Mécanismes de défense des plantes : interview d'Agnès Brosset

Hello, today I am with Agnès Brosset. Well, not exactly with Agnès, I will explain why. But first, I will do the introductions. Agnès did her PhD thesis in Finland on the theme of wild plants and cultivated from the Brassicaceae family. The thesis was published in 2022 and it aims to determine if wild plants have volatile constituents that are different from cultivated plants and to see if it impacts their defense response against herbivores, against air pollution. And this is the kind of discussion that interests us a lot, us lovers of edible and medicinal plants, because it will help us answer the following question. Are wild plants richer, more medicinal than cultivated plants? That's a good question. So I did two recordings with Agnès live and we had technical problems because she is currently in Sweden and I think she is at the end of the line because her internet was not good at all. And during an interview, we laughed a lot because she had mosquito problems and I'm not lying to you, a mosquito literally came to land on the camera to obscure it. So I thought it was a joke at first, you know, the Marseillais mosquito that was bigger than a rat. But no, it was a big mosquito. So we decided to record separately. So I'm going to ask the questions on my side and then Agnès will answer them on her side. We're going to do the editing, it's not live at all, but it's not important because you're going to see the quality content and that's what's important. Let's go, let's go. Agnès, before we start, I'd like you to tell us a little bit more about your story. How did you end up in Finland to do a thesis on this particular subject? So what brought me to plants was a rather indirect way, because originally I wanted to be in entomology, that is, to work with insects. And when I started my biology studies, at that time, the master's degree that was most specialized in insects was that of the University of Toulouse, which was specialized in the relationship between plants and insects. I went to do my master's thesis in Sweden. And there, in fact, I found myself in a working environment that I really liked, since we were on a very horizontal organization, rather than vertical, where we tutored each other, everyone in their place, whether you're a teacher, a student, a master's student, whatever, everyone is on the same line and listens. And in fact, after that, I said to myself, no, but I want to continue my studies in the north, and especially my doctorate in the north. And I met James Bland, who is a professor who works at the University of Kropio in Finland, who, when I finished my master's degree, was looking for someone for a doctorate. And his doctorate was specialized on better knowing the differences in plant defense between wild plants and cultivated plants. And I immediately had a good feeling with James. And so I felt that he was someone I could work with. And I embarked on the subject, although it was not directly connected to insects. And it was in fact in these five years of doctorate on wild and cultivated plants that I really developed my interest in plants, herbivores, wild plants, etc. We start with your favorite subject, the defense mechanisms of plants. So the plant cannot escape a predator, it cannot move in space, of course. How does it defend itself against insects, against herbivores, small and large? So in plants, there are several types of defense. First, there is the constitutive defense. That is to say, it is a defense that is always present in the plant. What is very easy to understand is the spines, the hairs, the fibers, for example. They are constantly present and they will limit any herbivore to feed on the plant. For example, like the rosette, which will be difficult to eat or little appreciated by their spines. And then there will also be chemical defenses that will be constitutive. And often they are odors. And thanks to the odors they emit, these odors can send a message to some herbivores to indicate whether a plant is edible, digestible, toxic or not. And so it will allow plants to really limit the quantity, the type of herbivores that can come and eat on it. But some herbivores have learned to specialize in certain plants and will still come and eat on the plant despite these constitutive defenses. For this, the plant has a second type of defense that we will call direct induced defenses. That is, they will be emitted, induced by the plant, only when the specialized herbivore will come and eat on it. In the context of my study, it was in particular insects. And when insects come to eat on plants, they break cells. By breaking these cells, it sends a message to the plants that will come and respond by emitting new molecules that were not present in the constitutive defense, which can be, for example, more toxic or different from those that were there at the beginning. It is a little more specialized against the insect that is eating. And so, for example, in mustard, there are molecules called glucosinolates that give this spicy mustard taste. And when an insect comes to eat, these glucosinolates are released and then they are put in contact with an enzyme that will convert them into a new molecule that is emitted, induced, only when a herbivore comes