MOLECULAR CLONING AND BIOCHEMICAL CHARACTERIZATION OF GALACTOSE-1-PHOSPHATE URIDYLYLTRANSFERASE FROM GRACILARIA CHANGII (RHODOPHYTA)(1)

Identification of Indicator Genes for Agar Accumulation in Gracilariopsis lemaneiformis (Rhodophyta)

Agar, as a seaweed polysaccharide mainly extracted from Gracilariopsis lemaneiformis, has been commercially applied in multiple fields. To investigate factors indicating the agar accumulation in G. lemaneiformis, the agar content, soluble polysaccharides content, and expression level of 11 genes involved in the agar biosynthesis were analysed under 4 treatments, namely salinity, temperature, and nitrogen and phosphorus concentrations. The salinity exerted the greatest impact on the agar content. Both high (40‱) and low (10‱, 20‱) salinity promoted agar accumulation in G. lemaneiformis by 4.06%, 2.59%, and 3.00%, respectively. The content of agar as a colloidal polysaccharide was more stable than the soluble polysaccharide content under the treatments. No significant correlation was noted between the two polysaccharides, and between the change in the agar content and the relative growth rate of the algae. The expression of all 11 genes was affected by the 4 treatments. Furthermore, in the cultivar 981 with high agar content (21.30 ± 0.95%) compared to that (16.23 ± 1.59%) of the wild diploid, the transcriptional level of 9 genes related to agar biosynthesis was upregulated. Comprehensive analysis of the correlation between agar accumulation and transcriptional level of genes related to agar biosynthesis in different cultivation conditions and different species of G. lemaneiformis, the change in the relative expression level of glucose-6-phosphate isomerase II (gpiII), mannose-6-phosphate isomerase (mpi), mannose-1-phosphate guanylyltransferase (mpg), and galactosyltransferase II (gatII) genes was highly correlated with the relative agar accumulation. This study lays a basis for selecting high-yield agar strains, as well as for targeted breeding, by using gene editing tools in the future.

Breeding of New Strains of Gracilariopsis lemaneiformis with High Agar Content by ARTP Mutagenesis and High Osmotic Pressure Screening

ARTP (atmospheric and room temperature plasma mutagenesis) mutagenesis was tried on G. lemaneiformis, and mutagenesis conditions were confirmed. An osmotic pressure screening program was established. Mutants were identified and characterized of relevant physiological traits. The aim of the study is to try to use ARTP mutagenesis and osmotic pressure screening for the breeding of high-agar G. lemaneiformis. Treatment time of 46 s was found to be an optimal mutagenesis time. The mutagenized spores were initially screened with 58‰ salinity artificial seawater, and then, the surviving spores were screened twice with 60‰ salinity artificial seawater in their vertical growth phase and branch growth phase, respectively. Four fast-growing and hypertonic resistance gametophytes were selected. The actual photosynthetic efficiency [Y(PSII)], photochemical quenching (qL), and non-photochemical quenching (NPQ) of four mutants were measured. The values of Y(PSII) and qL of HAGL-X3 and HAGL-X5 were higher than those of the control in the early stage of salt stress. NPQs of HAGL-X3 and HAGL-X5 were higher than control in most of the times. The growth rates of the four mutants were higher than that of the control. HAGL-X4 was the highest. The agar content was measured; HAGL-X5 displayed the highest agar content among the tested strains. HAGL-X5 was more in line with expectations, because of its high agar content and good hypertonic resistance. In this study, the mutant of G. lemaneiformis with high agar content was obtained by the procedure, which provided a certain reference for the selection of G. lemaneiformis strains with high agar content.

