Reactive oxygen species responsible for plant immune function

Plants receive various stresses from the environment. For example, excessive or insufficient sunlight, dryness, and pathogens can also cause stress. Methods for investigating what happens in the plants in responding to such stresses include an analysis of changes in substances in the plants.

For example, when the soil becomes acidic, aluminum that is abundant in the soil dissolves and becomes aluminum ion. However, it is a harmful substance for the plants.

It was known that the reactive oxygen species accumulated in roots exposed to aluminum ion.

Therefore, in order to investigate the difference in the amount of the reactive oxygen species between roots that had absorbed aluminum ions and those that had not, we were conducting an experiment to extract the reactive oxygen species by grinding the roots and measuring their amount.

However, since grinding the roots also produces the reactive oxygen species, measuring the amount of the reactive oxygen species using this experimental method does not allow us to say that it is only due to the stress of aluminum ions.

So, we came up with a way to measure the chemicals exuding from the roots. In fact, the plant roots not only soak up what is in the environment, but also exude a variety of substances.

It was found that the plant transiently released a large amount of the reactive oxygen species when invaded by pathogens or other things that are harmful to plants, including aluminum ions.

After all, the plants kill harmful things by oxidizing them with the reactive oxygen species. This is the function called plant immunity, which is indispensable for plants.

For humans, reactive oxygen species are said to be involved in aging and lifestyle-related diseases and have an image as harmful substances. However, in fact, they are also substances that play a role in our immune system. When bacteria or other pathogens invade the cells of the body, the cells cover them with a membrane and release the reactive oxygen species into the membrane vesicles to kill them.

In other words, we should not treat reactive oxygen species as a villain by focusing only on one aspect related to aging and disease, but it is beneficial for us to understand their multifaceted functions and characteristics and use them effectively.

In this way, analytical chemistry is research that visualizes and understands the unknown and invisible aspects of substances through chemical analysis.

For example, it seems that even reactive oxygen species have a function that we do not yet understand.

Plants controlling reactive oxygen species as needed

In the first place, reactive oxygen species are generated by the reaction of oxygen molecules with electrons. For example, when an extra electron is attached to an oxygen molecule, it changes to a substance called a superoxide anion radical. If another electron is attached to it, it changes to hydrogen peroxide, and if another electron is attached to it, it changes to a hydroxyl radical.

These, including singlet oxygen, are called reactive oxygen species (ROS), which are highly reactive and cause various oxidative reactions. In some cases, they serve as an immunization function and are involved in aging and lifestyle-related diseases in other cases.

Furthermore, it was said that in plants, when root hairs grow, the reactive oxygen species are released. It also increases and decreases in small amounts in 15- to 30-second cycles.

Also, the plants have an enzyme that releases reactive oxygen species from their roots, and it is known that plants that are deficient in this enzyme do not grow root hairs.

In fact, it is still unclear why such a phenomenon takes place. However, it is considered that reactive oxygen species, which are transiently released in large amounts to exert immune functions during stress caused by harmful substances, are normally involved in healthy root growth while being produced in small quantities.

In other words, it is considered that the plants control the reactive oxygen species for use as needed.

We, having an image of reactive oxygen species as harmful substances, tend to think of reactive oxygen species as nuisances that arise on their own in the body.

Indeed, many living things, including humans, breathe oxygen. Plants not only inhale carbon dioxide for photosynthesis, but also breathe oxygen. This is because living things can get a lot of energy from oxygen breathing.

In humans, the mitochondria in the cells use oxygen to generate energy. In the process, as mentioned above, electrons are often attached to the oxygen molecules. Thus, the reactive oxygen species are formed, which are controlled by antioxidant enzymes.

On the other hand, plants need a lot of enzymes and energy to produce plant hormones, which have a variety of effects, and this also takes time.

In contrast, the reactive oxygen species have a very simple structure and can probably be easily generated quickly and in large quantities by plants. For this reason, it is considered to be utilized in various ways.

Of course, no matter how much it is used for immunity, leaving a large amount of the reactive oxygen species generated in place will endanger cells. Therefore, enzymes for removing the reactive oxygen species are also necessary, and it is considered that these and other factors are well controlled.

Seeing things from multiple perspectives can lead to new discoveries

Of course, plant cells and animal cells are different. It is not possible for humans to do exactly what the plants do. However, we can consider what plants do as one functional mechanism and think about its application.

In other words, it is important not to look at only one aspect of reactive oxygen species, for example, and assume that they are harmful substances or nuisances. Seeing things from multiple perspectives, rather than one, can broaden our understanding and allow us to see new possibilities in them.

For example, as mentioned above, even if we wanted to measure exudates without grinding the plant roots, it would be very difficult to analyze root exudates from the soil since roots are usually found in the soil.

However, there was a professor at our university who was studying hydroponics, so he came up with the idea of using hydroponic liquids for analysis. Then the research expanded from there.

In normal hydroponics, a pump is used to pump oxygen into the hydroponic solution. This is because the roots will rot in an environment that lacks oxygen.

However, the professor at our university did not use a pump, but rather created a condition where there was air between the surface of the hydroponic solution and the above-ground parts of the plants and carried out hydroponic cultivation.

When the amount of oxygen in this tank is measured, it is found that the area near the surface is rich in dissolved oxygen, but the closer you get to the bottom, the less oxygen there is.

Then, the roots of the plants grow near the water surface and do not extend to the bottom of the tank. However, the plants will still grow well enough. Leafy greens, for example, are harvested in sufficient quantities.

This makes the cultivation method very energy efficient compared to ordinary hydroponics, which requires the installation of a pump and electricity.

In other words, we tend to think that the more energy we put into growing the roots, the more nutrients the plant will be able to absorb, which will increase the yield of leaves and fruits. But when we look at the energy performance of the harvest, there is an efficient balance.

In fact, it is hard for us to realize that we are only looking at things from the side that is taken for granted, but if we can see things from multiple perspectives, we can still find new discoveries and possibilities there.

Actually, it is a fact that reactive oxygen species are involved in the mechanism of aging. This is because the reactive oxygen species accelerate cell damage.

It has been found that such an increase in reactive oxygen species is due to abnormalities in the mitochondria. The reason for the increase in abnormal mitochondria is that the function known as autophagy, which is responsible for the metabolic turnover of cytoplasmic components, declines.

So, if we think about effective anti-aging, there are ways to avoid decreasing the function of autophagy. In fact, such research is underway.

If we look at aging from multiple perspectives, rather than just demonizing reactive oxygen species, we will be able to come up with various ways to deal with it.

For this reason, we think that analytical chemistry, which makes visible what we cannot actually see, is an important and very interesting field.

* The information contained herein is current as of February 2021.
* The contents of articles on 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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