Identification of the Carotenoid Isomerase Provides Insight into Carotenoid Biosynthesis, Prolamellar Body Formation, and Photomorphogenesis

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. 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 -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 -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 darkgrown 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.

Carotenoids in nature: insights from plants and beyond

“…Despite the block from cis-to an all-trans-configuration in crtiso mutants, carotenogenesis can proceed in chloroplasts via photoisomerisation, but there is a delay in the greening of etiolated seedlings and a substantial reduction in lutein in Arabidopsis as well as some degree of chlorosis in tomato and rice (Isaacson et al 2002;Park et al 2002;Fang et al 2008;Wei et al 2010;Chai et al 2011). Perhaps, CRTISO plays important roles in non-green tissues (e.g.…”

“…phytoene, phytofluene, z-carotene and neurosporene in addition to prolycopene) instead of the typical xanthophyll profile in nonphotosynthetic tissues such as flowers (e.g. petals), the inner pericarp of green fruit and etiolated seedlings (Isaacson et al 2002;Park et al 2002). The function of these cis-carotenoids remains largely unknown and it is easy to speculate that they could play a role as novel signalling molecules.…”

“…The production of all-trans-lycopene from 15-cisphytoene involves four enzymes, phytoene desaturase (PDS), z-carotene isomerase (z-ISO), z-carotene desaturase (ZDS) and carotenoid isomerase (CRTISO) as well as a lightmediated photoisomerisation (Fig. 2) (Bartley et al 1999;Park et al 2002;Isaacson et al 2004;Breitenbach and Sandmann 2005;Dong et al 2007;Chen et al 2010). PDS mRNA transcript levels are slightly upregulated during photomorphogenesis via the phytochrome-mediated pathway, which could imply that PDS plays a rate-limiting role in the generation of 9,15,9 0 -tricis-z-carotene (z-carotene) Qin et al 2007).…”

Timing and Biosynthetic Potential for Carotenoid Accumulation in Genetically Diverse Germplasm of Maize

“…Given that CrtISO expression controls pathway flux in endosperm, one should question whether engineering with a bacterial gene interferes with endogenous regulatory mechanisms of isomerization, as hinted by recent evidence of epigenetic control of CrtISO in Arabidopsis (Cazzonelli et al, 2009). Evidence that an isomerase controls flux partitioning in photosynthetic tissue is suggested by reduced lutein observed in CrtISO mutants and in plants overexpressing bacterial CrtI (Misawa et al, 1994;Park et al, 2002).…”

Golden Rice – five years on the road – five years to go?

“…The bacterial CRTI transgene in particular was suspected to be the cause of this phenomenon because this desaturase differs from the plant-type desaturases, not only structurally but also in the nature of the lycopene produced: all-trans in the bacterial system but specifically poly-cis when produced by plant-type carotene desaturases (Figure 2) [15]. In plants, this requires an additional enzyme, the recently identified carotene cis-trans-isomerase (CRTISO) [16,17], to enable cyclization and the production of all-trans b-carotene. The artificial accumulation of all-trans lycopene in daffodil flowers by the application of the cyclase inhibitor CPTA [2-(p-chlorophenylthio)triethylammonium chloride] led to altered expression levels of carotenogenic mRNAs and seemingly underscored the relevance of such a feedback mechanism [18].…”