S-Assimilation Influences in Carrageenan Biosynthesis Genes during Ethylene-Induced Carposporogenesis in Red Seaweed Grateloupia imbricata

The synthesis of cell-wall sulfated galactans proceeds through UDP galactose, a major nucleotide sugar in red seaweed, whilst sulfate is transported through S-transporters into algae. Moreover, synthesis of ethylene, a volatile plant growth regulator that plays an important role in red seaweed reproduction, occurs through S-adenosyl methionine. This means that sulfur metabolism is involved in reproduction events as well as sulfated galactan synthesis of red seaweed. In this work we study the effects of methionine and MgSO4 on gene expression of polygalactan synthesis through phosphoglucomutase (PGM) and galactose 1 phosphate uridyltransferase (GALT) and of sulfate assimilation (S-transporter and sulfate adenylyltransferase, SAT) using treatment of ethylene for 15 min, which elicited cystocarp development in Grateloupia imbricata. Also, expressions of carbohydrate sulfotransferase and galactose-6-sulfurylase in charge of the addition and removal of sulfate groups to galactans backbone were examined. Outstanding results occurred in the presence of methionine, which provoked an increment in transcript number of genes encoding S-transporter and assimilation compared to controls regardless of the development stage of thalli. Otherwise, methionine diminished the transcript levels of PGM and GALT and expressions are associated with the fertilization stage of thalli of G. imbricata. As opposite, methionine and MgSO4 did not affect the transcript number of carbohydrate sulfotransferase and galactose-6-sulfurylase. Nonetheless, differential expression was obtained for sulfurylases according to the development stages of thalli of G. imbricata.

The Cell Wall Polysaccharides Biosynthesis in Seaweeds: A Molecular Perspective

Cell wall polysaccharides (CWPS) of seaweeds play crucial roles in mechanical shear resistance, cell-cell adhesion and the interactions with changeable marine environments. They have diverse applications in food, cosmetics, agriculture, pharmaceuticals and therapeutics. The recent boost of multi-omics sequence analysis has rapidly progressed the mining of presumed genes encoding enzymes involved in CWPS biosynthesis pathways. In this review, we summarize the biosynthetic pathways of alginate, fucoidan, agar, carrageenan and ulvan in seaweeds referred to the literatures on published genomes and biochemical characterization of encoded enzymes. Some transcriptomic data were briefly reported to discuss the correlation between gene expression levels and CWPS contents. Mannuronan C-5 epimerase (MC5E) and carbohydrate sulfotransferase (CST) are crucial enzymes for alginate and sulfated CWPS, respectively. Nonetheless, most CWPS-relevant genes were merely investigated by gene mining and phylogenetic analysis. We offer an integrative view of CWPS biosynthesis from a molecular perspective and discuss about the underlying regulation mechanism. However, a clear understanding of the relationship between chemical structure and bioactivities of CWPS is limited, and reverse genetic manipulation and effective gene editing tools need to be developed in future.

Overexpression of UDP-sugar pyrophosphorylase leads to higher sensitivity towards galactose, providing new insights into the mechanisms of galactose toxicity in plants

Galactose toxicity (Gal-Tox) is a widespread phenomenon ranging from Escherichia coli to mammals and plants. In plants, the predominant pathway for the conversion of galactose into UDP-galactose (UDP-Gal) and UDP-glucose is catalyzed by the enzymes galactokinase, UDP-sugar pyrophosphorylase (USP) and UDP-galactose 4-epimerase. Galactose is a major component of cell wall polymers, glycolipids and glycoproteins; therefore, it becomes surprising that exogenous addition of galactose leads to drastic root phenotypes including cessation of primary root growth and induction of lateral root formation. Currently, little is known about galactose-mediated toxicity in plants. In this study, we investigated the role of galactose-containing metabolites like galactose-1-phosphate (Gal-1P) and UDP-Gal in Gal-Tox. Recently published data from mouse models suggest that a reduction of the Gal-1P level via an mRNA-based therapy helps to overcome Gal-Tox. To test this hypothesis in plants, we created Arabidopsis thaliana lines overexpressing USP from Pisum sativum. USP enzyme assays confirmed a threefold higher enzyme activity in the overexpression lines leading to a significant reduction of the Gal-1P level in roots. Interestingly, the overexpression lines are phenotypically more sensitive to the exogenous addition of galactose (0.5 mmol L^-1 Gal). Nucleotide sugar analysis via high-performance liquid chromatography-mass spectrometry revealed highly elevated UDP-Gal levels in roots of seedlings grown on 1.5 mmol L^-1 galactose versus 1.5 mmol L^-1 sucrose. Analysis of plant cell wall glycans by comprehensive microarray polymer profiling showed a high abundance of antibody binding recognizing arabinogalactanproteins and extensins under Gal-feeding conditions, indicating that glycoproteins are a major target for elevated UDP-Gal levels in plants.

