Plants have unique mechanisms to adapt to harsh environments
Plants are exposed to various environmental stresses such as high temperatures, low temperatures, ultraviolet ray, and drought. Unlike animals, plants cannot move from the place they once germinate. Therefore, each plant has specific mechanisms which other living things do not have, to adapt to the environment they are located in. It is considered that they can grow even in severe environments such as deserts and high mountains because they have acquired a mechanism of adaptation in the process of evolution. Plants are adopting a survival strategy which is completely different from animals that can move.
For example, under an environment below freezing, water which plants take up will turn to ice and destroy their cells. However, when plants growing in high mountains such as dicentra feel the coldness, in their cells they store components such as compatible solutes which prevent water from freezing and adapt to temperatures below zero. Moreover, normal plants will die if the salt concentration is high in the soil because they take in salt, and then water escapes from their cells. However, common ice plants with unique saltiness can survive because they have organs to discharge salt that they took in.
A type of rice called floating rice is another example. Terrestrial plants will wither if they become submerged for a long time because they cannot breathe. This rice, which vegetates in places such as Southeast Asia, where water levels rise during rainy seasons, suddenly starts growing when submerged, and breathes by sticking out its ear above water.
Furthermore, water lilies can grow on water, and welwitschia (Japanese name: ‘kisotengai,’ meaning ‘totally unexpected’), which is a gymnosperm plant living in the desert area of southwest Africa, stretches its roots deep under the ground even in a completely dry environment and can survive for 2,000-3,000 years with a very minimal amount of water.
If we could apply such power that these plants have, we might be able to create crops that grow under severe environments where global warming has advanced. In our laboratory, we try to find out how plants adapt to environmental stress at the genetic level. By focusing on genes that function when they encounter environmental stress, we should be able to ascertain the factor related to environmental adaptation. By altering them, we are trying to develop plants that are resistant to environmental stress in our research.
Clarifying common factors related to various stress resistance
When plants are environmentally stressed, their growth sometimes slows down. It is considered that this happens because by choosing not to grow they are reallocating their energy necessary for growth to stress adaptation. We found out that the genes ANAC044 and ANAC085 are playing a role in suspending the growth. Moreover, these two genes function to stop the growth whenever the plants are stressed, whether it is caused by high temperature, low temperature, or salinity. Conversely, if these genes do not exist, plants will not stop growing whatever stress they may encounter.
Most terrestrial plants have ANAC044 and ANAC085. On the other hand, plants living in the water do not have them. In the water, not to mention water itself, nutrients are abundant, temperature is more stable than on land, and there is less effect from ultraviolet rays. It is an environment which is relatively easier for plants to live in. Originally, plants came into being in water. However, by becoming terrestrial, they started to encounter various stresses, and thus, I think, they acquired these genes.
If they are resistant to specific stresses but weak against others, the environmental conditions that enable them to be cultivated become limited. In addition, it is ideal for plants that genes related to stress resistance only function when they are stressed. If they are always in full operation mode, growth in normal times will be harmed.
I would like to develop crops that are resistant to multiple stresses as well as specific stresses. This is possible if we can find out the factors related to stress resistance which commonly function when they process various stresses at the same time. Currently, I am working on the research with the hope for application.
Not only applying to crops but also reducing CO2 by greening
Some plants keep living for a long time. For instance, the tree age of Jomon Sugi (cedar tree) in Yakushima Island is estimated to be several thousand years. Unlike animals such as humans, why can they live so long? This is because the maintenance mechanism of stem cells is different between humans and plants.
Stem cells are, as the name shows, the undifferentiated cells that are the stems of the cells. They have the capacity to create new stem cells by dividing or differentiate to various cells with specific functions. In the case of humans, stem cells, which are the origin of everything, differentiate to diverse stem cells of blood, internal organs, skin, etc. However, once they differentiate, they will not return to the original stem cells, and in due course, they will not divide anymore, and thus humans will grow old and reach the end of their lives.
On the other hand, while plants are also creating roots, flowers, leaves from stem cells, they can permanently keep creating those stem cells, and moreover, unlike humans, they can turn their differentiated cells back to the original undifferentiated cells. Because of this, plants can not only live long but also acquired a highly advanced regenerative capacity.
For example, if we take out a cell and culture it in appropriate conditions, it is even possible to regenerate the original individual. Weeds come out again from the same place where we pulled them out, trees develop side buds from the stub that we cut off, side buds that are cut off from the plucked flower start growing when they are stuck into the soil. Such power is a distinctive characteristic of plants. This is impossible if we remove everything from the roots. However, even with a slight remainder, stem cells are easily regenerated, and everything returns as it was.
When plants encounter life-threatening critical stress in the process of stem cell differentiation, they kill their stem cells themselves, and the special cells located at the center, called quiescent center cells in the case of roots, will function. Quiescent center cells are very resistant to stress, and they hardly divide in normal times. However, as soon as stem cells die, they suddenly start to divide, and start supplying new stem cells one after another. In order to activate this series of processes, we found out in our research that plant hormones are involved.
We have expectations for applying this regenerative mechanism to agriculture as well. For instance, grafting to encourage better growth by connecting separate individuals could be achieved efficiently with a greater variety of species. Conversely, if we can prevent regeneration, it could lead to use as herbicides. Moreover, its stress resistance and regenerative capacity may lead to the development of plants which can overcome global warming.
According to an estimate, the global population will increase to 10 billion by 2050, and that will require as much as 60% more food than now. It is impossible to achieve this if we continue with conventional agriculture. Moreover, the global average temperature is predicted to rise by as much as 3.3-5.7°C by the end of the current century. If we can develop plants which can grow in places such as deserts and coastal areas, that can also lead to reductions in CO2. By developing plants that are stress resistant, I hope that we can not only resolve the food shortage problem but also contribute to mitigating global warming.
* The information contained herein is current as of February 2024.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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