Isolation and characterization of a sodium-dependent phosphate transporter gene in Dunaliella viridis

Novel Phosphate Transporter-B PvPTB1;1/1;2 Contribute to Efficient Phosphate Uptake and Arsenic Accumulation in As-Hyperaccumulator Pteris vittata

Arsenic (As) contamination in soil poses a potential threat to human health via crop uptake. As-hyperaccumulator Pteris vittata serves as a model plant to study As uptake and associated mechanisms. This study focuses on a novel P/AsV transport system mediated by low-affinity phosphate transporter-B 1 family (PTB1) in P. vittata. Here, we identified two plasma-membrane-localized PTB1 genes, PvPTB1;1/1;2, in vascular plants for the first time, which were 4.4-40-fold greater in expression in P. vittata than in other Pteris ferns. Functional complementation of a yeast P-uptake mutant and enhanced P accumulation in transgenic Arabidopsis thaliana confirmed their role in P uptake. Moreover, the expression of PvPTB1;1/1;2 facilitated the transport and accumulation of As in both yeast and A. thaliana shoots, demonstrating a comparable AsV uptake capacity. Microdissection-qPCR analysis and single-cell transcriptome analysis collectively suggest that PvPTB1;1/1;2 are specifically expressed in the epidermal cells of P. vittata roots. PTB1 may play a pivotal role in efficient P recycling during phytate secretion and hydrolysis in P. vittata roots. In summary, the dual P transport mechanisms consisting of high-affinity Pht1 and low-affinity PTB1 may have contributed to the efficient P/As uptake in P. vittata, thereby contributing to efficient phytoremediation for As-contaminated soils.

New horizons in culture and valorization of red microalgae

Research on marine microalgae has been abundantly published and patented these last years leading to the production and/or the characterization of some biomolecules such as pigments, proteins, enzymes, biofuels, polyunsaturated fatty acids, enzymes and hydrocolloids. This literature focusing on metabolic pathways, structural characterization of biomolecules, taxonomy, optimization of culture conditions, biorefinery and downstream process is often optimistic considering the valorization of these biocompounds. However, the accumulation of knowledge associated with the development of processes and technologies for biomass production and its treatment has sometimes led to success in the commercial arena. In the history of the microalgae market, red marine microalgae are well positioned particularly for applications in the field of high value pigment and hydrocolloid productions. This review aims to establish the state of the art of the diversity of red marine microalgae, the advances in characterization of their metabolites and the developments of bioprocesses to produce this biomass.

Functional PTB phosphate transporters are present in streptophyte algae and early diverging land plants

Two inorganic phosphate (Pi) uptake mechanisms operate in streptophytes and chlorophytes, the two lineages of green plants. PHOSPHATE TRANSPORTER B (PTB) proteins are hypothesized to be the Na⁺ /Pi symporters catalysing Pi uptake in chlorophytes, whereas PHOSPHATE TRANSPORTER 1 (PHT1) proteins are the H⁺ /Pi symporters that carry out Pi uptake in angiosperms. PHT1 proteins are present in all streptophyte lineages. However, Pi uptake in streptophyte algae and marine angiosperms requires Na⁺ influx, suggesting that Na⁺ /Pi symporters also function in some streptophytes. We tested the hypothesis that Na⁺ /Pi symporters exist in streptophytes. We identified PTB sequences in streptophyte genomes. Core PTB proteins are present at the plasma membrane of the liverwort Marchantia polymorpha. The expression of M. polymorpha core PTB proteins in the Saccharomyces cerevisiae pho2 mutant defective in high-affinity Pi transport rescues growth in low-Pi environments. Moreover, levels of core PTB mRNAs of M. polymorpha and the streptophyte alga Coleochaete nitellarum are higher in low-Pi than in Pi-replete conditions, consistent with a role in Pi uptake from the environment. We conclude that land plants inherited two Pi uptake mechanisms - mediated by the PTB and PHT1 proteins, respectively - from their streptophyte algal ancestor. Both systems operate in parallel in extant early diverging land plants.

