In order to sustainably produce seaweed, it is necessary to understand the reproductive strategies of seaweeds, and then to properly conserve and increase the number of seaweed beds. It is thought that seaweeds strictly control the transition
from growth to reproduction by sensing changes in the external environment, but these control mechanisms are still not clear. By understanding the reproductive strategies of seaweeds and their reproductive mechanisms, I hope to contribute to the SDGs
(2 ZERO HUNGER and 14 LIFE BELOW WATER).
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, seaweedis 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.