Plants have their own mechanisms of resistance to pathogens
It is said that about 36% of the crops that would have been produced are being lost worldwide owing to disease, pests and weed damage, as mentioned in textbooks and other publications.
This is by no means a small amount, and it is not an issue which is recently focused on. In order to reduce such damage, human beings have been making various efforts since ancient times.
When a plant disease occurs, there is a primary factor (pathogen), a predisposition (susceptibility of plants to disease), and a trigger (environmental factor to develop disease). And in fact, if any one of these three factors are missing, the plant disease does not occur.
In light of this, human beings first focused on eliminating pathogens, the primary factor. As a result, chemical pesticides were found to be the most effective method.
However, there has been a growing concern that pesticides may affect the environment. Also, for example, the rice blast fungus, which causes the most important disease in rice, becomes resistant to various chemical pesticides and soon chemical pesticides lose their effectiveness.
Therefore, in plant pathology, we have learned about the mechanism of plant resistance to pathogens and have been making efforts to produce plants resistant to disease by utilizing this mechanism. The idea is to reduce not only the primary factor but also the predisposition to disease.
Plants do not have an immune system, which mammals have developed, to protect themselves from pathogens or other foreign enemies. Therefore, plants do not currently have the function to make antibodies, for example, which human beings use to fight coronaviruses. Instead, plants have developed their own mechanism.
When compared to animal cells, plant cells have cell walls. In fact, most microorganisms around plants cannot penetrate the plant cell walls. In other words, many of the microorganisms existing around us are mere microorganisms that cannot obtain nutrition from plants.
Still, some invade plant cell walls by melting, penetrating, or breaking them. Microorganisms which can obtain nutrition from plants are called plant pathogens.
Plants have preexisting antibacterial substances which directly attack pathogens. When a pathogen attaches itself to a plant, it enters the cell in about six hours. So, plants make and store some antibacterial substances, and attack the pathogen immediately. However, some pathogens persist and continue to invade. Plants thus have a mechanism in which they synthesize antibacterial substances stronger than the preexisting ones to attack pathogens. It’s a very effective means of attack.
Sometimes plants take bolder approaches. For example, plant viruses multiply by taking nutrients from the cells they invade. However, plants have a mechanism in which cells that have been invaded by foreign enemies actively die. That is, plant cells prevent the proliferation of invaders by dying.
Meanwhile, some pathogens have evolved by developing mechanisms for sneaking into plant cells without being detected by the plant. Because they act covertly, plants are unaware of the invasion of pathogens and thus cannot prevent the invasion by synthesizing antibacterial substances or by self-destruction.
Research on mechanisms in which plants can and cannot detect invaders has progressed in recent years and has shown that plants recognize molecules called PAMPs that are common to a variety of pathogens. It is a sensor that recognizes pathogens, allowing the plant to constantly monitor their invasion. At the same time, however, it has also been found that some pathogens can circumvent the sensors.
In this way, plants and pathogens have evolved through repetitive interactions. This is called coevolution. Plants have acquired various mechanisms against rapidly evolving pathogens and built the complex defense mechanisms that they have today.
Even though plants are unable to move and appear defenseless at first glance, they protect themselves by resisting pathogens in various ways.
Genetic modification technology learnt from nature
In order to further increase the resistance of plants, human beings have conducted breeding for resistance by crossing since ancient times. It is done by artificially selecting a disease-resistant paternal side and maternal side to create a disease-resistant variety.
However, in most cases, you cannot get the one you want even if you cross a disease-resistant variety with a good-tasting variety. Crossbreeding takes a lot of time and effort.
Most of the crops we eat today have been bred through crossbreeding, which is the result of trial and error over a long period of time.
However, more than 100 years ago, plant pathologists discovered that a bacterium called Agrobacterium was introducing its genes into plants. When the gene is introduced to a plant, the plant starts to make an amino acid that Agrobacterium likes.
Researchers had begun to artificially use this mechanism, and genetic modification technology was developed in the 1980s. In short, this is a technology to create a target variety by introducing genes, such as those related to tastiness, from outside into plant cells.
As a result, a transformation method which specifically introduces resistance genes into a target variety has become possible.
There are many people who feel anxious about this kind of technology, but intrinsically, a child is born by exchanging genes on the paternal and maternal sides. Genetic modification is thus a natural phenomenon. Crossbreeding is an application of the natural phenomenon.
Furthermore, in nature, there are organisms which introduce genes and intentionally induce genetic modification, and human beings have studied those mechanisms and applied them.
In recent years, genetic modification technology has evolved into genome editing technology.
This is the technology in which one of the four bases, A, T, G, and C, which make up the DNA, the body of a gene, is removed and changed to shape the gene as intended.
Many people may be concerned about this genome editing, but this technology is also an application of natural phenomena.
In fact, organisms have a mechanism to repair DNA when it is damaged. On that occasion, it may be repaired in a different form from the original DNA. If one of the four bases is replaced, it becomes a gene with a different function.
Even so, this phenomenon is also a part of the repair which organisms do by themselves.
New breeding technology using this technology is being advanced and is about to be applied to breeding for resistance.
TAQing system – The latest technology which our university is also working on
One of the latest types of research on new molecular breeding methods is the TAQing system.
In genome editing, which is currently being actively studied, the key is to cut the target DNA precisely. However, the resistance to plant pathogens cannot be enhanced by the simple modification of a single gene. The mechanism of resistance is formed by a complex network of multiple genes.
However, it takes time and effort to modify and verify genes one by one. The TAQing system began with the idea of what would happen if multiple genes or DNA regions were cut at once. Therefore, we can expect a bigger change.
Also, genetic modification in nature occurs during meiosis, where paternal and maternal genes are passed on to offspring. In the TAQing system, on the other hand, genetic modification is induced not only during meiosis but also at the somatic level.
Plants naturally have many similar genes, and if genes in somatic cells are damaged, they try to repair them with similar genes. This serves as a chance to acquire new traits.
However, the TAQing system is a new kind of research field, and our research team is still at the basic research phase.
Although most of the attempts thus far have failed, human beings have developed various technologies through the accumulation of trial and error. We believe that results in breeding crops for disease resistance will be achieved by such steady research and accumulated knowledge, which will lead to the reduction of crop damage.
Research is a succession of failures, but as it spreads and accumulates like the bottom of a pyramid, a high peak will remain as an achievement. In my opinion, it’s the same for business and other various works you do.
We researchers keep moving on to the top without being discouraged by one or two failures, thinking that they are necessary steps to reach the top.
* The information contained herein is current as of March 2021.
* 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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