Abstract
One of the drawbacks in improving the aroma properties of tomato (Solanum lycopersicum) fruit is the complexity of this organoleptic trait, with a great variety of volatiles contributing to determine specific quality features. It is well established that the oxylipins hexanal and (Z)-hex-3-enal, synthesized through the lipoxygenase pathway, are among the most important aroma compounds and impart in a correct proportion some of the unique fresh notes in tomato. Here, we confirm that all enzymes responsible for the synthesis of these C6 compounds are present and active in tomato fruit. Moreover, due to the low odor threshold of (Z)-hex-3-enal, small changes in the concentration of this compound could modify the properties of the tomato fruit aroma. To address this possibility, we have overexpressed the omega-3 fatty acid desaturases FAD3 and FAD7 that catalyze the conversion of linoleic acid (18:2) to linolenic acid (18:3), the precursor of hexenals and its derived alcohols. Transgenic OE-FAD tomato plants exhibit altered fatty acid composition, with an increase in the 18:3/18:2 ratio in leaves and fruits. These changes provoke a clear variation in the C6 content that results in a significant alteration of the (Z)-hex-3-enal/hexanal ratio that is particularly important in ripe OE-FAD3FAD7 fruits. In addition to this effect on tomato volatile profile, OE-FAD tomato plants are more tolerant to chilling. However, the different behaviors of OE-FAD plants underscore the existence of separate fatty acid fluxes to ensure plant survival under adverse conditions.
Highlights
One of the drawbacks in improving the aroma properties of tomato (Solanum lycopersicum) fruit is the complexity of this organoleptic trait, with a great variety of volatiles contributing to determine specific quality features
In Arabidopsis (Arabidopsis thaliana), three genes, FAD3, FAD7, and FAD8, encode the enzymes that participate in the synthesis of hexadecatrienoic acid (16:3) and 18:3 from dienoic fatty acids
Tomato aroma compounds derived from the oxylipin pathway are synthesized via the subsequent
Summary
One of the drawbacks in improving the aroma properties of tomato (Solanum lycopersicum) fruit is the complexity of this organoleptic trait, with a great variety of volatiles contributing to determine specific quality features. Metabolic engineering offers an ideal solution to improve the aroma in tomato fruit by increasing the levels of (Z)-hex-3-enal that provides the highly valued fresh notes To this end, one possible strategy would be to increase the 13-LOX activity involved in the generation of short-chain aldehyde precursors. It is known that modification of the unsaturation degree of the fatty acids is a significant adaptive feature in response to temperature stress (Somerville and Browse, 1991; Iba, 2002) This increase in the trienoic fatty acids present in membrane lipids upon exposure to chilling temperatures is supposed to maintain the required membrane fluidity and to reduce membrane damage, ensuring the numerous processes that take place at cell membranes. Since most trienoic acids are present in the thylakoid membranes, where the photosynthetic machinery is found, variation of their unsaturation degree at low temperatures could play an important role in maintaining the photosynthetic capacity of the plants
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