Fatty acid desaturases (FADs) are enzymes that act in the chloroplast or the endoplasmic reticulum (ER) to incorporate double bonds into the acyl chains of fatty acids, and recent evidence indicates that at least one of these enzymes, FAD7, also influences plant resistance to aphids. FAD7 is an enzyme in the chloroplast that is found throughout the plant kingdom and that desaturates 16- and 18-carbon fatty acids (FAs) with two double bonds (dienoic acids) to generate FAs with three double bonds (trienoic acids). In tomato (Solanum lycopersicum) and the model plant Arabidopsis thaliana, mutants with impaired FAD7 function are more resistant to aphids than wild-type controls. Compared to wild-type plants, these mutants have increased 18-carbon FAs with one double bond (C18:1), higher 16- and 18-carbon FAs with two double bonds (C16:2 and C18:2), and lower 16- and 18-carbon FAs with three double bonds (C16:3 and C18:3). These changes in FA composition are most pronounced in galactolipids which are abundant in the chloroplast, but loss of function of FAD7 also has a modest effect on phospholipids from the endoplasmic reticulum (ER). In addition, loss of function of FAD7 also influences foliar profiles of C6 volatiles, a class of FA derivatives that have previously been reported to contribute to aphid resistance in potato and other plant species. The goal of this project was to investigate which of these changes in fatty acid metabolism contribute to aphid resistance in Arabidopsis and tomato mutants with impaired FAD7 activity. To this end, we compared aphid performance on a panel of mutant plant lines with variation in FA composition or C6 volatile production. Our results suggested that aphid resistance in plants with impaired FAD7 activity is independent of C6 volatile synthesis, because 1) the aphid-resistant fad7 mutant in Arabidopsis also carries a mutation in the hydroperoxide lyase (HPL) gene, which is required for C6 volatile synthesis; and 2) suppressing expression of HPL in a FAD7-impaired tomato line did not compromise aphid resistance in this line. Analysis of aphid resistance and FA profiles in a panel of Arabidopsis mutants with impairments in different FADs also indicated that aphid resistance was impacted by FAD activity in the ER as well as in the chloroplast. This suggested that C18 rather than C16 FAs plays a determining role in aphid resistance, since C16 FAs are exclusively synthesized in the chloroplast whereas C18 FAs are produced in both subcellular compartments. Furthermore, resistance appeared to be associated with high C18:2 levels, because mutant lines with high C18:2 displayed resistance to aphids, but other lines with low C18:2 were susceptible even if they had high C18:1 and low C18:3 levels. Potentially, C18:2 or its derivatives could contribute to defensive signaling or synthesis of defensive metabolites to combat aphid infestations. This study aids in identifying new sources of plant resistance to aphids, and advances our understanding of how FA metabolism modulates plant defenses against insects.
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