High-affinity nitrate/nitrite transporter genes (Nrt2) in Tisochrysis lutea: identification and expression analyses reveal some interesting specificities of Haptophyta microalgae

Microalgae have a diversity of industrial applications such as feed, food ingredients, depuration processes and energy. However, microalgal production costs could be substantially improved by controlling nutrient intake. Accordingly, a better understanding of microalgal nitrogen metabolism is essential. Using in silico analysis from transcriptomic data concerning the microalgae Tisochrysis lutea, four genes encoding putative high-affinity nitrate/nitrite transporters (TlNrt2) were identified. Unlike most of the land plants and microalgae, cloning of genomic sequences and their alignment with complementary DNA (cDNA) sequences did not reveal the presence of introns in all TlNrt2 genes. The deduced TlNRT2 protein sequences showed similarities to NRT2 proteins of other phyla such as land plants and green algae. However, some interesting specificities only known among Haptophyta were also revealed, especially an additional sequence of 100 amino acids forming an atypical extracellular loop located between transmembrane domains 9 and 10 and the function of which remains to be elucidated. Analyses of individual TlNrt2 gene expression with different nitrogen sources and concentrations were performed. TlNrt2.1 and TlNrt2.3 were strongly induced by low NO3 (-) concentration and repressed by NH4 (+) substrate and were classified as inducible genes. TlNrt2.2 was characterized by a constitutive pattern whatever the substrate. Finally, TlNrt2.4 displayed an atypical response that was not reported earlier in literature. Interestingly, expression of TlNrt2.4 was rather related to internal nitrogen quota level than external nitrogen concentration. This first study on nitrogen metabolism of T. lutea opens avenues for future investigations on the function of these genes and their implication for industrial applications.

Changes in gene expression of Prymnesium parvum induced by nitrogen and phosphorus limitation

Prymnesium parvum is a globally distributed prymnesiophyte alga commonly found in brackish water marine ecosystems and lakes. It possesses a suite of toxins with ichthyotoxic, cytotoxic and hemolytic effects which, along with its mixotrophic nutritional capabilities, allows it to form massive Ecosystem Disruptive Algal Blooms (EDABs). While blooms of high abundance coincide with high levels of nitrogen (N) and phosphorus (P), reports of field and laboratory studies have noted that P. parvum toxicity appears to be augmented at high N:P ratios or P-limiting conditions. Here we present the results of a comparative analysis of P. parvum RNA-Seq transcriptomes under nutrient replete conditions, and N or P deficiency to understand how this organism responds at the transcriptional level to varying nutrient conditions. In nutrient limited conditions we found diverse transcriptional responses for genes involved in nutrient uptake, protein synthesis and degradation, photosynthesis, and toxin production. As anticipated, when either N or P was limiting, transcription levels of genes encoding transporters for the respective nutrient were higher than those under replete condition. Ribosomal and lysosomal protein genes were expressed at higher levels under either nutrient-limited condition compared to the replete condition. Photosynthesis genes and polyketide synthase genes were more highly expressed under P-limitation but not under N-limitation. These results highlight the ability of P. parvum to mount a coordinated and varied cellular and physiological response to nutrient limitation. Results also provide potential marker genes for further evaluating the physiological response and toxin production of P. parvum populations during bloom formation or to changing environmental conditions.

The transcriptome and proteome of the diatom Thalassiosira pseudonana reveal a diverse phosphorus stress response

Phosphorus (P) is a critical driver of phytoplankton growth and ecosystem function in the ocean. Diatoms are an abundant class of marine phytoplankton that are responsible for significant amounts of primary production. With the control they exert on the oceanic carbon cycle, there have been a number of studies focused on how diatoms respond to limiting macro and micronutrients such as iron and nitrogen. However, diatom physiological responses to P deficiency are poorly understood. Here, we couple deep sequencing of transcript tags and quantitative proteomics to analyze the diatom Thalassiosira pseudonana grown under P-replete and P-deficient conditions. A total of 318 transcripts were differentially regulated with a false discovery rate of <0.05, and a total of 136 proteins were differentially abundant (p<0.05). Significant changes in the abundance of transcripts and proteins were observed and coordinated for multiple biochemical pathways, including glycolysis and translation. Patterns in transcript and protein abundance were also linked to physiological changes in cellular P distributions, and enzyme activities. These data demonstrate that diatom P deficiency results in changes in cellular P allocation through polyphosphate production, increased P transport, a switch to utilization of dissolved organic P through increased production of metalloenzymes, and a remodeling of the cell surface through production of sulfolipids. Together, these findings reveal that T. pseudonana has evolved a sophisticated response to P deficiency involving multiple biochemical strategies that are likely critical to its ability to respond to variations in environmental P availability.

