During photomorphogenesis in higher plants, a coordinated increase occurs in the chlorophyll and carotenoid contents. The carotenoid level is under phytochrome control, as reflected by the light regulation of the mRNA level of phytoene synthase (PSY), the first enzyme in the carotenoid biosynthetic pathway. We investigated PSY protein levels, enzymatic activity and topological localization during photomorphogenesis. The results revealed that PSY protein levels and enzymatic activity increase during de-etiolation and that the enzyme is localized at thylakoid membranes in mature chloroplasts. However, under certain light conditions (e.g., far-red light) the increases in PSY mRNA and protein levels are not accompanied by an increase in enzymatic activity. Under those conditions, PSY is localized in the prolamellar body fraction in a mostly enzymatically inactive form. Subsequent illumination of dark-grown and/or in far-red light grown seedlings with white light causes the decay of these structures and a topological relocalization of PSY to developing thylakolds which results in its enzymatic activation. This light-dependent mechanism of enzymatic activation of PSY in carotenoid biosynthesis shares common features with the regulation of the NADPH:protochlorophyllide oxidoreductase, the first light-regulated enzyme in chlorophyll biosynthesis. The mechanism of regulation described here may contribute to ensuring a spatially and temporally coordinated increase in both carotenoid and chlorophyll contents.

Growth inhibition of Lemna gibba plantlets by the bleaching herbicide, SC‐0051 (2‐(2‐chloro‐4‐methanesulfonylbenzoyl)‐1,3‐cyclohexanedione)) was alleviated by the addition of homogentisic acid to the growth medium. Homogentisic acid is a key intermediate in the biosynthesis of tyrosine‐derived plant quinones as well as in tyrosine metabolism. The herbicide prevented the incorporation of radioactivity from [14C]tyrosine into lipophilic plant metabolites and, in rat liver extracts, the herbicide inhibited the conversion of tyrosine to homogentisic acid. The enzyme p‐hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) from both Zea mays seedlings and liver tissues, was found to be subject to strong inhibition by SC‐0051. Inhibition of plant quinone biosynthesis is a new mode of herbicidal action. One of the consequences of quinone depletion in plants in vivo is apparently an indirect inhibition of phytoene desaturation. The enzyme phytoene desaturase itself, however, is not afflicted by the herbicide.

Under stress conditions such as high light intensity or nutrient starvation, cells of the unicellular alga Dunaliella bardawil overproduce ␤-carotene, which is accumulated in the plastids in newly formed triacylglycerol droplets. We report here that the formation of these sequestering structures and ␤-carotene are interdependent. When the synthesis of triacylglycerol is blocked, the overproduction of ␤-carotene is also inhibited. During overproduction of ␤-carotene no up-regulation of phytoene synthase or phytoene desaturase is observed on the transcriptional or translational level, whereas at the same time acetyl-CoA carboxylase, the key regulatory enzyme of acyl lipid biosynthesis, is increased, at least in its enzymatic activity. We conclude that under normal conditions the carotenogenic pathway is not maximally active and may be appreciably stimulated in the presence of sequestering structures, creating a plastid-localized sink for the end product of the carotenoid biosynthetic pathway.Carotenoids are vitally important in all photosynthetic membranes because of their ability to prevent photooxidative damage and to harvest light (for a recent review, see Frank and Cogdell, 1996). Underscoring their importance, chlorophylls and carotenoids are synthesized in a quantitatively and qualitatively coordinated manner in chloroplasts. Whenever this balance is strongly changed in favor of carotenoids, the plastid ultrastructure is also changed and, concomitantly, chlorophylls are degraded. The resulting chromoplasts are photosynthetically inactive, yellow to red in color, and differentiate in specialized plant organs such as petals, roots, and fruits. Chromoplasts are morphologically characterized by the absence of thylakoids and by the presence of newly formed structures in which the overproduced carotenoids are sequestered (for a recent review of chromoplast development, see Camara et al., 1995). These carotenoid-bearing structures may be plastoglobules (lipid droplets in most carotenoid-bearing flower petals), crystals (e.g. in Lycopersicon esculentum fruits), fibrils/tubules (e.g. in Capsicum annuum fruits), or membranes (e.g. in Narcissus pseudonarcissus petals). These structures probably prevent products from overloading the chromoplast membranes, the site of carotenoid formation.To investigate the apparently strict interdependence of carotenoid overproduction and the formation of sequestering structures, we exploited the unicellular alga Dunaliella bardawil as a laboratory system inducible for enhanced carotenoid formation. When exposed to stress conditions such as high light intensity or nutrient starvation, two stereoisomers of ␤-carotene, all-trans and 9-cis ␤-carotene, accumulated, reaching up to 10% of the cell's dry weight, with the pigment being deposited into plastid lipid globules as the sequestering structure (Ben Amotz et al., 1982, 1988 Ben Amotz and Avron, 1983; Jeminez and Pick, 1994). These lipid structures are stabilized and maintained in size by a peripherally associated 38-kD protein (Caroten...

In chloroplasts, carotenoids are essential pigments involved in photosynthesis. During-photomorphogenesis, a coordinated increase in the amounts of chlorophylls and carotenoids, in conjugation with other components, leads to the formation of a functional photosynthetic apparatus. To investigate the regulation of carotenoid biosynthesis during this process at the molecular level, GGPS, PSY and PDS cDNAs have been cloned from white mustard (Sinapis alba L). GGPS encodes a key enzyme in plastid isoprenoid metabolism, while the products of PSY and PDS catalyse the subsequent steps in carotenoid biosynthesis. Due to the low mRNA levels of the genes involved, the use of a RT-PCR protocol was necessary to measure gene expression during photomorphogenesis. With light, there is an up-regulation of PSY expression, the first gene within the carotenoid biosynthetic pathway, while PDS and GGPS expression levels remain constant. Treatment with different light qualities reveals a phytochrome-mediated regulation of PSY expression in developing white mustard seedlings. To obtain more detailed information on the light-regulation, Arabidopsis thaliana wild-type and phytochrome mutants were utilized. Continuous far-red and red light both increase the expression of PSY in wild-type seedlings, demonstrating that both light-labile and light-stable phytochromes are involved in PSY regulation. The response to far-red light is completely abolished in the phyA mutant, showing that PHYA mediates the increase in PSY transcript levels under these light conditions. In the phyB mutant, the red light response is normal, indicating that PSY expression is not controlled by PHYB but by other light-stable phytochromes. Measurement of chlorophylls and carotenoids under the same light regimes shows that the up-regulation of PSY expression does not necessarily result in an increase of the carotenoid content. Only those light conditions which allow chlorophyll biosynthesis lead to a significant increase of the carotenoid content. Therefore, it is proposed that up-regulation of PSY mRNA levels leads to an increased capacity for the formation of carotenoids. However, this only takes place under light conditions leading to protochlorophyllide photoconversion.

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