Abstract
Colored apocarotenoids accumulate at high concentrations in few plant species, where display a role in attraction of pollinators and seed dispersers. Among these apocarotenoids, crocins accumulate at high concentrations in the stigma of saffron and are responsible for the organoleptic and medicinal properties of this spice. Phytoene synthase and Orange protein are key for carotenoid biosynthesis and accumulation. We previously isolated four phytoene synthase genes from saffron with differential roles in carotenoid and apocarotenoid biosynthesis. However, the implications of Orange genes in the regulation of apocarotenoid accumulation are unknown. Here, we have identified two Orange genes from saffron, with different expression patterns. CsOr-a was mainly expressed in vegetative tissues and was induced by light and repressed by heat stress. Both CsOr-a and CsOr-b were expressed in stigmas but showed a different profile during the development of this tissue. The interactions of CsOr-a and CsOr-b were tested with all the four phytoene synthase proteins from saffron and with CsCCD2. None interactions were detected with CCD2 neither with the phytoene synthase 2, involved in apocarotenoid biosynthesis in saffron. The obtained results provide evidence of different mechanisms regulating the phytoene synthase enzymes in saffron by Orange for carotenoid and apocarotenoid accumulation in saffron.
Highlights
Colored apocarotenoids accumulate at high concentrations in few plant species, where display a role in attraction of pollinators and seed dispersers
In the case of Arabidopsis and other Brassicaceae only one phytoene synthase enzyme (PSY) gene (Arabidopsis genome Initiative, 2000) is found in the genome, while two or more PSY genes have been reported in other plants, including the grasses and other crops of economic importance[16,17]
Crocus sativus accumulates exceptional high levels of polar apocarotenoids known as crocins, which confer the color to the saffron spice[5]
Summary
Colored apocarotenoids accumulate at high concentrations in few plant species, where display a role in attraction of pollinators and seed dispersers. The Or gene is present as a small gene family with at least two members, named Or-a and Or-b in different plant species[7] This gene was discovered originally in cauliflower (Brassica oleracea; BoOr) where triggers the biogenesis of chromoplasts in non-green tissues[7] without changing carotenoid biosynthetic gene expression[8]. The presence of different PSY enzymes in some of these plants is associated to a specific specialization for the high production of carotenoids in chromoplast-containing tissues, to stress responses, or with the establishment of mycorrhization[17]. Crocins biosynthesis in saffron involves a chromoplast-specific carotenoid biosynthetic pathway[20,21,22], where CsPSY2 is the enzyme catalyzing the first committed step in the carotenoid biosynthesis for the production of the colored apocarotenoids that accumulate in the stigma of saffron, together with CsLCY-B2, CsBCH2 and CCD2 enzymes[20,21,23]. CsPSY1a and CsPSY1b are mainly expressed in photosynthetic tissues, but are involved in stress responses, while CsPSY3 is related to strigolactones production[17]
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