Sexual Reproduction via a 1-Aminocyclopropane-1-Carboxylic Acid-Dependent Pathway Through Redox Modulation in the Marine Red Alga Pyropia yezoensis (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.

The role of plant hormones on the reproductive success of red and brown algae

Seaweeds or macroalgae are important primary producers that serve as a habitat for functioning ecosystems. A sustainable production of macroalgae has been maintained by a diverse range of life cycles. Reproduction is the most dynamic change to occur during its life cycle, and it is a key developmental event to ensure the species' survival. There is gradually accumulating evidence that plant hormones, such as abscisic acid and auxin, have a role on the sporogenesis of brown alga (Saccharina japonica). Recent studies reported that 1-aminocylopropane-1-carboxylic acid, an ethylene precursor, regulates sexual reproduction in red alga (Neopyropia yezoensis) independently from ethylene. In addition, these macroalgae have an enhanced tolerance against abiotic and biotic stresses during reproduction to protect their gametes and spores. Herein, we reviewed the current understanding on the regulatory mechanisms of red and brown algae on their transition from vegetative to reproductive phase.

Phospholipase D activation is required for 1-aminocyclopropane 1-carboxylic acid signaling during sexual reproduction in the marine red alga Neopyropia yezoensis (Rhodophyta)

BACKGROUND

1-aminocyclopropane 1-carboxylic acid (ACC) is the immediate precursor of the plant hormone ethylene. However, recent studies have suggested that ACC also acts as a signaling molecule to regulate development and growth independently from ethylene biosynthesis. In red algae, ACC stimulates the switch from a vegetative to a sexual reproductive phase. However, despite evidence that ACC signaling in plants and algae is widespread, the mechanistic basis of the ACC signaling pathway remains unknown.

RESULTS

We demonstrate that exogenous ACC increased the activity of phospholipase D (PLD) and induced the accumulation of PLD transcripts in the marine red alga Neopyropia yezoensis. The product of PLD, the lipid second messenger phosphatidic acid (PA), also increased in response to ACC. Furthermore, the pharmacological inhibition of PLD by 1-butanol blocked ACC-induced spermatangia and carpospore production, but the inactive isomer t-butanol did not. In addition, 1-butanol prevented ACC-induced growth inhibition and inhibited transcript accumulation of genes upregulated by ACC, including extracellular matrix (ECM)-related genes, and alleviated the transcriptional decrease of genes downregulated by ACC, including photosynthesis-related genes.

CONCLUSIONS

These results indicate that PLD is a positive regulator of sexual cell differentiation and a negative regulator of growth. This study demonstrates that PLD and its product, PA, are components of ACC signaling during sexual reproduction in N. yezoensis.

Transcriptome Profiling in the Marine Red Alga Neopyropia yezoensis Under Light/Dark Cycle

Many organisms are subjected to a daily cycle of light and darkness, which significantly influences metabolic and physiological processes. In the present study, Neopyropia yezoensis, one of the major cultivated seaweeds used in "nori," was harvested in the morning and evening during light/dark treatments to investigate daily changes in gene expression using RNA-sequencing. A high abundance of transcripts in the morning includes the genes associated with carbon-nitrogen assimilations, polyunsaturated fatty acid, and starch synthesis. In contrast, the upregulation of a subset of the genes associated with the pentose phosphate pathway, cell cycle, and DNA replication at evening is necessary for the tight control of light-sensitive processes, such as DNA replication. Additionally, a high abundance of transcripts at dusk encoding asparaginase and glutamate dehydrogenase imply that regulation of asparagine catabolism and tricarboxylic acid cycle possibly contributes to supply nitrogen and carbon, respectively, for growth during the dark. In addition, genes encoding cryptochrome/photolyase family and histone modification proteins were identified as potential key players for regulating diurnal rhythmic genes.

