The present invention relates to the field of genes isolated from Arabidopsis that code for enzymes that are involved in -oxidation of very long chain fatty acids, particularly, in conversion of .omega.-hydroxy fatty acid to fatty aldehyde. The invention also includes methods of producing transgenic plants with an altered fatty acid composition. Furthermore, the present invention relates to assays making use of these enzymes as herbicide targets to identify inhibitors of said enzymes, thereby providing herbicidally active compounds.