Floridean Starch and Floridoside Metabolic Pathways of Neoporphyra haitanensis and Their Regulatory Mechanism under Continuous Darkness

Floridean starch and floridoside are the main storage carbohydrates of red algae. However, their complete metabolic pathways and the origin, function, and regulatory mechanism of their pathway genes have not been fully elucidated. In this study, we identified their metabolic pathway genes and analyzed the changes in related gene expression and metabolite content in Neoporphyra haitanensis under continuous dark conditions. Our results showed that genes from different sources, including eukaryotic hosts, cyanobacteria, and bacteria, were combined to construct floridean starch and floridoside metabolic pathways in N. haitanensis. Moreover, compared with those in the control, under continuous dark conditions, floridean starch biosynthesis genes and some degradation genes were significantly upregulated with no significant change in floridean starch content, whereas floridoside degradation genes were significantly upregulated with a significant decrease in floridoside content. This implies that floridean starch content is maintained but floridoside is consumed in N. haitanensis under dark conditions. This study elucidates the "floridean starch-floridoside" metabolic network and its gene origins in N. haitanensis for the first time.

Comparative genomics reveals differences in algal galactan biosynthesis and related pathways in early and late diverging red algae

Red algae are a major source of marine sulfated galactans. In this study, orthologs and inparalogs from seven red algae were analyzed and compared with the aim to discover differences in algal galactan biosynthesis and related pathways of these algae. Red algal orthologs for putative carbohydrate sulfotransferases were found to be prevalent in Porphyridium purpureum, Florideophytes and Bangiophytes, while red algal orthologs for putative chondroitin sulfate synthases, sulfurylases, and porphyranases /carrageenases were found exclusively in Florideophytes and Bangiophytes. The acquirement of these genes could have happened after the divergence from Cyanidiales red algae. Cyanidiales red algae were found to have more number and types of putative sulfate permeases, suggesting that these genes could have been acquired in adaptation to the environmental stresses and biogeochemistry of respective habitats. The findings of this study shed lights on the evolution of different homeostasis mechanisms by the early and late diverging red algal orders.

Phosphoglucomutase Is Not the Target for Galactose Toxicity in Plants

Plants synthesize a number of different oligomeric or polymeric sugars containing galactose. During growth and development some of these carbohydrates are metabolized or remodeled releasing galactose as a breakdown product. All plants have established recycling pathways for such sugars, for which they seem to have a limited capacity to cope with. Exceeding these limits results in sugar toxicity, which is observed already at concentrations as low as 1 mmol·l^-1 for galactose. The mechanism of galactose toxicity is poorly understood but it seems plausible that the enzymes involved in carbohydrate metabolism also might be the targets responsible for the adverse effects. Data from yeast and bacteria suggests that the enzyme phosphoglucomutase (PGM) is inhibited by galactose-1-phosphate. To test this hypothesis for plants we expressed recombinant cytosolic PGM3 from Arabidopsis in E. coli. Intriguingly, the enzyme was not inhibited by galactose-1-phosphate at physiological concentrations. Furthermore, PGM3 did not convert galactose-1-phosphate to galactose-6-phosphate, which was suggested as the inhibitory mode of action in yeast. In addition, metabolite levels in Arabidopsis roots were analyzed for their galactose-1-phosphate concentration by means of GC-MS. Seedlings grown on MS-media with sucrose contained less than 10 nmol·g FW^-1 of galactose-1-phosphate. However, seedlings from plates, in which the sucrose was replaced by galactose, showed a strong increase of Gal-1-P to levels of up to 200 nmol·g FW^-1.

