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Reproductive Mechanisms in Red Algae × SDGs
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to the SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
The proper conservation and cultivation of seaweed beds, as well as an understanding of breeding strategies, are necessary for the sustainable production of seaweed. Because the successful breeding of seaweed requires its reproduction at the optimal time, seaweed is believed to tightly regulate its transition from growth to reproduction by sensing changes in the external environment. However, the mechanisms of controlling such a transition are unknown. Thus, our laboratory is conducting research on the reproductive mechanisms of seaweed, focusing on the red alga Pyropia/Neopyropia, a seaweed that is actively cultivated in the Asian region for its use as a raw material. In addition, based on our findings, we are conducting research on the reproductive mechanisms of the red alga Agarophyton vermiculophyllum, which is an important raw material for agar.
The life cycle of Neopyropia yezoensis
- The main type of seaweed used as a raw material in Japan is Neopyropia yezoensis, and its gametophyte (thallus) is edible. When the gametophyte matures, it forms the antheridium and carpogonium, which are the male and female reproductive organs, respectively. Fertilization occurs when immobile sperm released from the antheridium adheres to the trichogyne formed at the tip of the carpogonium. After fertilization, repeated cell division results in the formation and release of spores called carpospores. After adhering to the substrate, these germinate and become sporophytes (filamentous bodies). Sporophytes are thought to sneak into the shells of mollusks and grow. At maturity, they form conchosporangiums, from which conchospores are released and germinate to form gametophytes. In addition to this, the gametophytes release asexual spores called monospores, which become gametophytes (clones) when germinated.
Research on sexual reproduction mechanisms in Pyropia/Neopyropia
It was found that treatment of Neopyropia yezoensis gametophyte with 1-aminocyclopropanecarboxylic acid (ACC), a precursor of ethylene that is one of the plant hormones in land plants, promotes sexual reproduction. On the other hand, when the same experiment was conducted with ethephon, which is an ethylene generator, the promotion of sexual reproduction, observed with ACC treatment, was absent. Therefore, it was clarified that ethylene, which is known to control fruit maturation, does not act on Neopyropia yezoensis, and its precursor acts as a plant hormone.
In addition, ACC treatment was shown to promote the synthesis of ascorbic acid (vitamin C), which increases oxidative stress tolerance. By culturing ACC-untreated algae in a medium containing hydrogen peroxide (H2O2), oxidative stress causes the pigment to disappear and cells die (left), while ACC-treated algae can survive (right). We believe that this mechanism protects the photosynthetic device from oxidative stress, such as high water-temperature and long-day conditions found in the environment in early spring when Neopyropia yezoensis matures.
• Development of gene function analysis technology in Pyropia/Neopyropia
We are also working on the development of gene transfer technology, which is an important technology as one of the molecular biology methods. For stable expression of the gene introduced in the cells of Neopyropia yezoensis, it is important to use a promoter derived from Neopyropia yezoensis gene, a region that controls gene expression, and to modify the DNA sequence of the transgene to match Neopyropia yezoensis (codon optimization). Currently, we are applying this technology to develop a genome editing technology that can easily destroy the target gene.
The gene construct above using the codon-modified hygromycin resistance gene (Pyaph7) as a selection marker was introduced into Neopyropia yezoensis cells by the particle gun method. Then, by selecting with an antibiotics hygromycin, the target gene (such as PyGUS gene) can be stably expressed. The blue-stained algae are those expressing the PyGUS gene, while the red ones are not.
Comprehensive gene expression analysis using the next-generation sequencer has made it possible to obtain information on genes whose expression increases during sexual reproduction induction by ACC treatment. These genes are expected to function in the formation of sexual germ cells and to contribute to the development of oxidative stress tolerance. Therefore, we plan to analyze the functions of these genes using genome editing, which is currently under development.
Based on the findings obtained from Neopyropia yezoensis, we would like to elucidate the reproductive mechanism of other useful red algae. First, we plan to examine whether the effect of ACC can be seen in other red algae.
Red algae are considered to be primitive eukaryotes based on the fact that fossils similar in morphology and reproductive organs to existing red algae have been discovered in the strata from more than 1 billion years ago, as well as on the molecular phylogenetic analysis using DNA base sequences. Therefore, we believe that knowledge of the reproductive mechanism of red algae is important not only for contributing to their sustainable production but also for understanding the evolution of the reproductive mechanism of eukaryotes.