15-cis-phytoene desaturases (PDS, plant-type phytoene desaturases) (EC 1.3.5.5, 15-cis-phytoene:plastoquinone oxidoreductase), are enzymes involved in the carotenoid biosynthesis in plants and cyanobacteria. Phytoene desaturases are membrane-bound enzymes localized in plastids and introduce two double bonds into their colorless substrate phytoene by dehydrogenation and isomerize two additional double bonds. This reaction starts a biochemical pathway involving three further enzymes (zeta-carotene isomerase, zeta-carotene desaturase and carotene cis-trans isomerase) called the poly-cis pathway and leads to the red colored lycopene. The homologous phytoene desaturase found in bacteria and fungi (CrtI) converts phytoene directly to lycopene by an all-trans pathway.