The characterization of two peroxiredoxin genes in Dunaliella viridis provides insights into antioxidative response to salt stress

Peroxiredoxins (Prxs), a group of antioxidant enzymes, are an important component of the oxidative defense system and have been demonstrated to function as peroxidases, sensors of H(2)O(2)-mediated signaling and/or chaperones. In this study, a cDNA library was constructed from a halotolerant alga, Dunaliella viridis, and was used in a functional complementation screen for antioxidative genes in an oxidative sensitive yeast mutant. Two Prx genes, DvPrx1 and DvPrx2, were obtained from this screen. These two genes were classified as type II Prx and 2-Cys Prx based on amino acid sequence and phylogenetic analysis. When over-expressed in yeast cells, both Prx genes were able to confer better oxidative tolerance and decrease the level of reactive oxygen species (ROS). Subcellular localization experiments in tobacco cells revealed that both DvPrx1 and DvPrx2 were localized in the cytosol. The transcription of DvPrx1 and DvPrx2 can be induced by hypersalinity shock, but is not obviously affected by treatment with high levels of oxidant. Our results shed light on the function and regulation of Prx genes from Dunaliella and their potential roles in salt tolerance.

Molecular cloning and characterization of a vacuolar H+₋pyrophosphatase from Dunaliella viridis

The halotolerant alga Dunaliella adapts to exceptionally high salinity and possesses efficient mechanisms for regulating intracellular Na(+). In plants, sequestration of Na(+) into the vacuole is driven by the electrochemical H(+) gradient generated by H(+) pumps, and this Na(+) sequestration is one mechanism that confers salt tolerance to plants. To investigate the role of vacuolar H(+) pumps in the salt tolerance of Dunaliella, we isolated the cDNA of the vacuolar proton-translocating inorganic pyrophosphatase (V-H(+)-PPase) from Dunaliella viridis. The DvVP cDNA is 2,984 bp in length, codes for a polypeptide of 762 amino acids and has 15 transmembrane domains. The DvVP protein is highly similar to V-H(+)-PPases from other green algae and higher plant species, in terms of its amino acid sequence and its transmembrane model. A phylogenetic analysis of V-H(+)-PPases revealed the close relationship of Dunaliella to green algal species of Charophyceae and land plants. The heterologous expression of DvVP in the yeast mutant G19 (Δena1-4) suppressed Na(+) hypersensitivity, and a GFP-fusion of DvVP localized to the vacuole membranes in yeast, indicating that DvVP encodes a functional V-H(+)-PPase. A northern blot analysis showed a decrease in the transcript abundance of DvVP at higher salinity in D. viridis cells, which is in contrast to the salt-induced upregulation of V-H(+)-PPase in some plants, suggesting that the expression of DvVP under salt stress may be regulated by different mechanisms in Dunaliella. This study not only enriched our knowledge about the biological functions of V-H(+)-PPases in different organisms but also improved our understanding of the molecular mechanism of salt tolerance in Dunaliella.

Expression of the 26S proteasome subunit RPN10 is upregulated by salt stress in Dunaliella viridis

Green algae of the genus Dunaliella can adapt to hypersaline environments and are considered model organisms for salinity tolerance. In an EST analysis in Dunaliella viridis under salt stress, we isolated a salt-inducible cDNA coding for the 26S proteasome subunit RPN10, designated DvRPN10. The DvRPN10 cDNA is 1472 bp and encodes a polypeptide of 377 amino acids. The DvRPN10 protein shares a high similarity to orthologs from other species. The function of DvRPN10 was confirmed by complementation of the yeast Deltarpn10 mutant. Q-PCR analysis of D. viridis cells grown in different salinities revealed that the transcript level of DvRPN10 increased in proportion to the external salinity within a range of 0.5-3 M NaCl, but decreased significantly at extremely high salinities (4-5 M NaCl). When a salinity shock of 1-3 M NaCl was applied to D. viridis cells, DvRPN10 mRNA levels remained steady during the first 36 h, and then gradually elevated to the level observed at 3 M NaCl. The gene structure of DvRPN10 was revealed by sequencing of a BAC clone containing this gene. Possible transcription factor binding sites related to stress tolerance were found in the promoter region of DvRPN10. The expression of DvRPN10 in response to the external salinity suggests that RPN10-mediated protein degradation plays a role in the salinity tolerance of D. viridis.

