Meet the authorInterview on 20 March 2022

Masanori Hiraoka (Kochi University, Japan)
See Kinoshita Y. et al. (2022) Cytologia 87, 17-22

What is your background and motivation for this experiment?
Currently, plants are an important key to realize a low-carbon economy and a resource
recycling bioeconomy. Plants build their bodies by collecting carbon from CO2 released into
the environment by photosynthesis. The plant biomass thus produced becomes anindustrially usable material again. The development of technology for producing plantbiomass with a high efficiency and low energy input is required. We are working on the growth
physiology of algae, which have the highest growth rates among plants. There are two typesof algae, unicellular microalgae and multicellular macroalgae, and microalgae with a highgrowth rate have been explored to optimize the economics of the biomass production
process, subsequently less research effort has beenprioritized to macroalgae which have relatively low growth
rates. However, microalgae require much energy to beharvested from the liquid medium, and their industrial use islimited. On the other hand, macroalgae can be easilycollected from liquid media in a net or similar structure.
Highly efficient algal biomass production systems can beconstructed if both advantages of micro- and macroalgae
are utilized. From this point of view, our research theme isto elucidate a mechanism of switching between unicellular
proliferation and multicellular proliferation of algae in orderto improve the efficiency of algae biomass production.
Is it possible to switch the growth mode of multicellularplants and make them proliferate in a unicellular state likemicroalga? In 2017, we discovered a phenomenon in which
the somatic cells of the marine leafy multicellular alga

Fig. 1. Young leafy multicellular plantof Monostroma latissimum cultured in
the presence of the morphogens.

Gayralia oxysperma and the closely
relatedspeciesMonostroma

latissimum proliferate exponentiallylike microalgae in a unicellular state.
Normally, in the sea, both species
grow in the form of thin blades with asingle cell layer of several tens ofcentimeters in length (Fig. 1). It is
known that morphogenesis-inducing
factors (morphogens) such as thallusin

Fig. 2. Amorphous aggregates of Gayralia oxysperma cells
cultured in the absence of the morphogens.

produced by specific marine bacteria induce the development of the multicellularity of thesealgae. However, in laboratory culture with artificial seawater in the absence of morphogens
both species lose their typical leafy morphology and make an amorphous aggregate of cells(Fig. 2). Previous studies have focused on the multicellularity induced by morphogens, so
the cell masses that failed to develop multicellularization had been treated as "abnormal"
cells and subsequently excluded from studies. I wondered what would happen if I continued
to culture a small amount of the abnormal cell mass,
so I added inorganic nutrients to the medium lacking
morphogens and cultured it in a flask with aeration.
Then, in about a week, the medium of the flask turned
into a dense green suspension (Fig. 3). This is themoment when I first realized that a multicellular plantcan vigorously proliferate exponentially in a unicellularstate. I was surprised by their flexible growth potential.
The present study was designed to investigate inmoredetailhow

thisunicellularproliferation

phenomenon occurs. In particular, we investigatedthe developmental potential of two forms of cells,
blade cells and rhizoid cells, that are found in cellmasses that have failed to become multicellular.

Fig. 3. Unicellular proliferation of multicellular
Monostroma in 300mL flask.

What is the appeal and importance of Gayralia oxysperma as an experimentalmaterial?
Volvocales are well-studied as algae that model the evolution from single cell tomulticellular organisms. However, it is not possible to isolate somatic cells from themulticellular bodies of the Volvocales and proliferate them as unicellular organisms like

Chlamydomonas which is a unicellular member of the Volvocales. On the other hand,
Gayralia oxysperma is a multicellular plant that is easily induced to grow in a unicellular state
by a simple method of culturing in a medium lacking morphogens. Using such a method, it ispossible to isolate and examine the somatic or blade cells that have increased in this way
one by one. Furthermore it is also possible to carry out comparative studies in the case ofmulticellular proliferation and the case of unicellular proliferation in a cloned cell population
having the same genome. Such Gayralia and related species are expected to become newmodel organisms for studies on evolution, growth physiology, and developmental biology ofmulticellular plants.
What were your challenges or innovations in publication of this paper?
This is the first paper focusing on developmental ability of the cell masses that failed todevelop multicellularization in Gayralia. Morphologically there are at least two types of theGayralia cells. The round blade cells that make up the leaf part that occupies most of themulticellular form, and the elongated rhizoid cells that make up the small holdfast. These twotypes of cells were isolated from the cell mass and their developmental potential wasexamined. The blade cells divided to produce blade cells and/or rhizoid cells. This indicatesthat the blade cells retain totipotency. On the other hand, the rhizoid cells did not have ability
to divide but only have the ability to elongate. Surprisingly, they continued to lengthen beyond
1 mm and some of them became a giant cell up to a maximum of 3 mm. Currently, we areinvestigating the limit of how far such cells will lengthen. The findings obtained in this study
will contribute to the field of developmental biology of plant multicellularization, but may be
industrially applicable. For example, cellulose nanofiber (CNF) is attracting attention as anext-generation industrial material for the bioeconomy, but if the giant rhizoid cells can bemass-produced, it could be used as a raw material for CNF. In addition, such millimeter-sizedcells can be easily collected on a fine mesh, implying various potential bioindustrialapplications.