The liverwort oil body is formed by redirection of the secretory pathway
The cell is the basic unit of life and is surrounded by a confining membrane called the plasma membrane. Inside the cell, there are various organelles bound by the membrane, each with a variety of different functions. How these organelles, each with different functions, developed during evolution is still unknown. This phenomenon has fascinated many researchers.
In the study published in Nature Communications, the evolutionary relationship between two different organelles in liverwort cells is revealed: the cell plate, which divides cells during cell division, and the oil body, which serves as a reservoir for various chemicals.
The mechanism of exchanging proteins and lipids between organelles is important for cellular activities and is called "membrane traffic. A research group led by Professor Takashi Ueda and Assistant Professor Takehiko Kanazawa at the National Institute of Basic Biology has focused on a group of proteins, SYP1, that play a key role in membrane traffic, using the liverwort Marchantia polymorpha as a research target.
They found that MpSYP12A, a protein belonging to the SYP1 group of liverworts, is necessary for the formation of cell plates. They also found that MpSYP12B, a protein very similar to MpSYP12A, acts only in cells containing oil bodies and targets the membranes of oil bodies.
By studying how MpSYP12B is localized to the membranes of oil bodies, the team showed that during oil body formation, the direction of the secretory pathway, which is normally sent along the plasma membrane and the extracellular space, is redirected to the oil bodies.
Professor Takashi Ueda, leader of the research group, said, "Although a theoretical framework has been proposed for the acquisition of organelles through the expansion and new functionalization of mechanical components of membrane traffic, it is still almost unknown how these organelles arose in the course of evolution. Our findings provide strong empirical support for this hypothesis, namely the organelle paralogy hypothesis."
In addition, the team identified the master regulator of oil body formation, MpERF13. loss of MpERF13 function resulted in the complete loss of oil bodies, while enhanced MpERF13 function resulted in the overproduction of oil bodies throughout the plant.
The group also investigated the biological significance of the oil bodies, which was previously not well understood. Dr. Takehiko Kanazawa conducted an experiment in which starving pill bugs were placed in petri dishes containing liverworts without oil bodies or with overproduced oil bodies, and their herbivory was compared.
Reflecting on his undergraduate days, Dr. Takehiko Kanazawa, the first author of the article, said, "When I started studying this plant, I had eaten some liverwort. It tasted really awful. This extreme experience helped me come up with the idea of exploring the biological functions of the oil body with the pharmacophore assay."
Liverworts without oil bodies were eaten by pill bugs, but liverworts with increased numbers of oil bodies were not, suggesting that oil bodies function to prevent herbivores such as arthropods.
Oil bodies aggregate a variety of compounds with divergent biological activities, some of which have been reported to have antibacterial, anticancer, and/or antiviral activities. The mechanisms of oil body formation and the genomes involved in oil body formation revealed in this study could be applicable to the efficient production of such useful compounds.
