Faced with “pervasive” microorganisms, what “magical protective powers” ​​do plants have?

The past few years may be the time when immunology has been most popular among the public. Everyone from the age of 99 to the age of just learning to walk probably knows about vaccines. If they pay a little attention, they may also know about antibodies, immune cells, antibiotics, etc. People are also aware that germs can be killed with disinfectants and protected against by wearing masks. Of course, if you really can't afford to offend, you can hide and avoid these risky places.

So, an interesting question arises: Plants cannot move around like us humans, and they don’t have hospitals, so what do they do when faced with the ubiquitous microorganisms in nature? So, do plants have immunity?

The answer is yes. To survive in nature, plants naturally have immunity, but it is not the same as the immunity of vertebrates. So what is plant immunity? Let's talk about it.

01. What is plant immunity like?

Plants do not have an adaptive immune system, which is what we often call immunity. However, plants still have specific immunity and establish immune memory, which can be summarized into three aspects: plant disease avoidance, disease resistance and disease tolerance.

Why? Is it a bit hard to understand? Let's take a typical example, my country's kingdom of plants - Xishuangbanna, which is the place with the highest biodiversity in my country. As the largest tropical area on the Chinese mainland, it has the largest tropical rainforest and the highest biodiversity in the country. A quarter of the country's animals and one sixth of the plants grow here. There are more than 5,000 species of tropical plants alone. It can be said to be one of the best areas for studying plants.

The unique biodiversity in Xishuangbanna is also related to the unique climate and geographical environment. On the one hand, like many tropical regions, Xishuangbanna presents a typical tropical two-season climate style of rainy season and dry season. In the rainy season, the local area has abundant rainfall, while in the dry season, Xishuangbanna has more dry season fog due to the influence of the terrain and climate. For example, the following figure shows the statistics of foggy days in Xishuangbanna [1]:

We can see that in the dry season from October to April of the following year, the proportion of foggy days in Xishuangbanna is quite high, especially in Mengla, where even in April there are one-third of foggy days. This high fog environment also provides Xishuangbanna with additional water and chemical component input, allowing Xishuangbanna to maintain a high humidity during the dry season, thus allowing local plants to grow better.

Of course, high humidity is not only conducive to plant growth, but also means that the natural enemies of plants - microorganisms - are also very suitable. After all, humidity is a very critical factor for microorganisms.

For example, both are tropical regions. Researchers compared the soil microbial diversity in Hainan and Xishuangbanna and found that the soil microbial diversity in Xishuangbanna was significantly higher [2].

It can be seen from this that Xishuangbanna has a high plant diversity, but it also faces more complex and diverse microorganisms, so local plants must have a stronger ability to deal with microorganisms.

02. How do plants deal with microorganisms?

So how do plants deal with microbes?

First of all, plants have natural physical disease resistance , which is similar to our innate immunity. We humans have innate barriers such as skin, and so do plants, with natural protection such as bark. Of course, not only that, many plants have also developed a series of natural protective layers, such as cuticle, wax layer, cork layer and other structures, which can effectively resist the invasion of microorganisms and serve as the first line of defense.

Secondly, plants also have natural antibacterial ingredients, which are the key to plant antibacterial properties.

Although the plant epidermis can resist many microbial invasions, once the microorganisms invade the plant body, other antibacterial methods are needed. For plants, they mainly rely on various antibiotics, which are antibacterial ingredients secreted by the plant itself. There are many types. For example, many plants have unique smells, and the most common ones are aromatic plants. These plants can extract aromatic oily liquids, which are the broad-spectrum antibacterial ingredients of the plant. This is how monoterpenes are extracted from a plant. They are widely distributed in many plant species, and the antibacterial properties of this ingredient have been fully studied [3].

As shown in the figure above, different doses of terpinene-4-ol have an inhibitory effect on Pseudomonas fluorescens. It can exert an antibacterial effect even in trace amounts, and its 2uL/mL dose is equivalent to the antibacterial effect of 50ug/mL of the common antibacterial antibiotic kanamycin (positive control). In other words, terpinene has a more excellent antibacterial effect at a lower concentration.

There have also been many studies on the antibacterial mechanism of terpinene-4-ol . Terpinene-4-ol can effectively destroy the cell wall of bacteria, exposing the cell membrane, and further change the permeability of the cell membrane, allowing intracellular ions to be excreted, thereby achieving a bactericidal effect.

For example, there is another type of protein component secreted by plants: lysozyme. This is a type of glycoside hydrolase, which is different from bacteriostatin. It is a biologically active protease that can specifically act on the cell wall of microorganisms, thereby widely exerting resistance to pathogens and achieving a bactericidal effect.

