I am interested in how plants adapt to and evolve in the space environment. Previous spaceflight experiments have confirmed that, as long as the environment is controlled with the right hardware, seeds can germinate, the resulting seedlings can grow, and the mature plants can bloom and bear fruit in space. However, the degree of growth is a different matter. A microgravity environment has a great impact on plant growth and development, and it eventually affects plant yield.
When plants migrated from the sea approximately 450 million years ago, they became land organisms. To do so, however, they had to overcome various environmental stresses, such as drought, in their terrestrial life. In order to avoid such stresses, sessile terrestrial plants evolved strategies to perceive light, water and gravity, and to respond to them by changing their growth orientation. Among these adaptive strategies, is gravimorphogenesis, in which plant growth is influenced by gravity. Examples of this phenomenon are: gravitropism, where roots grow downward and stems grow upward; circumnutation, where the stem or the root tips display helical or spiral movement (for example, a climbing vine shows remarkable circumnutation); and peg formation, which helps cucurbitaceous seedlings shed their seed coats . The space environment is an ideal place to study these mechanisms of gravity-dependent growth in the development of plants.
Figure 1 (courtesy of Professor Takayuki Hoson of Osaka City University)
Gravitropism is a bending response, accomplished by differential growth of plant organs in response to gravity. On the space shuttle flight STS-95, which included Astronaut Chiaki Mukai, experiments were conducted to compare ground-grown and space-grown Arabidopsis and rice. On Earth, aerial parts of the plant (shoots) grow upward while roots grow downward. However, the experiments showed that in a microgravity environment, the growth direction is unregulated, and some roots even extend in the same direction as the aerial stems (Figure 1). In the case of root gravitropism, the hypothesis is that gravity is perceived by root cap cells, called columella, which are found at the root tips. Within the columella cells, starch-filled amyloplasts settle due to gravity, causing a change in the flow of the plant hormone auxin.
In essence, auxin characteristically flows in a fixed direction, from an aerial shoot, including the apical meristem and young leaves, towards the roots, through a central cylinder. After flowing down to the root tips by this polar transport, auxin begins to flow in the opposite direction, as if making a U-turn, along the roots. When roots are inclined and given gravitational stimulus, however, U-turning auxin tends to go downward instead of upward. As a result, the concentration of auxin increases in the lower part of the elongation zone in the inclined roots, causing a differential growth between the lower part and the upper part; the growth rate of the lower part decreases compared to that of the upper part causing the root to bend downward. This is how plant roots on Earth grow downward in response to gravity. However, in microgravity, amyloplasts do not settle within the root cap cells, so gravity is not perceived, nor is asymmetric auxin distribution induced. This is why, presumably, growth direction is uncontrolled in space.
You have probably seen Morning Glory vines growing upward, spiraling around a pole. This is thanks to circumnutation, which also has something to do with gravity. Previous studies have shown that stem circumnutation requires an endodermis, surrounding vascular tissue and made up of gravisensing cells. In a nutshell, without the so-called SCARECROW gene, which is essential for the proper differentiation of endodermal cells, the Morning Glory cannot sense gravity, and as a result, cannot circumnutate - its vines cannot twine. This indicates that circumnutation and spiral growth are gravity-dependent phenomena. I am very much looking forward to seeing whether circumnutation, or twining of vine plants, can be observed in the weightlessness of space.
Figure 2 (courtesy of Tohoku University)
Figure 3 (courtesy of Tohoku University)
Peg formation on cucurbits - the plant family that includes cucumbers, melons and squash - is also influenced by gravity. A peg, which is a small protuberance, develops immediately after germination in the transition zone between root and stem (Figure 2). On Earth, the downward growth (gravitropism) of the roots results in a curvature at the transition zone. When seeds germinate in a horizontal or inclined position, a peg develops on the lower, concave side of the bending transition zone at an early stage of seedling growth. As such, it had been presumed that peg formation was regulated by gravity. When we germinated cucumber seeds in space (Figure 3), a peg formed on each side of the transition zone, showing that pegs develop with or without gravity. In other words, cucumber seedlings have the innate ability to develop two pegs, but on Earth, the seedlings suppress peg formation on the upper side of the inclined transition zone in response to gravity, which causes unilateral placement of the peg in cucumber seedlings. This gravity regulation has something to do with auxin, the plant hormone I mentioned earlier.
To sum up, plant life depends on gravity, and auxin transport, which is regulated by gravity, plays an important role. It is thought that in the weightlessness of space the absence of gravity to regulate auxin transport results in abnormal growth and development of plants. However, exactly how gravity regulates auxin transport remains unknown. When this mechanism is understood, it will not only improve plant production on Earth, but will also help with plant cultivation in space. So, it is very important to perform space experiments that will clarify the mechanisms of plant growth and development.
Dr. Hideyuki Takahashi
Professor, Graduate School of Life Sciences, Tohoku University
In 1982, Dr. Takahashi received his Ph.D. in Agriculture from the Graduate School of Agricultural Science at Tohoku University, and a postdoctoral fellowship at the Department of Biology at Wake Forest University, in North Carolina, USA. He was a research associate at the Institute for Agricultural Research at Tohoku University from 1985 to 1987, and at the Institute of Genetic Ecology at Tohoku University in 1988. The following year he was a visiting fellow at the Department of Biology of the University of North Carolina at Chapel Hill (North Carolina, USA). Subsequently, Dr. Takahashi was appointed associate professor, and as of 1996, a full professor, at the Institute of Genetic Ecology at Tohoku University. He has been in his current position since 2001.