Putative trehalose biosynthesis proteins function as differential floridoside-6-phosphate synthases to participate in the abiotic stress response in the red alga Pyropia haitanensis

Divergent responses of an armored and an unarmored dinoflagellate to ocean acidification

Dinoflagellates, both armored and unarmored, with distinct cell wall difference, are being affected by elevated CO2-induced ocean acidification (OA). However, their specific responses to OA are not well understood. In this study, we investigated the physiological and molecular response of the armored species Prorocentrum obtusidens and the unarmored species Karenia mikimotoi to OA over a 28-day period. The results show that the two species responded differently to OA. Cell growth rate, particulate organic carbon (POC) content, and the activities of C4 pathway enzymes decreased in P. obtusidens under future acidified ocean condition (pH 7.8, 1000 μatm pCO2), but the activities of carbonic anhydrase (CA), ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), and superoxide dismutase (SOD) increased. Whereas cell growth rate, contents of Chl a and PON, and SOD activity altered insignificantly in K. mikimotoi, but contents of POC and total carbohydrate, and the activity of RubisCO increased while the activities of CA and C4 pathway enzymes decreased. Transcriptomic analysis indicates that genes associated with antioxidative response, heat shock protein, proteasome, signal transduction, ribosome, and pH regulation were up-regulated in P. obtusidens but down-regulated in K. mikimotoi. Notably, the synthesis of soluble organic matter (i.e., spermidine and trehalose) was enhanced in K. mikimotoi, thereby regulating intracellular pH and improving stress resistance. This study highlights the divergent response of the armored and unarmored dinoflagellates to OA, with the unarmored dinoflagellate exhibiting a higher ability to withstand this stressor. Therefore, caution should be exercised when predicting the behavior and the eventual fate of dinoflagellates in the future acidified ocean.

Physiological activities, transcriptomes and metabolomes of Pyropia yezoensis conchocelis unveil the roles of pyPGK, pyBCKDHA, and pyDLD in response to freshwater soaking

Freshwater soaking of the conchocelis is often used to reduce yellow spot, white spot, and mud red disease in Pyropia yezoensis. However, the understanding of physiological, transcriptomic, and metabolomic changes for the conchocelis under freshwater stress remains limited. Here, we comprehensively explored the dynamic changes of physiological activities, transcriptomes, and metabolomes of the conchocelis under three points of freshwater stress (0 h, 4 h, and 24 h) and one point of seawater recovery (R2h). We found that the content of photosynthetic pigments, soluble proteins, and photosynthesis performance significantly responded to freshwater stress. Metabolomic analysis identified a total of 24 metabolites, including 15 DAMs, suggesting the metabolites changes in the conchocelis in response to freshwater stress. Additionally, comparative transcriptome and metabolome analyses identified a black co-expression module that was strongly correlated with the DAMs. Furthermore, this module was predominantly enriched in carbohydrate and amino acid metabolism pathways. We found that PyDLD, PyPGK, and PyBCKDHA were key genes in hub-networks, which are potentially involved in changes of leucine, valine, isoleucine, lactate, and floridoside during freshwater stress. These findings reveal the genetic basis of the dynamic changes of physiological activities, transcriptome, and metabolome in the Py. yezoensis conchocelis during freshwater soaking for disease control.

Single-cell transcriptome sequencing revealing the difference in photosynthesis and carbohydrate metabolism between epidermal cells and non-epidermal cells of Gracilariopsis lemaneiformis (Rhodophyta)

