Analysis of ethylene-induced gene regulation during carposporogenesis in the red seaweed Grateloupia imbricata (Rhodophyta)

Identification of reproductive sex-biased gene expression in Asparagopsis taxiformis (lineage 6) gametophytes

The sub-tropical red seaweed Asparagopsis taxiformis is of significant interest due to its ability to store halogenated compounds, including bromoform, which can mitigate methane production in ruminants. Significant scale-up of aquaculture production of this seaweed is required; however, relatively little is known about the molecular mechanisms that control fundamental physiological processes, including the regulatory factors that determine sexual dimorphism in gametophytes. In this study, we used comparative RNA-sequencing analysis between different morphological parts of mature male and female A. taxiformis (lineage 6) gametophytes that resulted in greater number of sex-biased gene expression in tips (containing the reproductive structures for both sexes), compared with the somatic main axis and rhizomes. Further comparative RNA-seq against immature tips was used to identify 62 reproductive sex-biased genes (59 male-biased, 3 female-biased). Of the reproductive male-biased genes, 46% had an unknown function, while others were predicted to be regulatory factors and enzymes involved in signaling. We found that bromoform content obtained from female samples (8.5 ± 1.0 mg·g-1 dry weight) was ~10% higher on average than that of male samples (6.5 ± 1.0 mg·g-1 dry weight), although no significant difference was observed (p > 0.05). There was also no significant difference in the marine bromoform biosynthesis locus gene expression. In summary, our comparative RNA-sequencing analysis provides a first insight into the potential molecular factors relevant to gametogenesis and sexual differentiation in A. taxiformis, with potential benefits for identification of sex-specific markers.

H2O2 drives the transition from conchocelis to conchosporangia in the red alga Pyropia haitanensis with promotion facilitated by 1-Aminocyclopropane-1-carboxylic acid

The Bangiales represent an ancient lineage within red algae that are characterized by a life history featuring a special transitional stage from diploid to haploid known as the conchosporangia stage. However, the regulatory mechanisms governing the initiation of this stage by changes in environmental conditions are not well understood. This study analyzed the changes in phytohormones and H2O2 content during the development of conchosporangia. It also compared the gene expression changes in the early development of conchosporangia through transcriptome analysis. The findings revealed that H2O2 was shown to be the key signal initiating the transition from conchocelis to conchosporangia in Pyropia haitanensis. Phytohormone analysis showed a significant increase in 1-aminocylopropane-1-carboxylic acid (ACC) levels during conchosporangia maturation, while changes in environmental conditions were found to promote the rapid release of H2O2. H2O2 induction led to conchosporangia development, and ACC enhanced both H2O2 production and conchosporangia development. This promotive effect was inhibited by the NADPH oxidase inhibitor diphenylene iodonium and the H2O2 scavenger N, N'-dimethylthiourea. The balance of oxidative-antioxidative mechanisms was maintained by regulating the activities and transcriptional levels of enzymes involved in H2O2 production and scavenging. Transcriptome analysis in conjunction with evaluation of enzyme and transcription level changes revealed upregulation of protein and sugar synthesis along with modulation of energy supply under the conditions that induced maturation, and exogenous ACC was found to enhance the entire process. Overall, this study demonstrates that ACC enhances H2O2 promotion of the life cycle switch responsible for the transition from a vegetative conchocelis to a meiosis-preceding conchosporangia stage in Bangiales species.

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.

Screening on the Presence of Plant Growth Regulators in High Biomass Forming Seaweeds from the Ionian Sea (Mediterranean Sea)

The use of seaweed as plant biostimulants is a solution for sustainable agriculture. The present study aims to quantify and compare the presence of plant growth regulators (PGRs) in four genetically labeled macroalgae growing in the Ionian Sea. Species were selected because they produce abundant biomass, disturbing ecological equilibrium and anthropic activities. We measured the content of gibberellic acid (GA3), kinetin (KN), indoleacetic acid (IAA), abscisic acid (ABA) and indole butyric acid (IBA). The method applied was modified from the literature to obtain simultaneously different PGRs from seaweed biomass in a shorter period of time. Among results, it is notable that Hypnea corona Huisman et Petrocelli (Rhodophyta) showed higher GA3 concentration, while in Spyridia filamentosa (Wulfen) Harvey (Rhodophyta), higher KN, IBA, IAA and ABA contents were recorded. The latter species displayed an interesting profile of PGRs, with an IAA value comparable with that reported in Ascophyllum nodosum (Linnaeus) Le Jolis (Ochrophyta), which is currently used as a source of plant biostimulants in agriculture. Macroalgae thrive abundantly in nutrient-rich environments, such as anthropized coastal areas affecting human economic activities. Consequently, environmental agencies are forced to dredge algal thalli and discard them as waste. Any use of unwanted biomass as an economic product is highly desirable in the perspective of ecosustainable development.

Expression of Genes Related to Carrageenan Synthesis during Carposporogenesis of the Red Seaweed Grateloupia imbricata

Carrageenan, the foremost constituent of extracellular matrix of some rhodophyta, is a galactan backbone with a different number of sulphate groups attached. Variations of degree of sulphation are associated with different types of carrageenans, which vary according to seaweed life cycles, and have consequences for the exploitation of this raw material. In this work, we used three well-recognised stages of development thalli and two stages of cystocarp maturation to analyse genes that encode addition and elimination of sulphate groups to cell-wall galactan of the red seaweed Grateloupia imbricata. Expressions of carbohydrate sulfotransferase and galactose-6 sulfurylase and genes encoding stress proteins such as cytochrome P450 and WD40, were examined. Results showed that transcript expression of carbohydrate sulfotransferase occurs at all stage of thalli development. Meanwhile galactose-6 sulfurylase expressions displayed different roles, which could be related to a temporal regulation of cystocarp maturation. Cytochrome P450 and WD40 are related to the disclosure and maturation of cystocarps of G. imbricata. Our conclusion is that differential expression of genes encoding proteins involved in the sulphation and desulphation of galactan backbone is associated with alterations in thalli development and cystocarp maturation in the red seaweed Grateloupia imbricata. Exploitation of industry-valued carrageenan will depend on insight into gene mechanisms of red seaweeds.