Plants: The Master Chemists of Our Planet

Imagine your legs were buried in the ground and you were not able to move or to talk. What would you do to eat, grow, and defend yourself? This situation is more common than you think: It is part of the daily life of plants. Plants have developed many strategies not only to survive, but also to interact with other plants, animals, and microorganisms around them. Some of these strategies involve using chemical compounds that work as messages from the plant to its surroundings. Scientists have developed creative methods to estimate what and how much of a chemical is inside a plant. If we can identify the great diversity of plant compounds, in the future we might be able to better understand how plants grow and interact with their environments. Also, we may be able to use those compounds to make medicines and to produce healthier, tastier vegetables.


PLANTS PRODUCE CHEMICALS TO GROW AND INTERACT WITH THEIR SURROUNDINGS
Plants are sessile, which means they cannot move and are permanently restricted to the spot where they germinate. As plants grow, they must cope with an environment that changes all the time (sometimes too dry, some others too wet). Think of the changes in seasons, or the di erent weather conditions throughout the day and night. As if that is not enough, plants also need to have strategies to interact with their neighbors, to attract animals that help them move their seeds or pollen from one place to another, and to defend themselves from animals that want to eat them. One of the strategies that plants use to do all of that is to produce a great number of chemicals, known as metabolites. Plant metabolites are METABOLITE Small chemical compound that helps plants grow and interact with other organisms. small chemical compounds that help plants grow and interact with other organisms.
It is estimated that, taken together, all plants produce between , and million metabolites [ ]. To study them, scientists have classified the metabolites into two groups: primary and specialized metabolites. Primary metabolites are found in all plants and help the plants grow, develop and reproduce. One of the most well-known groups of primary metabolites are carbohydrates, which provide plants with energy to grow. Specialized metabolites are unique to di erent plants and they help plants interact with other organisms. We will focus on specialized metabolites in the remainder of this article.
Have you ever wondered how a plant can defend itself? Plants and herbivore (plant-eating) insects have lived together for millions of years, and during this long time, plants have developed toxic defense compounds. For example, when an insect starts eating the leaves of a cabbage, the plant increases the amounts of toxic specialized metabolites called glucosinolates, which are then converted into even GLUCOSINOLATES Specialized metabolites with pungent taste, present in plants, such as broccoli, radish, and mustard. more toxic compounds [ ]. These compounds make the cabbage's leaves taste very unpleasant, discouraging the insects from eating them ( Figure A). Glucosinolates are very familiar to us, as they give broccoli and radishes their characteristic flavors. At the same time, insects have created ways to tolerate those toxic compounds, allowing them to keep eating those "poisonous" plants. This continuous battle between plants and insects has resulted in the evolution of new specialized metabolites.
In other cases, plants need to attract animals to pollinate their flowers so that they can produce seeds to secure their reproduction ( Figure  A). Pollinators are not only essential for plant reproduction, but they are also important for humans, as % of the world's food crops depend on pollinators for successful production [ ]. One of the strategies plants use to attract pollinators is to produce a sugary liquid called nectar. Bees and other insects visit plants to drink the nectar and while doing so, pollen attaches to their bodies. When the same insect visits other flowers, the pollen from the previous plants is released in the new flower, securing future seed production. In this way, plants use the visiting insects for their own benefit. Interesting fact: scientists have found that nectar does not only contain sugar, but also small amounts of ca eine [ ]. In high amounts, ca eine is bitter, and it works as a plant defense compound. However, in low amounts, it acts as a memory enhancer, stimulating insects to remember to come back for more nectar and further ensuring the plant's pollination process.

