Plants — Living Organisms of Mystery

Wonder of Storing Water for Growth

Yoshihisa Oda
Assistant Professor,
Department of Biological Sciences,
Graduate School of Science

It is yet to be determined how plants actually live.

Plant cells look like worms or grains of rice. They seem like long, thin, transparent living creatures that exude beautiful colors. Some plant cells are surrounded by blue threads and others by purple threads. A closer look, however, shows that while some cells are either only purple or blue, some cells are surrounded by both blue and purple threads. What exactly does this tell us about cells?

Plant cells are much larger than animal cells. Although some differences appear depending on the cells, plant body cells are usually long and thin (some are 100μm), whereas the average length of animal body cells is 10μm. Plants, especially trees, are generally bigger than animals. For example, an adult giraffe, the tallest animal on earth, is 6m tall, but some sequoias in North America grow as tall as 100m. Even though they are both living organisms, what causes this difference? Yoshihisa Oda, an associate professor at the Laboratory of Cellular Biochemistry in the Department of Biological Sciences, says, "Not much is known about plants such as flowers, grass, or trees although they have always been around human beings. In our laboratory, we are working to clarify how plants live."

Hard cell walls and holes permeable to water exist side-by-side

Cultivated plant cells changing into xylem cells: The blue fibrous structure is a cytoskeleton called actin fiber. The pink figure is a hard cell wall, which is characteristic of xylem cells. Once this cell wall is made, the cell dissolves its contents and the actin fiber (the cell in the middle) disappears.

How are trees able to stand on the ground as they grow older even though they do not have bones like animals? Knowledge about how plants live may be useful for improving agriculture in the future, and may be of help for making a better environment for plant growth. Moreover, since his laboratory belongs to the School of Science, Oda's research naturally also plays a role in the scientific pursuit of truth about the natural world.

We can learn about plant life by observing their cells. A crucial difference between animal and plant cells is that plant cells grow larger by absorbing water. Water can be stored in a place called a vacuole, which is located in a cell and looks like a bag. Plant cells also divide as do animal cells. Oda concentrates on the mechanisms that allow plants to grow bigger by absorbing water.

A tree is made of hard cells called xylem cells. The surfaces of cells are called cell walls. They are as solid as actual walls, and they are equivalent to animal bones. They become hard and support the tree. However, there are elements that are permeable to water in xylem cells; these elements are connected together into long tubes called xylem vessels. Their growth process is well known. While slender cell walls with big holes develop in young tissues so that the cells can grow larger, the holes become smaller in developed tissues and the cells become hard. Old trees that are several decades old become harder and stronger as xylem cells accumulate in them. Still, water can be carried up to the tree tops thanks to the xylem vessels that allow water to run through them.

How are xylem cells with holes in them created when there are also other cells that do not have holes? Which xylem cells grow with big holes and which genes work effectively for that? The key to answering these questions is the protein called a transcription factor that replicates xylem cells. Oda's group noticed that transcription factors seemed to change normal cells into xylem cells. Therefore, they thought that they could make xylem cells replicate in cells by using transcription factors. They succeeded in making transcription factors by dosing animal hormones on doctored genes.

Xylem cells make hard cell walls and water holes as they grow larger

The colors of the cells that look like worms in the image can be observed during the process of xylem cell wall generation. Oda's team has been culturing xylem cells in their laboratory. While being cultured, cells are in bits and pieces; they do not form any plant-like shapes. However, the cell growth can be clearly observed since cell divisions occur easily in culture fluid.

In plant cell division, unlike animal cell division, the cell membrane as well as the cell wall separate each cell into two. While an animal cell splits into two after being constricted in the middle, a plant cell forms a cell wall in the center of it. Furthermore, two cell walls are produced, which form a sandwich-like structure with cell membranes. Then, with the cell walls at the center, two different cells are produced.

When transcription factors are produced by adding animal hormones to these cultivated cells, xylem cells are steadily produced one after another. The fibrous form in the photo, which looks like a blue string, is a protein polymer called actin fiber; it is a kind of cytoskeleton. Actin fiber stimulates the growth of xylem cells and, as a result, hard cell walls are produced. In this image, we can see that the pink cell walls are increasing little by little.

In the image, one cell has become quite pink and is interspersed with black spaces. These spaces that have not turned pink will become holes that absorb water. Cytoskeletons disappear as xylem cells dissolve their contents; they become hollow and water runs through them. If xylem cells change completely into xylem vessels, they die as cells, just like the dead cells in tree rings and cut-down limber. Trees let water run through them, and they become very hard.

We could change trees as we wish

Details about how normal cells are changed into xylem cells have not been clarified yet. Oda expects that cell hardness can be controlled and tree properties changed freely if this mechanism is completely clarified. Soft trees with good breathability could be created in the future. Or it could be possible to grow trees more efficiently within a shorter period of time. Oda hopes that his research will lead to a clearer elucidation of the basic structure of life and to a firm understanding of this particular life mechanism of plants, and will show students the pleasure of stimulating one's intellectual curiosity.

Oda's research area is biological sciences, and the University of Tokyo also has related departments: the Department of Biophysics and Biochemistry focuses more on the molecular level of substances, and the Department of Bioinformatics and Systems Biology handles large-scale data from the life sciences using computer methodologies. The biological sciences study living organisms from the macro- to the micro-levels, including the ecosystem, making it the closest field to living organisms.