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Frontiers of Science

Fascinated by the Diversity and Mystery of Plants in Nature

Atsushi Kawakita

Professor, Koishikawa Botanical Garden, University of Tokyo Graduate School of Science

April 1, 2022

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The Koishikawa Botanical Garden, attached to the University of Tokyo’s Graduate School of Science, is the birthplace of modern botany in Japan and a global center for plant research. In his office in the Garden’s main building, regarded widely as a masterpiece of pre-war modernist architecture, with portraits of his predecessors lining the walls, Professor Atsushi Kawakita, the current Director of the Botanical Garden, says that plants are extremely mysterious organisms.

“We humans are animals, so we look at plants as if they were animals too, and try to apply our common wisdom about animals to plants. And we think, ‘Plants aren’t capable of doing anything that animals can.’ But plants are completely different organisms from animals, and if you set aside your common wisdom about animals and look at them from the perspective of plants, you’ll find many more surprises.”

Professor Kawakita specializes in plant ecology, with a particular focus on plant-insect interactions, or mutualistic relationships between insects and plants. Mutualisms are deeply enigmatic relationships between plants and insects in which they help each other survive, as if they have cross-generational purposefulness and memory. One example of a well-known relationship is the mutualism between the Glochidion acuminatum plant (Phyllanthaceae family) and the Epicephala moth (Epicephala genus).

Glochidion acuminatum is an evergreen tree that grows in the Ryukyu Islands of Japan. In May, it produces numerous male flowers at the base of its branches and female flowers at the tips. However, the plant is so plain and unprepossessing that Professor Kawakita says, “from a distance, this plant with its small green flowers doesn’t look like it’s blooming at all.”

“And yet there is an insect, known as the Epicephala moth, that carries pollen to Glochidion acuminatum. This moth rubs its proboscis repeatedly against the stamen, as if it is drinking nectar, but takes no nectar from the flower at all. Epicephala moths collect pollen, not nectar, and under the microscope, we can see that the proboscis of this moth is thickly coated with pollen. This pollen is in turn carried to the female flower of Glochidion acuminatum, where it is transferred to the pistil. After pollination (the deposition of pollen on the pistil), the Epicephala moth pierces the pistil with its ovipositor and lays a single egg in the ovary of the flower.”

We can understand the behavior if the insect was collecting nectar, but collecting pollen from male flowers to pollinate female flowers doesn’t seem to be of any direct benefit to the Epicephala moth. What is the meaning of this behavior, as if these moths had been subcontracted to transport pollen and pollinate the flower?

This phenomenon can be seen in November and December, when the fruits of Glochidion acuminatum ripen. The larva of the Epicephala moth, which grows by feeding on the developing ovules inside the fruit, bores an exit hole in the pericarp when the fruit is ripe and pupates on the ground. The fruit contains six seeds, but only two or three of these are eaten by the larvae of the Epicephala moth, and for Glochidion acuminatum, the remaining seeds are sufficient for its offspring.

So here’s the key. For the larvae to grow inside the fruit of Glochidion acuminatum, the Epicephala moth must transport the pollen to, and pollinate the pistil to help them produce fruit. On the other hand, Glochidion acuminatum has evolved special flowers and fruits that are adapted to the Epicephala moth for it to transport pollen and to pollinate its flowers. This is a win-win situation. What an amazing natural mechanism!

This is the truly mysterious relationship of mutualism, an interaction between insects and plants for their joint survival.

Wonders at the Interface between Plants and Insects

In fact, the first person to discover the mutualistic relationship between Glochidion acuminatum and the Epicephala moth was Professor Makoto Kato, a mentor of Kawakita’s when he was a student at Kyoto University.

“I had a lecturer who taught a fascinating course, and once he took our group to the mountains north of Kyoto to make up for a canceled class. His eyes twinkled as he told us all about this plant and its intriguing relationship to insects.”

The lecturer with the interesting story was Professor Makoto Kato, who inspired the young man to ride his bicycle every weekend to the mountains of Kyoto to collect plants.

“Almost every week, I’d go to the mountains and seek out plants whose names I didn’t know and look them up in a field guide. When I’d saved up enough money from my part-time job at a ramen noodle shop, I’d pitch my tent on Mount Kushigata in Yamanashi Prefecture or Mount Shirouma in Shinshu during summer vacation and stay on the mountain. Returning to my lodgings, the plants I’d collected would be tucked between the pages of my Shonen Jump manga. That’s because manga paper absorbs moisture really well (laughs)! More than anything else, I wanted to find plants that I hadn’t seen before, so I set my sights on places where such plants are likely to grow. I used the Seishun 18 Kippu discount train ticket all the time, and tried to go as far as I could on slow trains (laughs).”

A young man who had spent his junior and senior high school years obsessed with soccer suddenly became a botanist, roaming around the fields and mountains after an encounter with an interesting lecturer.

It was in his second year of university that young Kawakita made a new discovery that became the topic of his first paper.

“It was winter, and I asked Professor Kato what should I do at this time of year. He suggested that I check out a plant known as Balanophora, which grows in Kagoshima and the Ryukyu Islands, because the insects that carry its pollen were yet to be identified. I went to Okinawa Island with my tent and spent many nights in the mountains, constantly observing the flowers in the forest. It’s a tiny plant that grows to about 10 centimeters high above the ground.”

Holed up in the mountains for two weeks, he’d go down to the village and soak himself in a public bath once every three days. Other than that, he would settle down in front of the plant to observe it.

