Identification of the elusive carotenoid isomerase enzyme redefines carotenoid biosynthesis and reveals a role for carotenoids in the membrane dynamics of plastid development

Abstract

Carotenoids are essential photoprotective and antioxidant pigments synthesized by all photosynthetic organisms. Most carotenoid biosynthetic enzymes were thought to have evolved independently in bacteria and plants. In bacteria, a single enzyme catalyzes four desaturations leading from colorless phytoene to red lycopene, while in plants two desaturases, unrelated to the bacterial enzyme, are required. We have demonstrated that carotenoid desaturation in plants requires a third and different enzyme, the carotenoid isomerase (CrtS). Mutations disrupting CrtS in Arabidopsis cause accumulation of poly-cis carotenoids in the etioplast, the progenitor plastid of dark-grown plants and reveal a requirement for all-trans carotenoids in formation of the prolamellar body (PLB), the lattice of membranes that defines an etioplast. The absence of PLBs in CrtS mutants demonstrates a function for this unique structure in facilitating chloroplast development during the first critical days of seedling germination and photomorphogenesis.