The allocation of photoassimilates is considered as a key factor for determining plant productivity. The difference in photosynthesis and carbohydrate metabolism between source and sink cells provide the driven force for photoassimilates' allocation. However, photosynthesis and carbohydrate metabolism of different cells and the carbon allocation between these cells have not been elucidated in Gracilariopsis lemaneiformis. In the present study, transcriptome analysis of epidermal cells (EC) and non-epidermal cells (NEC) of G. lemaneiformis under normal light conditions was carried out. There were 3436 differentially expressed genes (DEGs) identified, and most of these DEGs were related to photosynthesis and metabolism. Based on a comprehensive analysis both at physiological and transcriptional level, the activity of photosynthesis and carbohydrate metabolism of EC and NEC were revealed. Photosynthesis activity and the synthesis activity of many low molecular weight carbohydrates (floridoside, sucrose, and others) in EC were significantly higher than those in NEC. However, the main carbon sink, floridean starch and agar, had higher levels in NEC. Moreover, the DEGs related to transportation of photoassimilates were found in this study. These results suggested that photoassimilates of EC could be transported to NEC. This study will contribute to our understanding of the source and sink relationship between the cells in G. lemaneiformis.

Genome-wide identification of the trehalose-6-phosphate synthase gene family in sweet orange (Citrus sinensis) and expression analysis in response to phytohormones and abiotic stresses

Background

Trehalose-6-phosphate synthase (TPS) is an essential enzyme for synthesizing trehalose and is a significant regulator of plant development and stress response. Sweet orange (Citrus sinensis) is an economically important fruit tree crop and a common transgenic material. At present, little information is available about the TPS gene family in sweet orange.

Methods

The TPS gene family were identified from sweet orange genome by bioinformatics analysis. Additionally, the expression of CisTPS genes was analyzed under phytohormones and abiotic stresses by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR).

Results

Here, eight TPS genes were identified and were found to be randomly distributed in five sweet orange chromosomes. TPS and trehalose-6-phosphate phosphatase (TPP) domains were observed in all CisTPS proteins. The phylogenetic tree showed that CisTPS genes were divided into two subfamilies, and genes in each subfamily had conserved intron structures and motif compositions. The cis-acting elements of CisTPS genes suggested their roles in phytohormone and stress responses. All CisTPS genes were ubiquitously expressed in roots, leaves, and stems, and six members were highly expressed in roots. Expression profiles showed that CisTPS genes exhibited tissue specificity and were differentially expressed in response to phytohormones and abiotic stresses. This study lays a foundation for revealing the functions of the TPS gene family in trehalose regulation in sweet orange, and provides a valuable reference for this gene family in other plants.

Physiological and multi-omics responses of Neoporphyra haitanensis to dehydration-rehydration cycles

BACKGROUND

Seaweeds in the upper intertidal zone experience extreme desiccation during low tide, followed by rapid rehydration during high tide. Porphyra sensu lato are typical upper intertidal seaweeds. Therefore, it is valuable to investigate the adaptive mechanisms of seaweed in response to dehydration-rehydration stress.

RESULTS

A reduction in photosynthetic capacity and cell shrinkage were observed when N. haitanensis was dehydrated, and such changes were ameliorated once rehydrated. And the rate and extent of rehydration were affected by the air flow speed, water content before rehydration, and storage temperature and time. Rapid dehydration at high air-flow speed and storage at - 20 °C with water content of 10% caused less damage to N. haitanensis and better-protected cell activity. Moreover, proteomic and metabolomic analyses revealed the abundance members of the differentially expressed proteins (DEPs) and differentially abundant metabolites (DAMs) mainly involved in antioxidant system and osmotic regulation. The ascorbic acid-glutathione coupled with polyamine antioxidant system was enhanced in the dehydration response of N. haitanensis. The increased soluble sugar content, the accumulated polyols, but hardly changed (iso)floridoside and insignificant amount of sucrose during dehydration indicated that polyols as energetically cheaper organic osmolytes might help resist desiccation. Interestingly, the recovery of DAMs and DEPs upon rehydration was fast.

CONCLUSIONS

Our research results revealed that rapid dehydration and storage at - 20 °C were beneficial for recovery of N. haitanensis. And the strategy to resist dehydration was strongly directed toward antioxidant activation and osmotic regulation. This work provided valuable insights into physiological changes and adaptative mechanism in desiccation, which can be applied for seaweed farming.