In fact, plants have very strong antibacterial ability, which is the basic guarantee for their survival in nature.

03. Are these natural antibacterial ingredients useful to humans?

Since plants have very strong antibacterial ability, is it possible to obtain antimicrobial ingredients from plants to serve humans? In fact, humans have already obtained a variety of antibacterial ingredients from plants. For example, Melaleuca alternifolia, which is widely cultivated in the world today, is such a classic case.

Melaleuca alternifolia is a tree of the Myrtaceae family, native to Australia. The local Aboriginal people would soak the leaves of Melaleuca alternifolia and apply them to the wounds to promote healing. During World War II, Australian soldiers' first aid kits also contained tea tree oil extracted from Melaleuca alternifolia to deal with trauma.

Since the antibacterial effect of Melaleuca alternifolia essential oil is widely known, scholars have begun to conduct scientific research on it. For example, as early as 1923, Australian scientist Dr. C. R. Penfold first studied the bactericidal effect of tea tree essential oil and found that tea tree essential oil can indeed significantly kill bacteria, with its bactericidal ability being 13 times that of phenol, which confirms the local traditional cognition [4].

Further research found that its core ingredient is terpinene-4-ol , which is also the plant antibacterial ingredient with obvious antibacterial effects mentioned earlier.

Due to the good antibacterial effect of tea tree essential oil, high-yield varieties rich in terpinene-4-ol were subsequently widely planted and further introduced to all parts of the world, from the United States, Southeast Asia to China. This excellent antibacterial plant has begun to be planted.

Nowadays, the essential oil extracted from Melaleuca alternifolia has been widely used in many fields. For example, in the field of biomedicine, the essential oil of Melaleuca alternifolia has excellent antibacterial ability. Its core ingredient, terpinene-4-ol, has been confirmed by many studies to have antibacterial effects and revealed its antibacterial mechanism. Therefore, it was successfully included in the European Pharmacopoeia in 2001 and approved by the US FDA as an over-the-counter drug.

At present, the research on terpinene-4-ol is not only in the field of antibacterial, but has even expanded to other disease areas, providing new options for the treatment of many diseases.

Of course, as an excellent antibacterial ingredient, it is not only used in medicine, but also has a wide range of uses in daily life, a typical example being sterilization in daily life, such as cleaning our clothes.

You may have noticed that when you hang clothes out to dry on rainy days, various odors always appear. The reason behind this is that the ubiquitous microorganisms use our clothes as food, multiply in large numbers due to the humid weather, and secrete various odorous substances.

Since we mentioned that plants have a wide variety of antibacterial substances, then, can we obtain these antibacterial ingredients from plants to help us eliminate bacteria? The answer is yes. It has been applied in practice.

Plant antibacterial ingredients have another advantage: they are non-irritating and can be used even on sensitive skin. A typical style of biological systems is balance. They will self-regulate their own functions. For example, it is impossible to over-secrete these immune substances for antibacterial purposes and affect their other functions. The result is a balance between antibacterial and growth, reproduction and many other functions. So we can see that plants can maintain normal functions while inhibiting bacteria, which is very useful for us. Many of our clothes are made from plant fibers, so they can better adapt to the antibacterial advantages of plants: inhibiting bacteria without excessively damaging plant fibers, which is the natural advantage of plant sterilization.

Over the years, we have been working hard to protect biodiversity. In fact, protecting biodiversity is also protecting ourselves. Just like the plant Melaleuca alternifolia, we have discovered the magical power of plant sterilization, which can help us resist diseases, improve our lives, and achieve a healthier and safer lifestyle.

[1] Peng Haiying, Shi Zhengtao, Tong Shaoyu. Climatological characteristics and influencing factors of fog in Xishuangbanna[J]. Geographical Research, 2020, 39(08): 1907-1919.

[2] Du Haonan, Wei Yaqing, Quan Fei, et al. Composition and diversity of rhizospheric bacterial communities in rubber plantations in Xishuangbanna and Hainan[J]. Journal of Western Forestry Science, 2022, 51(01): 77-83. DOI: 10.16473/j.cnki.xblykx1972.2022.01.012.

[3] Geng Yiming, Li Tingting, Li Jianrong, et al. Antibacterial activity and mechanism of terpinene-4-ol against Pseudomonas fluorescens[J]. Food Science, 2022, 43(01): 30-36.

[4] Wu Kegang. Essential oil plant Melaleuca alternifolia[J]. Life World, 2021(09):28-29.