to eat. These are molecules called isotoxinates, which are neurotoxic. I would like you to tell us a little story, that of the tetranics, these small aquariums that feed on lima beans. What did we discover in the 1980s about a strategy used by beans to protect themselves against this aquarium? And how did this open the door to other research on animal plant defenses? Indeed, in this study, researchers highlighted a new plant defense strategy. They highlighted the indirect induced defense in plants, that is to say that they do not act directly against the herbivore, but these molecules that are emitted send a message that is perceived by predators that will come to locate their prey thanks to the odors emitted by the plant. And this study really opened the door to hundreds of studies that were conducted on the subject, since this defense strategy has now been shown in several species of different plant families. So this study really opened the way to all the roles that these molecules induced by herbivores can have, especially in the communication between plants, and we will talk about that a little later. We will leave the animal world aside. We will now talk about defenses against aggressions that come from pollution. Ozone, for example, is a well-known pollutant. We talked a lot about it in the 90s and 2000s, with the famous hole in the ozone layer of the stratosphere, because this layer filters UV rays and therefore acts as a protection, but at another level, in the lower layers, it is a very unstable and harmful molecule if it exceeds certain thresholds. And we clearly see the increased concentrations due to human activity. What do plants do to protect themselves against this pollutant? You should know that ozone is a pollutant that is not directly emitted by human activity, but it is induced by a chemical reaction between the pollutants emitted by human activities, the sun and certain odors. And so we need these three factors to form ozone. Sun, chemical or natural odors, and pollutants, nitrogen oxides. And when we have these three present, we form ozone. And so ozone varies throughout the year depending on the availability in the sun. This is quite important to understand why ozone is toxic for plants, since we will have a lot of sun in spring and summer. And so it is also when the concentrations in ozone will be the most important. At the same time, plants are in a period of growth and activity. And that's why it makes them quite harmful to them, since it happens at the same time. And in response to ozone, plants have two strategies. Ozone will have the effect on plants to induce premature aging. That is to say that when it enters the cells, it oxidizes the cells. And often we find plants that are a little yellowish. And so to protect themselves, the plants will come and close their stomachs. By closing their stomachs, they also limit photosynthesis and therefore the potential entry of carbon and energy. So often, in fact, ozone will induce premature aging of plants and then limit their growth. And then also to defend themselves against that, they will also have the antioxidant factory. There will be a lot of antioxidant enzymes that will come and catalyze the oxidation reactions that will be put in place by the plants. And then the plants will also start to emit molecules such as terpene, which react with ozone, which allows to tamp down and limit oxidative stress in plants. According to what you explain to us in your thesis, an increase in ozone, an aggression that comes from pollution, can cause an increase in defenses against insects. How do we explain this? What is the link between these different types of aggression? So first of all, the results when we have herbivores plus ozone, unlike the herbivore alone, they are quite variable between the varieties. And then which compounds are studied in particular. Sometimes we have an increase in some compounds, then sometimes we have a decrease in defense against some compounds. But it is true that in my study, we noticed that in general, we had a lot more terpene that was emitted when we had herbivores plus ozone than herbivores alone. And in fact, that comes with what I said before, it is to understand how plants respond to ozone. And so, compared to ozone, they have this ability to start the antioxidant enzyme and to emit molecules like terpene, which will react with ozone and therefore limit the presence of ozone in the cells. They actually tamp down, limit the oxidative reaction. So I think that's why they are often more emitted when there is ozone in the air. And which actually adds up to the response that the plant has to the herbivore. Let's talk about plant-to-plant interaction through different messages sent from one plant to another. What is it today about this network of communication between plants? We talk a lot about mycorrhizal networks, for example. Are we talking about plants of the same kind, of the same family, of the same order? Who manages to communicate? In other words, do we communicate just between family members or between neighbors? Or can we even communicate with strangers? So I'm going to start with the communication below the ground, so the mycorrhizae. You should know that they are mushrooms. We have about 80% of the plants on earth that make associations