Ectopic expression of bacterial 1-aminocyclopropane 1-carboxylate deaminase in Chlamydomonas reinhardtii enhances algal biomass and lipid content under nitrogen deficit condition

1-Aminocyclopropane-1-carboxylate (ACC) deaminase is a well-known bacterial producing enzyme that helps plants to overcome stress conditions by modulating ethylene biosynthesis. However, the functional role of ACC deaminase and ethylene in microalgae during stress remains to be explored. In this study, to investigate the role of ACC deaminase (acds) from Pseudomonas putida UW4 in enhancing the biomass and lipid content of Chlamydomonas under nitrogen deficit condition. The synthetic codon-optimized acds gene was cloned into vector pChlamy_4 and introduced into Chlamydomonas. Results indicated that Chlamydomonas-expressing acds lines showed significant tolerance to nitrogen-deficit by reducing the ethylene content. The biomass, chlorophyll content and photosynthetic activity of acds-expressing lines were significantly increased during nitrogen deficit condition. Moreover, the intracellular lipid and fatty acid content were much higher in acds-expressing lines than the wild-type. In terms of stress alleviation, the transgenic lines displayed increased antioxidant enzymes, reduced ROS and lipid peroxidation levels.

Distinct Functions of Ethylene and ACC in the Basal Land Plant Marchantia polymorpha

Ethylene is a gaseous phytohormone involved in various physiological processes, including fruit ripening, senescence, root hair development and stress responses. Recent genomics studies have suggested that most homologous genes of ethylene biosynthesis and signaling are conserved from algae to angiosperms, whereas the function and biosynthesis of ethylene remain unknown in basal plants. Here, we examined the physiological effects of ethylene, an ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) and an inhibitor of ethylene perception, silver thiosulfate (STS), in a basal land plant, Marchantia polymorpha. M. polymorpha plants biosynthesized ethylene, and treatment with high concentrations of ACC slightly promoted ethylene production. ACC remarkably suppressed the growth of thalli (vegetative organs) and rhizoids (root-hair-like cells), whereas exogenous ethylene slightly promoted thallus growth. STS suppressed thallus growth and induced ectopic rhizoid formation on the dorsal surface of thalli. Thus, ACC and ethylene have different effects on the vegetative growth of M. polymorpha. We generated single and double mutants of ACC synthase-like (ACSL) genes, MpACSL1 and MpACSL2. The mutants did not show obvious defects in thallus growth, ACC content and ethylene production, indicating that MpACSL genes are not essential for the vegetative growth and biosynthesis of ACC and ethylene. Gene expression analysis suggested the involvement of MpACSL1 and MpACSL2 in stress responses. Collectively, our results imply ethylene-independent function of ACC and the absence of ACC-mediated ethylene biosynthesis in M. polymorpha.

Ethylene-independent functions of the ethylene precursor ACC in Marchantia polymorpha

The plant hormone ethylene has many roles in growth and development¹. In seed plants, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is converted into ethylene by ACC oxidase (ACO), and treatment with ACC induces ethylene responses². However, non-seed plants lack ACO homologues^3-8, which led us to examine the relationship between ACC and ethylene in the liverwort Marchantia polymorpha. Here, we demonstrate that ACC and ethylene can induce divergent growth responses in Marchantia. Ethylene increases plant and gemma size, induces more gemma cups and promotes gemmae dormancy. As predicted, Mpctr1-knockout mutants display constitutive ethylene responses, whereas Mpein3-knockout mutants exhibit ethylene insensitivity. Compared with the wild type, Mpctr1 gemmae have more and larger epidermal cells, whereas Mpein3 gemmae have fewer and smaller epidermal cells, suggesting that ethylene promotes cell division and growth in developing gemmae. By contrast, ACC treatment inhibits gemma growth and development by suppressing cell division, even in the Mpein3-knockout alleles. Knockout mutants of one or both ACC SYNTHASE (ACS) gene homologues produce negligible levels of ACC, have more and larger gemma cups, and have more-expanded thallus branches. Mpacs2 and Mpacs1 Mpacs2 gemmae also display a high frequency of abnormal apical notches (meristems) that are not observed in ethylene mutants. These findings reveal that ethylene and ACC have distinct functions, and suggest that ACC is a signalling molecule in Marchantia. ACC may be an evolutionarily conserved signal that predates its efficient conversion to ethylene in higher plants.