“The insects might come at any time, so I’d have to be on watch day and night, go to bed, wake up, and start watching again. I waited for a long time, feeling sleepy, and then an insect came. All at once I was excited, taking photographs and staring intently to see what it was doing. The moment was captivating. That’s why I don’t mind staying awake to watch plants: in fact I enjoy it.”

This research continued into his junior year, and Kawakita wrote a paper on his new discovery that a moth called the pyralid moth is responsible for pollinating Balanophora.

“Many people think that research on plants focuses only on plants, and research on insects focuses only on insects, but there are many phenomena at the interface between plants and insects that are yet to be detected by anyone. My mentor Professor Kato has been collecting shells since he was a child, so he understands plants, insects, and marine invertebrates. And now he is discovering new species of plants, insects, shellfish, and fish. The fact that he was able to notice the mutualistic relationship between Glochidion acuminatum and the Epicephala moth was due to his deep knowledge of insects as well as plants. I benefited greatly by being taught by someone like that.”

The Importance of First-hand Observations

However, documenting the natural historical relationships between plants and insects is not the main focus of Professor Kawakita’s research. Why are there so many different forms of plants? What kind of evolutionary path did a plant take to develop a particular form? Professor Kawakita is also seeking to answer these and other fundamental questions.

“The flowers of Glochidion acuminatum are so small and plain that even a botanist would have to be a bit eccentric to even think of studying them. But these flowers, which look as if they are vestigial, were actually the result of a unique evolutionary pathway that led to a mutualistic relationship with a particular insect that is the Epicephala moth. This is just one example of how the characteristics of flowers are inevitably determined by their relationships with animals, such as insects, or their environment.”

For example, Codonopsis lanceolata, a member of the bellflower family, has bell-shaped flowers that point downward. Only wasps are attracted to these flowers and pollinate them by carrying pollen on their backs. The flowers contain copious amounts of nectar, but for some reason ants never come to collect it. Curious, I investigated this phenomenon with one of my students and found out that the surface of the Codonopsis lanceolata flower was so slippery that ants could not get a foothold on it; they would simply fall off. In other words, the flowers have evolved to have a slippery surface to prevent ants from stealing the nectar, because they are not useful for pollination.

It’s not only flowers; the shapes of leaves are also mysterious.

The leaves of the Isodon umbrosus var. hakusanensis plant, which belongs to the Lamiaceae family, have an unusual shape, with a deep notch on each side. We believe that this leaf shape helps to confuse the insect known as the leaf-rolling weevil. The leaf-rolling weevil folds a leaf in two and rolls it up from the tip to make a ‘cradle’ for its young. However, this weevil does not try to roll up the leaves of Isodon umbrosus.

The leaf-rolling weevil walks over a leaf, ‘surveying’ it before building its cradle. After making sure that there is nothing amiss with the shape or size of the leaf, it begins to make the cradle. However, because of the deep notches in the leaves of the Isodon umbrosus, the leaf-rolling weevil is unable to walk straight ahead along the edge of the leaf. So they know that this is not a normal leaf and therefore not suitable for making a cradle.

Professor Kawakita says that although many such strange relationships between insects and plants have been found, there is still much more we don’t know.

“None of our research is cutting-edge, but there is so much that we don’t know and don’t realize about plant diversity, even at the level of the naked eye. For example, ginkgo leaves, cherry leaves, and maple leaves have completely different shapes, but we still don’t know why they do. Wouldn’t it be interesting if we could understand what is ‘commonplace’ about plants in relation to insects?”

Understanding the evolutionary pathways of plant morphology and the driving forces of evolution naturally demands a worldwide perspective, and Professor Kawakita has traveled to many countries and regions as if he were a young man again, roaming in the fields and mountains.

“New Caledonia is very interesting because it is home to a unique group of plants and the oldest plant lineages in the world. Peru in South America has a completely different flora. Whenever I’m stuck in the lab I always get itchy feet, just as I did when I was a student. The me in the lab is like an imposter (laughs). As there’s a limit to what you can come up with seated at your bench, I think about something that someone else is doing somewhere in the world, but those thoughts don’t go any further. When you actually go to research a plant in situ, you see other plants growing next to it and notice something different, and the research that grows from there can become a valuable topic. My main aim is to understand the diversity of plants in nature, but the path to understanding is by no means systematic.”

And so it is again that we meet at the end of our journey with marvel and mystery.

“The only way to appreciate these strange relationships between plants and animals in reality is to believe that they are the result of evolution, and when I think about that process, there are many times when I wonder if they could have come about only through a series of mutations. Mount Everest was formed over a tremendous amount of time by the collision of tectonic plates. We may not be able to follow the individual steps of this continuous process, but living things change continually on a similar time scale, giving us a view of incredibly wonderful and fascinating phenomena that are the result of evolution.”

His message to students was, of course, delivered in inimitable Kawakita style.

“We all see a lot of things on the Internet these days, but it’s an artificial world built on the knowledge of others. You should actually go to places and see things for yourself first-hand while you are still a student; it’s the best way to spend your student days.”

The gleam in his eyes was that of a young man who once roamed throughout the mountains of Kyoto on his bicycle.

※Year of interview:2022
Interview/Text: Minoru Ota
Photography: Junichi Kaizuka

Atsushi Kawakita
Professor, Koishikawa Botanical Garden, University of Tokyo Graduate School of Science
Graduated from the Faculty of Science, Kyoto University in 2002 and completed his doctorate at the Graduate School of Human and Environmental Studies, Kyoto University in 2007. After working as an Assistant Professor at the same Graduate School and an Associate Professor at Kyoto University’s Center for Ecological Research, he was appointed as a Professor at the Koishikawa Botanic Garden, University of Tokyo Graduate School of Science, in 2018.
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