Induced β-Carotene Synthesis Driven by Triacylglycerol Deposition in the Unicellular AlgaDunaliella bardawil1

Abstract

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...

Phytoene Desaturase Is Localized Exclusively in the Chloroplast and Up-Regulated at the mRNA Level during Accumulation of Secondary Carotenoids in Haematococcus pluvialis (Volvocales, Chlorophyceae),

“…Support for this point of view comes from Linden (1999), who cloned ␤-carotene hydroxylase from H. pluvialis and observed an increase of the corresponding mRNA in parallel to astaxanthin synthesis. Contrary findings have been reported in Dunaliella bardawil (Rabbani et al, 1998). On stress induction, this alga massively accumulates ␤-carotene in intraplastidic droplets.…”

“…One reason for this striking difference between the regulation patterns of carotenogenesis in these two Volvocales might be the different localization of the final product. The intraplastidic accumulation in D. bardawil seems to be regulated by the presence of the sequestering structures (Rabbani et al, 1998). In contrast, the extraplastidic accumulation in H. pluvialis might allow the common transcriptional control of biosynthesis of SC and cytoplasmic lipid vesicle material.…”

“…In the green alga Dunaliella bardawil, both mRNA and protein level of PDS remained constant during the massive ␤-carotene synthesis on high-light conditions. In this unicellular alga the secondary ␤-carotene accumulation in intraplastidic lipid droplets is controlled by the formation of the sequestering structures (Rabbani et al, 1998).…”

Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances

“…The formation of a C18 fatty acid consumes approximately 24 NADPH derived from the electron transport chain, which is twice that required for synthesis of a carbohydrate or protein molecule of the same mass, and thus relaxes the overreduced electron transport chain under high light or other stress conditions. The TAG synthesis pathway is usually coordinated with secondary carotenoid synthesis in algae (Rabbani et al, 1998;Zhekisheva et al, 2002). The molecules (e.g.…”

Metabolite Profiling and Integrative Modeling Reveal Metabolic Constraints for Carbon Partitioning under Nitrogen Starvation in the Green Algae Haematococcus pluvialis

“…The composition of these reserve molecules depends on the environmental conditions and is interconnected with the content of other classes of metabolite compounds (e.g. carotenoids) (4,5).…”