Cold stress tolerance of the intertidal red alga Neoporphyra haitanensis

BACKGROUND

Red algae Porphyra sensu lato grow naturally in the unfavorable intertidal environment, in which they are exposed to substantial temperature fluctuations. The strategies of Porphyra to tolerate cold stress are poorly understood.

RESULTS

Herein, investigations revealed that chilling and freezing induced alterations in the physiological properties, gene transcriptional profiles and metabolite levels in the economically important red algae species, Neoporphyra haitanensis. Control samples (kept at 20 °C) were compared to chilled thalli (10 and 4 °C) and to thalli under - 4 °C conditions. Chilling stress did not affect the health or photosynthetic efficiency of gametophytes, but freezing conditions resulted in the arrest of growth, death of some cells and a decrease in photosynthetic activity as calculated by Fv/Fm. Transcriptome sequencing analysis revealed that the photosynthetic system was down-regulated along with genes associated with carbon fixation and primary metabolic biosynthesis. Adaptive mechanisms included an increase in unsaturated fatty acids levels to improve membrane fluidity, an increase in floridoside and isofloridoside content to enhance osmotic resistance, and an elevation in levels of some resistance-associated phytohormones (abscisic acid, salicylic acid, and methyl jasmonic acid). These physiochemical alterations occurred together with the upregulation of ribosome biogenesis.

CONCLUSIONS

N. haitanensis adopts multiple protective mechanisms to maintain homeostasis of cellular physiology in tolerance to cold stress.

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.

Evolution and function of carbohydrate reserve biosynthesis in parasitic protists

Nearly all eukaryotic cells synthesize carbohydrate reserves, such as glycogen, starch, or low-molecular-weight oligosaccharides. However, a number of parasitic protists have lost this capacity while others have lost, and subsequently evolved, entirely new pathways. Recent studies suggest that retention, loss, or acquisition of these pathways in different protists is intimately linked to their lifestyle. In particular, parasites with carbohydrate reserves often establish long-lived chronic infections and/or produce environmental cysts, whereas loss of these pathways is associated with parasites that have highly proliferative and metabolically active life-cycle stages. The evolution of mannogen biosynthesis in Leishmania and related parasites indicates that these pathways have played a role in defining the host range and niches occupied by some protists.

Identification of the trehalose-6-phosphate synthase gene family in Medicago truncatula and expression analysis under abiotic stresses

Trehalose-6-phosphate synthase (TPS) exerts important functions related to plant desiccation tolerance and responses to environmental stimuli. However, in Medicago truncatula, the TPS family has not been reported to date. This study found 11 MtTPS genes in the genome of M. truncatula, which could be divided into two subfamilies: Class I and Class II. All TPS family members have a TPS domain (Glyco transf_20) at the N-terminus and a TPP domain (Trehalose_PPase) at the C-terminus. Interestingly, the genetic structures differ between Class I and Class II, Class I members have more introns than Class II members. Furthermore, transcriptome and real-time PCR analysis showed that five MtTPS genes could be induced by drought, salt or cold. Specifically, MtTPS2, MtTPS8, MtTPS9, MtTPS11 were up-regulated under both drought and salt treatment, particularly, MtTPS8 and MtTPS9 can also be induced by cold, while MtTPS7 only responded to salt stress. In summary, this study provides the foundation for further research on TPS genes in M. truncatula and their regulatory function in response to abiotic stresses.

