Direct Infusion Mass Spectrometry of Oxylipin-Containing Arabidopsis Membrane Lipids Reveals Varied Patterns in Different Stress Responses

Direct infusion electrospray ionization triple quadrupole precursor scanning for three oxidized fatty acyl anions revealed 86 mass spectral peaks representing polar membrane lipids in extracts from Arabidopsis (Arabidopsis thaliana) infected with Pseudomonas syringae pv tomato DC3000 expressing AvrRpt2 (PstAvr). Quadrupole time-of-flight and Fourier transform ion cyclotron resonance mass spectrometry provided evidence for the presence of membrane lipids containing one or more oxidized acyl chains. The membrane lipids included molecular species of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, digalactosyldiacylglycerol, monogalactosyldiacylglycerol, and acylated monogalactosyldiacylglycerol. The oxidized chains were identified at the level of chemical formula and included C18H27O3 (abbreviated 18:4-O, to indicate four double bond equivalents and one oxygen beyond the carbonyl group), C18H29O3 (18:3-O), C18H31O3 (18:2-O), C18H29O4 (18:3-2O), C18H31O4 (18:2-2O), and C16H23O3 (16: 4-O). Mass spectral signals from the polar oxidized lipid (ox-lipid) species were quantified in extracts of Arabidopsis leaves subjected to wounding, infection by PstAvr, infection by a virulent strain of P. syringae, and low temperature. Ox-lipids produced low amounts of mass spectral signal, 0.1% to 3.2% as much as obtained in typical direct infusion profiling of normal-chain membrane lipids of the same classes. Analysis of the oxidized membrane lipid species and normal-chain phosphatidic acids indicated that stress-induced ox-lipid composition differs from the basal ox-lipid composition. Additionally, different stresses result in the production of varied amounts, different timing, and different compositional patterns of stress-induced membrane lipids. These data form the basis for a working hypothesis that the stress-specific signatures of ox-lipids, like those of oxylipins, are indicative of their functions.