Identification of the carotenoid isomerase provides insight into carotenoid biosynthesis, prolamellar body formation, and photomorphogenesis.

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. For example, in bacteria, a single enzyme (CrtI) catalyzes the four desaturations leading from the colorless compound phytoene to the red compound lycopene, whereas plants require two desaturases (phytoene and zeta-carotene desaturases) that are unrelated to the bacterial enzyme. We have demonstrated that carotenoid desaturation in plants requires a third distinct enzyme activity, the carotenoid isomerase (CRTISO), which, unlike phytoene and zeta-carotene desaturases, apparently arose from a progenitor bacterial desaturase. The Arabidopsis CRTISO locus was identified by the partial inhibition of lutein synthesis in light-grown tissue and the accumulation of poly-cis-carotene precursors in dark-grown tissue of crtISO mutants. After positional cloning, enzymatic analysis of CRTISO expressed in Escherichia coli confirmed that the enzyme catalyzes the isomerization of poly-cis-carotenoids to all-trans-carotenoids. Etioplasts of dark-grown crtISO mutants accumulate acyclic poly-cis-carotenoids in place of cyclic all-trans-xanthophylls and also lack prolamellar bodies (PLBs), the lattice of tubular membranes that defines an etioplast. This demonstrates a requirement for carotenoid biosynthesis to form the PLB. The absence of PLBs in crtISO mutants demonstrates a function for this unique structure and carotenoids in facilitating chloroplast development during the first critical days of seedling germination and photomorphogenesis.

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