These volatiles include C6 aldehydes, alcohols, esters, and lactones. Polyunsaturated fatty acids (PUFAs), such as linoleic acid (18:2) and linolenic acid (18:3), are the main precursors for aroma-related volatiles of peach fruit generated via the lipoxygenase (LOX) pathway or β-oxidation [3]. For the LOX pathway, 18:2 and 18:3 are firstly oxidized to the fatty acid hydroperoxides, which are subsequently cleaved by hydroperoxide lyase to form hexanal and hexenal, respectively. It has been reported that the addition of exogenous PUFAs led to an increase in hexanal and (E)-2-hexenal of fruit [5,6,7], implying that modification of fatty acid composition may affect fruit volatile profiles
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