Chemical Languages: The Way Plants Talk to Themselves and Others
Plants are more active than we think. Imagine a honeybee with a beautiful flower. What is the first thing that comes to your mind? That the bee is busy collecting food from the flower? Or that the flower is busy attracting the bee to spread its pollen around? Most likely we would think the bee is more active because it behaves more like us: always searching for tasty food. Plants, on the other hand, just sit there and appear to do nothing. You are right that plants can’t walk around; but that doesn't mean they can’t react to things around them.
The bee orchid is an excellent example. As its name suggests, the flower of the bee orchid not only dresses up like a female bee is resting on it, but also smells like female bees. A male bee can be easily lured by the enticing color, shape, and scents of the orchid, which pretends to be a female bee. By the time he realizes the "female bee" is fake and angrily flies off, the bee orchid has packed a full bag of pollen and stuck it on the back of the male bee. When the male bee is fooled by another bee-like orchid, he unknowingly helps deliver the pollen from one flower to another. Bee orchids have a brilliant strategy to find mates and reproduce! The colors and scents they produce are the deceptive message they send out to recruit match-maker pollinators.
In fact, the language in the plant world is the hundred-thousands of metabolites (small molecules). Metabolites essential for the normal growth of all plants are called primary metabolites. Here are some examples of primary metabolites. Like us, plants need energy to live; but unlike us, they can’t walk to a restaurant to order food. Plants have to make their own food energy by doing photosynthesis during the day. To make the food, plants first use light-absorbing molecules like chlorophylls (also the molecules that makes plant green) to capture energy from the sun and then use the energy to fuel the process of turning carbon dioxide into sugars. Sugars are great for storing energy and carbon. The breakdown of sugars not only releases energy for plants to use, but also provides the building blocks for making other molecules, such as amino acids that make up all kinds of proteins. Can all body parts in plants make their own food energy? No. Think about roots, flowers, and fruits that are not green; although they cannot do photosynthesis, they want sugars. What can they do? Perhaps send a message to leaves and ask them to deliver some extra sugars? Actually, sugars and many other signaling molecules can travel throughout the plant body. The “highway” system for the transportation of these metabolites is a tissue called phloem. Distantly located organs like leaves and roots can communicate with each other by exchanging metabolites via the phloem.
The environment around a plant is never static. Over time, plants acquire the capabilities to make a diverse array of specialized metabolites to communicate with the outside world. The reason why we call them “specialized” is because these metabolites are unique for certain plant species. Just like people living in different places speak different languages, plants living in different places are exposed to different climates, match makers, and attackers. They need to convey specific messages at a specific time, which is critical for their reproduction and survival.
Many plants can send out misleading messages to their attackers by using specialized metabolites. Terpenes are a group of such metabolites that are responsible for many aromas. One type of terpene, menthol, is made in mint. Menthol may smell refreshing to us; but for some herbivores, the smell of menthol is so terrible that they just don't bother trying to bite the plant. Some plants get annoyed and deliver fatal messages when they are chewed by herbivores. Cassava is one such plant that produces cyanogenic glycosides. These metabolites are not toxic on their own; but when the plant is crushed by an herbivore, cyanogenic glycosides will be broken down to release poisonous substances that seriously harm the herbivore.
The showy colors of many plants come from another class of specialized metabolites called anthocyanins. One example would be the red leaves in autumn, which are typically seen in deciduous plants that shed leaves every year, such as maple. When the plants sense the drop in temperature, they want to express themselves in a different way. The chlorophylls in the leaves decrease while the anthocyanins accumulate. As a result, leaves gradually turn from green to red. One reason for the color change is because the metabolism slows down in cold weather so leaves receive more sunlight than they need. The red pigment anthocyanin may serve as a sun screen to protect the leaves from excess sunlight. Another possibility is that the red pigment may serve as a warning signal to deter herbivores from settling on the leaves. Anthocyanins are also present in many colorful flowers and fruits. Next time when you are attracted by the vivid colors and pleasant aromas of some fruits in a grocery store, you may hear these little creatures murmuring “please pick me and help spread the seeds!”