Physiological characterization and stress-induced metabolic responses of Dunaliella salina isolated from salt pan

A Dunaliella strain was isolated from salt crystals obtained from experimental salt farm of the institute (latitude 21.46 N, longitude 72.11 degrees E). The comparative homology study of amplified molecular signature 18S rRNA, proves the isolated strain as D. salina. The growth pattern and metabolic responses such as proline, glycine betaine, glycerol, total protein and total sugar content to different salinity (from 0.5 to 5.5 M NaCl) were studied. The optimum growth was observed at 1.0 M NaCl and thereafter it started to decline. Maximum growth was obtained on 17th day of inoculation in all salt concentrations except 0.5 M NaCl, whereas maximum growth was observed on 13th day. There were no significant differences (P < 0.01) in chlorophyll a/b contents (1.0-1.16 +/- 0.05 microg chl. a and 0.2-0.29 +/- 0.01 microg chl. b per 10(6) cells) up to 2.0 M NaCl, however at 3.0 M NaCl a significant increase (2.5 +/- 0.12 microg chl. a and 0.84 +/- 0.4 microg chl. b per 10(6) cells) was observed which declined again at 5.5 M NaCl concentration (2.0 +/- 0.1 microg chl. a and 0.52 +/- 0.03 microg chl. b per 10(6) cells). Stress metabolites such as proline, glycine betaine, glycerol and total sugar content increased concomitantly with salt concentration. Maximum increase in proline (1.4 +/- 0.07 microg), glycine betaine (5.7 +/- 0.28 microg), glycerol (3.7 +/- 0.18 ml) and total sugar (250 +/- 12.5 microg) per 10(5) cells was observed in 5.5 M NaCl. A decrease in total protein with reference to 0.5 M NaCl was observed up to 3.0 M NaCl, however, a significant increase (P < 0.01) was observed at 5.5 M NaCl (0.19 +/- 0.01 microg per 10(5) cells). Inductive coupled plasma (ICP) analysis shows that intracellular Na(+) remained unchanged up to 2.0 M NaCl concentration and thereafter a significant increase was observed. No relevant increase in the intracellular level of K(+) and Mg(++) was observed with increasing salt concentration. Evaluation of physiological and metabolic attributes of Dunaliella salina can be used to explore its biotechnological and industrial potential.

Sequencing and analysis of a genomic fragment provide an insight into the Dunaliella viridis genomic sequence

Dunaliella is a genus of wall-less unicellular eukaryotic green alga. Its exceptional resistances to salt and various other stresses have made it an ideal model for stress tolerance study. However, very little is known about its genome and genomic sequences. In this study, we sequenced and analyzed a 29,268 bp genomic fragment from Dunaliella viridis. The fragment showed low sequence homology to the GenBank database. At the nucleotide level, only a segment with significant sequence homology to 18S rRNA was found. The fragment contained six putative genes, but only one gene showed significant homology at the protein level to GenBank database. The average GC content of this sequence was 51.1%, which was much lower than that of close related green algae Chlamydomonas (65.7%). Significant segmental duplications were found within this fragment. The duplicated sequences accounted for about 35.7% of the entire region. Large amounts of simple sequence repeats (microsatellites) were found, with strong bias towards (AC)(n) type (76%). Analysis of other Dunaliella genomic sequences in the GenBank database (total 25,749 bp) was in agreement with these findings. These sequence features made it difficult to sequence Dunaliella genomic sequences. Further investigation should be made to reveal the biological significance of these unique sequence features.

TMV recombinants encoding fused foreign transmembrane domains to the CP subunit caused local necrotic response on susceptible tobacco

With regard to the effects of various foreign peptides fused to the coat protein subunits on the infectivity of corresponding TMV recombinants, some of TMV recombinants were found to induce necrotic local lesions on the inoculated leaves of susceptible tobacco. This paper reported that there existed a group of TMV recombinants in which the fused foreign peptides contained a transmembrane domain according to the predictions by three programs of SOSUI, TMpred and DAS. Further studies showed for the first time that a foreign transmembrane domain in a fused peptide of the corresponding TMV recombinant would result in the local lesions on the susceptible tobacco leaves. In addition, it was concluded that none of the TMV recombinants that systematically infected susceptible tobacco contained a transmembrane domain in the coat protein subunits.