REGULATION OF THE CAROTENOID BIOSYNTHETIC PATHWAY IN THE HALOTOLERANT MICROALGA DUNALIELLA SALINA

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Nutrient limitation is the main regulatory factor for carotenoid accumulation and for Psy and Pds steady state transcript levels in Dunaliella salina (Chlorophyta) exposed to high light and salt stress

Publication . Coesel, Sacha; Baumgartner, Alexandra; Teles, Lícia; Ramos, Ana; Henriques, Nuno; Cancela, Leonor; Varela, J.

Dunaliella salina (Dunal) Teodoresco (1905) is a green unicellular alga able to withstand severe salt, light, and nutrient stress, adaptations necessary to grow in harsh environments such as salt ponds. In response to such growth conditions, this microalga accumulates high amounts of β-carotene in its single chloroplast. In this study, we show that carotenoid accumulation is consistently inhibited in cells grown in nutrient-supplemented media and exposed either to high-light or medium-low-light conditions. Likewise, carotenogenesis in cells shifted to higher salinity (up to 27% NaCl) under medium-low-light conditions is inhibited by the presence of nutrients. The steady-state levels of transcripts encoding phytoene syn- thase and phytoene desaturase increased substantially in D. salina cells shifted to high light or high salt under nutrient- limiting conditions, whereas the presence of nutrients inhibited this response. The regulatory effect of nutrient availability on the accumulation of carotenoids and mes- senger RNA levels of the first two enzymes committed to carotenoid biosynthesis is discussed.

2008Journal article

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Isolation and characterization of a stress-inducible Dunaliella salina Lcy-β gene encoding a functional lycopene β-cyclase

Publication . Ramos, Ana; Coesel, Sacha; Marques, Ana; Rodrigues, Marta; Baumgartner, Alexandra; Noronha, João; Rauter, Amélia; Brenig, Bertram; Varela, J.

The halotolerant green alga Dunaliella salina accumulates large amounts of β-carotene when exposed to various stress conditions. Although several studies concerning accumulation and biotechnological production of β-carotene have been published, the molecular basis and regulation of the genes involved in carotenoid biosynthesis in D. salina are still poorly known. In this paper, we report the isolation and regulation of the lycopene β-cyclase (Lcy-β) gene by abiotic stress. The function of this gene was determined by heterologous genetic complementation in E. coli. Gene expression and physiological analyses revealed that D. salina Lcy-β steady-state transcript and carotenoid levels were up-regulated in response to all stress conditions tested (salt, light and nutrient depletion). The results presented here suggest that nutrient availability is a key factor influencing carotenogenesis as well as carotenoid biosynthesis-related gene expression in D. salina.

2008Journal article

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Molecular and functional characterization of a cDNA encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase from Dunaliella salina

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Molecular and functional characterization of a cDNA encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase from Dunaliella salina

Publication . Ramos, Ana; Marques, A. R.; Rodrigues, M.; Henriques, Nuno; Baumgartner, Alexandra; Castilho, Rita; Brenig, Bertram; Varela, J.

In green algae,the final step of the plastidial methylerythritol phosphate (MEP) pathway is catalyzed by 4-hydroxy-3-methylbut-2-enyldiphosphate reductase(HDR; EC: 1.17.1.2),an enzyme proposed to play a keyrole in the regulation of isoprenoid biosynthesis. Here we report the isolation and functional characterization of a 1959- bp Dunaliella salina HDR (DsHDR) cDNA encoding a deduced polypeptide of 474 amino acid residues. Phylogenetic analysis implied a cyanobacterial origin for plant and algal HDR genes. Steady-state DsHDR transcript levels were higher in D. salina cells submitted to nutritional depletion, high salt and/or high light, suggesting that DsHDR may respond to the same environmental cues as genes involved in carotenoid biosynthesis.

2009Journal article

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Evolutionary origins and functions of the carotenoid biosynthetic pathway in marine diatoms

Publication . Coesel, Sacha; Obornik, Miroslav; Varela, J.; Falciatore, Angela; Bowler, Chris

Carotenoids are produced by all photosynthetic organisms, where they play essential roles in light harvesting and photoprotection. The carotenoid biosynthetic pathway of diatoms is largely unstudied, but is of particular interest because these organisms have a very different evolutionary history with respect to the Plantae and are thought to be derived from an ancient secondary endosymbiosis between heterotrophic and autotrophic eukaryotes. Furthermore, diatoms have an additional xanthophyll-based cycle for dissipating excess light energy with respect to green algae and higher plants. To explore the origins and functions of the carotenoid pathway in diatoms we searched for genes encoding pathway components in the recently completed genome sequences of two marine diatoms. Consistent with the supplemental xanthophyll cycle in diatoms, we found more copies of the genes encoding violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZEP) enzymes compared with other photosynthetic eukaryotes. However, the similarity of these enzymes with those of higher plants indicates that they had very probably diversified before the secondary endosymbiosis had occurred, implying that VDE and ZEP represent early eukaryotic innovations in the Plantae. Consequently, the diatom chromist lineage likely obtained all paralogues of ZEP and VDE genes during the process of secondary endosymbiosis by gene transfer from the nucleus of the algal endosymbiont to the host nucleus. Furthermore, the presence of a ZEP gene in Tetrahymena thermophila provides the first evidence for a secondary plastid gene encoded in a heterotrophic ciliate, providing support for the chromalveolate hypothesis. Protein domain structures and expression analyses in the pennate diatom Phaeodactylum tricornutum indicate diverse roles for the different ZEP and VDE isoforms and demonstrate that they are differentially regulated by light. These studies therefore reveal the ancient origins of several components of the carotenoid biosynthesis pathway in photosynthetic eukaryotes and provide information about how they have diversified and acquired new functions in the diatoms.

2008Journal article

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