Comparative Quantitative Proteomics Reveals the Desiccation Stress Responses of the Intertidal Seaweed NEOPORPHYRA haitanensis

Neoporphyra haitanensis is an economically important red seaweed that inhabits upper intertidal zones. The thallus tolerates extreme fluctuating environmental stresses (e.g., surviving more than 80% water loss during low tides). To elucidate the global molecular responses relevant to this outstanding desiccation tolerance, a quantitative proteomics analysis of N. haitanensis under different desiccation treatments as well as rehydration was performed. According to the clustering of expression patterns and the functional interpretation of the 483 significantly differentially expressed proteins, a three-stage cellular response to desiccation stress and subsequent rehydration was proposed. Stage I: at the beginning of water loss, multiple signal transduction pathways were triggered including lipid signaling, protein phosphorylation cascades, and histone acetylation controlling acetate biosynthesis to further modulate downstream hormone signaling. Protein protection by peptidyl-prolyl isomerase and ROS scavenging systems were also immediately switched on. Stage II: with the aggravation of stress, increases in antioxidant systems, the accumulation of LEA proteins, and the temporary biosynthesis of branched starch were observed. Multiple enzymes involved in redox homeostasis, including peroxiredoxin, thioredoxin, ascorbate peroxidase, superoxide dismutase, glutathione peroxidase, and glutathione reductase, were hypothesized to function in specific cellular compartments. Stage III: when the desiccated thalli had rehydrated for 30 mins, photosynthesis and carbon fixation were recovered, and antioxidant activities and protein structure protection were maintained at a high level. This work increases the understanding of the molecular responses to environmental stresses via a proteomic approach in red seaweeds and paves the way for further functional studies and genetic engineering.

Trehalose and (iso)floridoside production under desiccation stress in red alga Porphyra umbilicalis and the genes involved in their synthesis

The marine red alga Porphyra umbilicalis has high tolerance toward various abiotic stresses. In this study, the contents of floridoside, isofloridoside, and trehalose were measured using gas chromatography mass spectrometry (GC-MS) in response to desiccation and rehydration treatments; these conditions are similar to the tidal cycles that P. umbilicalis experiences in its natural habitats. The GC-MS analysis showed that the concentration of floridoside and isofloridoside did not change in response to desiccation as expected of compatible solutes. Genes involved in the synthesis of (iso)floridoside and trehalose were identified from the recently completed Porphyra genome, including four putative trehalose-6-phosphate synthase (TPS) genes, two putative trehalose-6-phosphate phosphatase (TPP) genes, and one putative trehalose synthase/amylase (TreS) gene. Based on the phylogenetic, conserved domain, and gene expression analyses, it is suggested that the Pum4785 and Pum5014 genes are related to floridoside and isofloridoside synthesis, respectively, and that the Pum4637 gene is probably involved in trehalose synthesis. Our study verifies the occurrences of nanomolar concentrations trehalose in P. umbilicalis for the first time and identifies additional genes possibly encoding trehalose phosphate synthases.

Floridoside Exhibits Antioxidant Properties by Activating HO-1 Expression via p38/ERK MAPK Pathway

Floridoside is a low-molecular-weight organic compound, which can be accumulated by red algae under stressful conditions to protect cells via its excellent antioxidant properties. In the present study, we investigated the antioxidant mechanism of floridoside toward human hepatocyte L-02 cells. We found that floridoside had no toxicity to L-02 cells, and no reactive oxidative species were induced by it either. However, the expression of hemoxygenase-1 (HO-1) protein was up-regulated upon exposure to floridoside, and two antioxidant enzymes, superoxide dismutase (SOD) and GSH-Px, were activated by floridoside. Moreover, we investigated the pathway involved in the production of these antioxidants, p38/extracellular signal-regulated kinase (ERK) MAPK-nuclear factor-erythroid-2-related factor 2 (Nrf2) pathway. ERK1/2 and p38 phosphorylation, nuclear translocation of Nrf2, and activation of ARE luciferase activity were observed upon exposure to floridoside. siRNA interference and inhibitor treatment suppressed the HO-1 expression and the phosphorylation of ERK1/2 and p38, respectively. These results indicated that floridoside exerted its antioxidant activity by activating HO-1 expression via p38/ERK MAPK-Nrf2 pathway in human hepatocyte L-02 cells.