HUMANS (AND OTHER ANIMALS) USE CHEMICALS PRODUCED BY PLANTS
Humans and other animals have learned to use chemicals produced by plants for their own benefit. Since ancient times, people have used plant metabolites as medicines, natural dyes, and ingredients in food and cosmetics, amongst many other uses ( Figure B).
One of the oldest plant extracts is opium, a mix of chemical compounds extracted from the plant Papaver somniferum, commonly known as the poppy, which was used as an antidote against snake and spider bites and scorpion stings. Today, morphine, one of the many chemicals found in opium, is prescribed to alleviate pain. Saponins are another well-known example of plant compounds used We are still far from identifying all plant metabolites and even further from understanding how plants produce them. However, in the last decades, technological developments have allowed scientists to discover more plant metabolites. In the next section, we will explore how scientists isolate and identify these substances.

HOW DO SCIENTISTS IDENTIFY AND STUDY PLANT CHEMICALS?
Since the extraction of metabolites. After some time, the solvent takes up the flavor and color of the metabolites contained in the co ee beans. The mixture is then filtered and the solid plant materials are discarded, while the liquid solvent contains an extract of plant metabolites.
Scientists have applied this same principle to extract and study many plant metabolites. To identify specific metabolites, scientists must consider their chemical and physical properties, such as whether the metabolites can dissolve in water or whether a di erent solvent is needed. Obtaining the filtered extract is the last step of the extraction process ( Figure A). The next steps are the separation and identification of the chemical compounds present in the extract.
Chromatography is a technique used to separate chemical compounds CHROMATOGRAPHY A separation technique commonly used to separate a mix of compounds.
( Figure B). The liquid mixture of metabolites to be separated is called the mobile phase (contained in the tube in Figure A). The mobile phase is then flowed through a second substance called the stationary phase (colored blue in Figure B). The metabolites in the mobile phase (plant extract) will interact with the stationary phase in di erent ways. Some metabolites will move slowly through the stationary phase and others will move more quickly, causing the various metabolites to separate. The di erent travel time of each metabolite is one of the signatures that scientists use to identify them.
Some plant metabolites can be easily identified using chromatography alone. However, plant metabolites can be extremely complex. This complexity makes their identification di cult, and other methods are sometimes required to identify them. Mass spectrometry is a

MASS SPECTROMETRY
A technique to measure the mass and charge of ions from molecules present in a solution.
technique that breaks metabolites down further and then separates the di erent parts (called ions) to detect how many of them are present in a chemical compound ( Figure C). A mass spectrometer kids.frontiersin.org is usually composed of three main chambers. In the first chamber, the metabolite is disintegrated into its essential parts, called ions. The ions race through the second chamber, called the mass analyzer, to reach the third chamber, called the detector. The mass of each ion and the time taken to travel through the mass analyzer is recorded by the detector, providing extremely specific information about the ion's identity. Chromatography and mass spectrometry can be combined in a single, powerful machine to detect very small amounts of specialized metabolites.

WHAT IS NEXT?
We have shown you just a few examples of the great diversity and uses of plant metabolites and explained how scientists isolate and identify them. Many plant metabolites have already been discovered and, in addition to being important to the plants that make them, some of the compounds are also useful to humans . There are If plant metabolites interest you, check out taxol, artemisinin, and betalains.
still plenty more plant metabolites to be discovered and explored, and every year scientists discover new ones. Understanding plant chemicals is not only exciting, but it also helps us to develop new medicines and agricultural resources. Although the functions of many plant metabolites are still not understood, these compounds represent a huge reservoir of potential applications. This great diversity of chemical compounds makes plants the master chemists of our planet. . doi: . /frym. .

CONFLICT OF INTEREST:
NC-Q is employed by Keygene N.V.
The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
COPYRIGHT © Carreno-Quintero and Cárdenas. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original To better understand how and why plants have become master chemists, I study plants' genes using molecular biology, and their chemical compounds using metabolomics. I have studied the biosynthesis of bitter compounds called steroidal alkaloids in potato and tomato plants, and now I study soap-like plant-defense compounds called saponins. The more we understand about how and why plants make chemical compounds, the easier it will be to develop useful products for a more sustainable society. *pdcardenas@plen.ku.dk