Chemical transformations of isoprenoids in plants and some bacteria and fungi lead to the production of various conjugated products, including carotenoids. Carotenoids can be cleaved to generate apocarotenoid precursors for signaling molecules such as abscisic and retinoic acids, and for the photosensory pigment retinal. The enzymes that catalyze the various transformations of carotenoids and apocarotenoids are closely related. This evolutionarily distant conservation is unexpected and intriguing. Many aspects of the metabolism of retinoids in vertebrates remain controversial and poorly understood. Because few chemical reactions are possible for this group of compounds, furthering our knowledge of isoprenoid transformation in plants could be beneficial to our understanding of how retinoids and carotenoids are transformed in vertebrates.