Genomic analyses of unique carbohydrate and phytohormone metabolism in the macroalga Gracilariopsis lemaneiformis (Rhodophyta)

BACKGROUND

Red algae are economically valuable for food and in industry. However, their genomic information is limited, and the genomic data of only a few species of red algae have been sequenced and deposited recently. In this study, we annotated a draft genome of the macroalga Gracilariopsis lemaneiformis (Gracilariales, Rhodophyta).

RESULTS

The entire 88.98 Mb genome of Gp. lemaneiformis 981 was generated from 13,825 scaffolds (≥500 bp) with an N50 length of 30,590 bp, accounting for approximately 91% of this algal genome. A total of 38.73 Mb of scaffold sequences were repetitive, and 9281 protein-coding genes were predicted. A phylogenomic analysis of 20 genomes revealed the relationship among the Chromalveolata, Rhodophyta, Chlorophyta and higher plants. Homology analysis indicated phylogenetic proximity between Gp. lemaneiformis and Chondrus crispus. The number of enzymes related to the metabolism of carbohydrates, including agar, glycoside hydrolases, glycosyltransferases, was abundant. In addition, signaling pathways associated with phytohormones such as auxin, salicylic acid and jasmonates are reported for the first time for this alga.

CONCLUSION

We sequenced and analyzed a draft genome of the red alga Gp. lemaneiformis, and revealed its carbohydrate metabolism and phytohormone signaling characteristics. This work will be helpful in research on the functional and comparative genomics of the order Gracilariales and will enrich the genomic information on marine algae.

Transcriptome profiling of sulfate deprivation responses in two agarophytes Gracilaria changii and Gracilaria salicornia (Rhodophyta)

Seaweeds survive in marine waters with high sulfate concentration compared to those living at freshwater habitats. The cell wall polymer of Gracilaria spp. which supplies more than 50% of the world agar is heavily sulfated. Since sulfation reduces the agar quality, it is interesting to investigate the effects of sulfate deprivation on the sulfate contents of seaweed and agar, as well as the metabolic pathways of these seaweeds. In this study, two agarophytes G. changii and G. salicornia were treated under sulfate deprivation for 5 days. The sulfate contents in the seaweed/agar were generally lower in sulfate-deprivated samples compared to those in the controls, but the differences were only statistically significant for seaweed sample of G. changii and agar sample of G. salicornia. RNA sequencing (RNA-Seq) of sulfate-deprivated and untreated seaweed samples revealed 1,292 and 3,439 differentially expressed genes (DEGs; ≥1.5-fold) in sulfate-deprivated G. changii and G. salicornia, respectively, compared to their respective controls. Among the annotated DEGs were genes involved in putative agar biosynthesis, sulfur metabolism, metabolism of sulfur-containing amino acids, carbon metabolism and oxidative stress. These findings shed light on the sulfate deprivation responses in agarophytes and help to identify candidate genes involved in agar biosynthesis.

Biosynthesis of agar in red seaweeds: A review

Agar is a jelly-like biopolymer synthesized by many red seaweeds as their major cell wall component. Due to its excellent rheological properties, it has been exploited commercially for applications in food, cosmetic, pharmaceutical, biomedical and biotechnology industries. Despite its multiple uses, the biosynthesis of this phycocolloid is not fully understood. The current knowledge on agar biosynthesis is inferred from plant biochemistry and putative pathways for ulvan and alginate biosynthesis in green and brown seaweeds, respectively. In this review, the gaps in our current knowledge on agar biosynthetic pathway are discussed, focusing on the biosynthesis of agar precursors, elongation of agar polysaccharide chain and side chain modification. The development of molecular markers for the screening of desired seaweeds for industrial exploitation is also discussed.