Deciphering plant messages: electrochemical ecology

Who can better understand the importance of chemical communication between plants than an agriculturalist?

My father was a School Teacher and is now retired and working full time on his hobby, agriculture, and for this reason, I decided to interview him. As he lives in Valencia (Spain) and I am currently living in Rio de Janeiro (Brazil), the interview took place via Skype. I called him after lunch (2 pm in Brazil) and I explained my idea about what I wanted to do with this article.

I told him that I needed his help to write an article about chemical communication between plants. This surprised him so I explained that his expertise as an agriculturalist would be most valuable for articles and publications. He then sat down in front of his computer, adjusted his earphones, and smoking one of his cigarettes started to listen to me as I explained the story of this article.

I, first of all presented him with the recently published article ¹ in RCS Advances, which was based on innovative studies of electrochemical methods, focusing on two key aspects related to the chemistry of plants: the chemical response of herbivores and the chemical communication between plants. This work was an international collaboration between the Universities of São Paulo and Valencia.

The publication was based on the fact, that when a plant is submitted to external stressors it produces defensive substances in response to stress. Interestingly, from the complex mixtures of molecules produced by the plant we can identify some volatile compounds (named Volatile Organic Compounds, or VOCs). I have been very interested in VOCs for quite a long time now, because they have several interesting functions in nature. For example, in the case of plants suffering aggressions the VOCs are responsible for repelling insects, or attracting parasitoids and predators that will attack the aggressor. The functions of these molecules are very important for a plant for two main reasons: i) plants cannot communicate like us, a plant cannot send a whatsapp to another, saying “Help! a beetle is eating me” and ii) the plants cannot run away to avoid the aggressor. For these two important reasons, the chemical responses to aggression, as well as the chemical communication between plants via VOCs, are very important tools for the survival of plants.

In the article, the plant of study is a widely spread American ornamental plant, commonly named “baby rubber” or “pepper face” (scientifically named Peperomia obustifolia). This Peperomia suffers strong aggression from the herbivore beetle Monoplatus sp.

A sample of undamaged adult specimens of “pepper face” was taken from the Chemistry Institute´s botanical garden at the University of São Paulo and each plant was put into separate plastic pots. They were then irrigated and incubated inside incubators, under controlled conditions (temperature, illumination and humidity). The plants were divided into three groups of ten individuals, each group in plastic boxes permeable to chemicals (VOCs) but not insects. The different groups were:

1. Plants isolated in a separate incubator: Undamaged Group.
2. Plants suffering herbivory by beetles: Herbivory Group.
3. Plants communicating with the Herbivory Group (in the same incubator as Herbivory Group, but without contact with beetles): Communication Group.

In the cited publication we described the first application of electrochemical techniques for monitoring the chemical signatures associated with the defense of plants against external stressors. The proposed method employed to study the plant reaction to stress, exploits the electrochemical activity of many natural products present in the plants. This study was developed through two different approaches:

1. Voltammetry of Immobilized Particles (VIMP): forming a microfilm of the ethanolic extract of the leaf (produced by maceration) at the extreme of the electrochemical sensor.
2. Scanning Electrochemical Microscopy (SECM): by direct contact with the whole leaf fragment.

By both methods we obtained an electrochemical response composed by a group of characteristic signals from the electrochemical composition of the plant (signals corresponding to the reduction/oxidation (redox) reactions experimented by the plant´s components).

One interesting aspect of this approach is that in the middle of this complex “soup” are molecules biosynthesized by the plant as response to an aggression, and only one fraction of them will be detectable by the electrochemical methods and the rest will be silent. Fortunately, many molecules involved in the plant´s response to stress are related to the response of the oxidative damage and are “redox” active and consequently detectable by the electrochemical methods.

Focusing on the electrochemical study of the different groups, we found that the profile of the undamaged “pepper face” presented a series of redox signals different to the ones presented by the Herbivory Group. These differences evidenced the chemical reaction of the plant to the aggression from the beetle. The main divergences are condensed in the signals corresponding to the oxidation of compounds, mainly due to changes in the composition of organic compounds related to the defense against oxidative stress (like poliphenols or terpenes). Once we detected the chemical differences between the undamaged plants and the Herbivory Group, we focused our attention on the chemistry of the last group, Peperomia, the Communication Group. Interestingly, the chemical composition of this group of plants is similar to the recorded electrochemical profile of the Herbivory Group and evidently, different from the Undamaged Group. This observation is an important probe into the existence of chemical communications by VOCs.

Having arrived at this point, I concluded my explanation. I then confirmed that my father had understood the whole picture by asking him to summarize the story for me. Once I was sure that he had grasped the idea, I asked him two questions. I asked first about the possible applications of this experiment in his daily life in his field of work and then, I asked if he could give me ideas or suggestions about future research in this scientific area.

The answers to both my questions gave me precious information. As I mentioned at the beginning my father is an agriculturist and, based on his experience, he told me that it would be very interesting to determine if the response of the plants to herbivory is selective to each type of insect or not. If it is the case and we can identify the molecules that are producing the chemical response, we can plan to obtain them on an industrial scale, with the objective of testing them as selective pesticides (that will attack one type of herbivore only, thus will be non-toxic for other insects). He highlighted the importance of preserving the diversity of the biosphere, mainly because several insects are necessary for polinization and the correct development of several plants.

Furthermore, my father gave me an interesting idea for future research. He said that we could add one additional control to our future experiments. I was surprised, so I asked him for a more detailed explanation. He continued by saying that we could test to see if an undamaged plant will change its chemical profile when in atmospheric contact with the insect. Suddenly I realized that maybe the chemical conversation by VOCs involves not only plant-plant communication, but also insect-plant communication. This important moment due to my father´s observations had opened up a new pathway for me to explore. Let´s follow my Dad´s advice for future experiments!