with mushrooms, which are called mycorrhizae. It's an association that's going to be symbiotic. Since the plant will be able to benefit from this association by having access to more nutrients, and the mushroom will benefit by gaining sugars that the plant will exchange for other nutrients, such as phosphorus, for example, or nitrogen. These mushrooms have been shown to be associated with a plant or a tree, but it has been shown that, for example, in a forest, they can also be associated with plants or trees. In fact, plants will use these mycorrhizal bridges to transmit chemical information, such as signals, defense messages, for example, or certain hormones. And then there is the communication above the ground, which is done with odors, which was in particular the communication that I studied. So there, as I was saying, when the insect comes to eat on a plant, we have these new compounds, these new odors that are emitted. These odors, in fact, within a single plant, will be perceived, if we imagine that an insect begins to eat on a plant, it will eat on a leaf, and so this leaf will emit these odors, which will be perceived by all the leaves that are around, which will also begin to respond to herbivory, to emit odors, and this is what we call communication within a plant. And then these odors, when the whole plant begins to defend itself, these odors will be emitted in the air, and so they can be perceived by plants that are around. And when they are perceived by plants that are around, these plants understand the message as a presence of herbivores, and therefore themselves, even before there is an insect that has begun to eat on them, they will also begin to emit these defense molecules. Sometimes we don't see it, we don't see that the neighboring plants respond, but when the insect comes to eat on one of the neighboring plants, when it had received the message beforehand, this plant responds faster and more intensely, so it is better defended. Then, when it comes to the question of whether we communicate better within a family or between strangers, there is a very good study of Terrain, which is quite rare, which was done in California on the Armoise tridentae. And the populations of Armoise tridentae, like many aromatic plants, is that they have chemotypes. Often we know the chemotypes with essential oils. For example, some seaweeds have a more abundant profile of essential oils than others in Borneol, others in Cineol, for example. Well, it's the same with the Armoise tridentae. And so, what they did is that they selected the odors of several plants, of several populations of plants, which presented different chemotypes, and they exchanged them, so they transferred them, on plants of the same chemotype or different chemotypes. And they showed that the more we look alike from a chemical point of view, so the closer the chemotypes were, the more the message sent a signal and that the plants responded to the message by defending themselves better for the rest of the season against herbivores. So here it shows us that the closer we are from a chemical point of view, the more communication is made and understood. So the more we speak the same chemical language, the better we understand each other and therefore the message is better transmitted to the neighbors. At this stage, we understand better how the wild plant defends itself against different aggressors. Let's now talk about domestication. We have been growing the plant for our own use for millennia. We take care of our plants. We water them, we fertilize the soil, we provide water and nutrients. So very comfortable conditions. In recent decades, we have also created a chemical arsenal to destroy predators, competing plants. An arsenal that is poisoning us, we know that. So that's the first point. And the second point is that we have also selected species for better yields. We made crossings, we created hybrids, we manipulated the genes of the plant. What was the consequence of this domestication, from the point of view of the richness of the plant's defense arsenal? So the domestication of plants is not new. It started about 12,000 years ago. But in fact, what really changed the morphology and chemistry of plants is really the green revolution of the 1960s. Where we selected plants for their yields. And what has been observed is that cultivated plants end up with a lot of physical defense. So for example, mustard. A wild mustard is often quite hairy, fibrous, and then dark green. While cultivated mustards were rather smooth, the leaves, and then a rather light green. So that's for everything that has a bit of morphological defense. So less hair, less fibrous. But also, already there, by the color of the pigments, we can see that there has been a change in chemistry as well. And indeed, we often end up with cultivated plants that are less rich in defense compounds. In particular, in mustard, it was clearly shown that there was less glucosinolate. And I was able to show that there was less isothiocyanate. So this new molecule that is formed when an insect comes to eat mustard, as a result of the reaction of glucosinolate. Often also, what I noticed is that cultivated plants, in terms of smell, may emit the same amount, but the molecules are less varied. Whereas plants, even