Maize Y9 Encodes a Product Essential for 15-cis-ζ-Carotene Isomerization

Abstract

Carotenoids are a diverse group of pigments found in plants, fungi, and bacteria. They serve essential functions in plants and provide health benefits for humans and animals. In plants, it was thought that conversion of the C40 carotenoid backbone, 15-cisphytoene, to all-trans-lycopene, the geometrical isomer required by downstream enzymes, required two desaturases (phytoene desaturase and z-carotene desaturase [ZDS]) plus a carotene isomerase (CRTISO), in addition to light-mediated photoisomerization of the 15-cis-double bond; bacteria employ only a single enzyme, CRTI. Characterization of the maize (Zea mays) pale yellow9 (y9) locus has brought to light a new isomerase required in plant carotenoid biosynthesis. We report that maize Y9 encodes a factor required for isomerase activity upstream of CRTISO, which we term Z-ISO, an activity that catalyzes the cis-to trans-conversion of the 15-cis-bond in 9,15,9#-tri-cis-z-carotene, the product of phytoene desaturase, to form 9,9#-di-cis-z-carotene, the substrate of ZDS. We show that recessive y9 alleles condition accumulation of 9,15,9#-tri-cis-z-carotene in dark tissues, such as roots and etiolated leaves, in contrast to accumulation of 9,9#-di-cis-z-carotene in a ZDS mutant, viviparous9. We also identify a locus in Euglena gracilis, which is similarly required for Z-ISO activity. These data, taken together with the geometrical isomer substrate requirement of ZDS in evolutionarily distant plants, suggest that Z-ISO activity is not unique to maize, but will be found in all higher plants. Further analysis of this new gene-controlled step is critical to understanding regulation of this essential biosynthetic pathway.

PSY3, a New Member of the Phytoene Synthase Gene Family Conserved in the Poaceae and Regulator of Abiotic Stress-Induced Root Carotenogenesis

“…However, it is possible to block the pathway using mutants that condition accumulation of pathway intermediates (Wurtzel, 2004;Matthews and Wurtzel, 2007). We therefore chose the maize y9 mutation, which blocks carotenoid biosynthesis in nonphotosynthetic tissues through interference with isomerization of z-carotene by z-carotene isomerase (Z-ISO; Li et al, 2007); homozygous mutants accumulate the Z-ISO substrate 9,15,9# z-carotene and earlier pathway intermediates (Fig. 1), which can be easily measured by HPLC.…”

“…All plant materials were stored at 280°C until analysis. For the salt treatment of y9 mutants, y9 seedlings were treated with 250 mM NaCl and roots were collected for HPLC analysis to measure carotenoid content, which was performed and quantified as previously described (Li et al, 2007). Values represent the mean of three RT-PCR replicates 6SD from five pooled plants.…”

“…ABA accumulation is a balance of biosynthesis and catabolism, the latter process primarily regulated by ABA 8# hydroxylase (ABA8ox; Kushiro et al, 2004;Saito et al, 2004). ABA biosynthesis, as derived from de novo biosynthesis in a given tissue, may be potentially limited by the plastid enzymes involved in producing the carotenoid (epoxycarotenoid) precursors (Li et al, 2007;Matthews and Wurtzel, 2007;Quinlan et al, 2007) and/ or the enzymes involved in conversion of specific epoxycarotenoids to ABA (Taylor et al, 2005;Fig. 1); factors affecting flux to ABA may vary according to tissue (photosynthetic versus nonphotosynthetic) and/ or to particular species (and possibly plant families), as we demonstrate in this article for maize.…”

“…The carotenoid biosynthetic pathway in maize and other grasses is complex compared to Arabidopsis and other dicots; many of the enzymes in the grasses are encoded by small gene families for which much ongoing investigation is focused on elucidating specific roles in carotenogenesis (Wurtzel, 2004;Li et al, 2007;Matthews and Wurtzel, 2007).…”

The Maize Phytoene Synthase Gene Family: Overlapping Roles for Carotenogenesis in Endosperm, Photomorphogenesis, and Thermal Stress Tolerance

“…The first committed and rate-controlling step in the plastid-localized carotenoid biosynthetic pathway (as recently revised; see Li et al, 2007) is mediated by the nuclear-encoded phytoene synthase (PSY); PSY catalyzes formation of 15-cis phytoene from two molecules of geranylgeranyl pyrophosphate (for review, see Matthews and Wurtzel, 2007). It is well known for many species that the PSY step affects flux to carot-enoids and much, though not all of the regulation is at the transcriptional level.…”