In situ molecular identification of the plastid ω3 fatty acid desaturase FAD7 from soybean:: Eevidence of thylakoid membrane localization

omega 3 fatty acid desaturases are the enzymes responsible for the synthesis of trienoic fatty acids in plants. These enzymes have been mainly investigated using molecular, biochemical, and genetic approaches but very little is known about their subcellular distribution in plant cells. In this work, the precise subcellular localization of the omega 3 desaturase FAD7 was elucidated by immunofluorescence and immunogold labeling using a monospecific GmFAD7 polyclonal antibody in soybean (Glycine max) photoautotrophic cell suspension cultures. Confocal analysis revealed the localization of the GmFAD7 protein within the chloroplast; i.e. signals from FAD7 and chlorophyll autofluorescence showed specific colocalization. Immunogold labeling was pursued on cryofixed and freeze-substituted samples for convenient preservation of antigenicity and ultrastructure of membrane subcompartments. Our data revealed that the FAD7 protein was preferentially localized in the thylakoid membranes. Biochemical fractionation of purified chloroplasts and western analysis of the subfractions further confirmed these results. These findings suggest that not only the envelope, but also the thylakoid membranes could be sites of lipid desaturation in higher plants.