if they emit the same amount of smell, the quality is different. We will have little diversity. Whereas in cultivated plants, we will have a great diversity of molecules. So it's not so much that we pamper them, that we come to give them nutrients, or that there are insects that are going to be controlled by pesticides, for example, which makes them sensitive and less defended. No, it's the fact that we have selected them, which means that today we have this difference of defense between wild and cultivated plants. What can we deduce from the beneficial effects of these domesticated plants compared to the benefits of the same wild plants? And here I am talking about the antioxidant, anti-inflammatory, antibacterial, fungal, stimulating, immune, etc. What I could also add here, in this question, from a point of view of the defense arsenal, is that herbivores, especially insects, what was observable was that since cultivated plants have less fiber, but are more caloric, often they have a lot more amides, a lot more sugar, we end up with plants that are easier to digest, more nutritious. So insects that come to eat cultivated plants often develop better, faster and bigger than those that develop on wild plants. Since the defense of plants and all these chemical molecules is reduced in cultivated plants, we end up with plants that, from a medicinal point of view, are less rich in this phytochemistry that we are looking for. For example, in bees, red bees like myrtle, it was clearly shown that we had less phenolic compounds, so much less antioxidants, much less flavonoids, tannins too, so much less rich from a medicinal point of view. How do we explain this? That is, the fact that their defense is also a remedy for us? I think there are several answers to this. First, the intrinsic property of some compounds, of these molecules. For the plant, some compounds will have the role of being antimicrobial, antiviral, antibacterial, for example. This role in plants will also have a role in other living species. The same goes for antioxidants. For example, in bees, it serves the plant as protection against sun rays and everything that can oxidize the plant. We will have the same answers in us. I also believe that there is the dose. For example, if we take tannins, in plants, they are really used to affect the digestive system of insects. Here, you have to imagine that for an insect, this dose is huge, and therefore it can really interfere with the assimilation of nutrients. Whereas in humans, this dose will be minimal, and therefore it will have an astringent effect on digestive tissues. I also believe that there is the coevolution between man and plants. We must not forget that man has fed on plants and what was in his environment for millions of years, the genus Homo in general, and therefore the human body has evolved to its optimal with what was in the environment, and therefore with the chemistry of the plants that were in the environment. I always take to explain this the example of vitamin C, since man was able to synthesize vitamin C, but this quality was lost since it was not necessary. It consumed energy for nothing, since man could have the amount of vitamin C in his food every day, and therefore it was useless for the body to produce its own vitamin C. And so that's how this ability was lost. I think this is one of the reasons that also explains that the defense components of plants can also be medicinal in humans. Today, we see a tension between wild plants and the planetary demand for medicinal plants, with a lot of marketing, sometimes exaggerated. So a demand that is already unbearable today in relation to wild plants. Today we see the message that wild plants are more nutritious, more medicinal. So we ask ourselves a lot of questions in our circles about this issue. I know it's a complex question, but how do you see things on your side? I think one of the solutions would be to try to cultivate wild plants, which are in danger today, to preserve them so that we can continue to access them without harming the wild populations. For my thesis, I used the word cultivated versus wild, but the real term would be domesticated versus wild, since domesticated really incorporates the fact that they are not just cultivated at home, but that they have been selected. And so it is really this selection for yield that has caused plants to lose their defense. So if tomorrow we take wild populations that we come to cultivate, but without touching them, without trying to select them for yield or something, we should be able to preserve their medicinal properties, but be able to have them next to us without taking the wild populations. From a small-scale, private point of view, I also believe that we can, for example, pick seeds in nature and plant them in our garden. To keep all the plants that are a little bad herbs, they are often medicinal and bring a lot of nutrients. It is to keep them in our garden, around us, to be able to access them without taking wild plants. It could be a direction to take to limit our impact on nature. Well, thank you Agnès for this super interesting discussion. And also thank you for being so patient with me, with the different recordings, with the internet connection problems and with these damn big mosquitoes. See you soon!