Communication is an essential part of human and animal life. Just imagine trying to survive in a world where communication doesn’t exist! While their methods of communication may not look the same as ours, plants also rely on being able to communicate with the rest of the world. Plants, despite having no sentience or ability to make noises or move quickly, interact with and respond to visiting insects and wildlife.
Since plants cannot move, they must constantly make little changes to adapt to their environment. Plants have systems that optimize water and nutrient usage, monitor light levels, and defend them from stressors. All of these systems rely on interaction with the world to respond properly. However, in the world of botany, ‘communication’ is used to describe the combination of hormone signalling and biotic interaction – in other words, a plant having a change in hormone levels causing a change in response to an interaction with another living thing. This blog is part of a series of posts called How Do Plants Work? and will go through some of the different levels of plant communication.
What is Hormonal Signalling?
Hormonal signalling is the basis of communication not only in plants but in animals as well. Hormones are signalling molecules designed to travel through the body or plant and start certain processes. They mainly influence growth, development, metabolism, reproduction, and mood (or stress levels in plants). In order for hormones to be sent to initiate a response, the plant first needs to receive the signal.
A signal, in this case, is any change in environment or interaction that causes one or more responses in the plant. Signals are captured from the outside world by receptors. There are many different types of receptors in plants, each specifically designed to receive a certain signal. The receptor then initiates a signal transduction. This is essentially just the process of getting the alert from the receptor to wherever it needs to go within the plant. Each plant response has its own specific signal transduction pathway, using many different molecules within cells. Signal transductions can range from being really slow (a wind-gnarled tree after years of wind) to quite fast (think Venus Flytrap).
Once the alert gets to the right place, it triggers the response. A response is any change that the plant makes because of the original signal. There are two main types of signalling:
- Non-cell autonomous signalling, where the signal and response happen in different cells, and
- Cell-autonomous signalling, where the signal and response happen in the same cell.
Cell-autonomous signalling causes significantly faster responses than Non-cell autonomous signalling. Guard cells, the cells in charge of opening and closing the stomata for photosynthesis, have the signal and the response occur in the same cell.
Biotic Interactions
A biotic interaction is when a plant comes in contact with a living thing, as opposed to an abiotic interaction, where a plant is affected by non-living things like weather and rockslides. Plants can have all sorts of biotic interactions, like being eaten by an animal or having a friendly relationship with bacteria.
Here are some common biotic interactions that plants might experience.
A. Mutualisms
Mutualisms, also known as symbiosis, are relationships between two different species where both individuals benefit. For these relationships to work, the plant has to be able to communicate with its partner on the chemical level. In many cases, basic information is shared between them through different molecules.
Various mutualistic relationships can be observed in invasive plants:
Pollinators are an example of a mutualistic relationship that many plants have. Himalayan Balsam is an invasive that has adapted to be very good at attracting pollinators. It produces tons of extremely sweet nectar and draws pollinators away from our native wildflowers.
Note that while Himalayan Balsam does attract pollinators, it has many negative effects on the environment, so in the long term we – and the pollinators! – would be better off growing native species than invasive Himalayan Balsam.
Mycorrhiza, the underground thread-like body of fungus, is very commonly found in mutualistic relationships with plants, especially woody ones. English Hawthorn, a member of the woody Rose family, is known to form mycorrhizal relationships. The fungi help the plant by acting as an extended root system and increasing the surface area that can be used to collect nutrients.
Nitrogen-fixing bacteria sometimes make themselves at home in certain plants’ roots, gaining shelter and nutrients from the plant in return for fixing Nitrogen from its natural state to a form that the plant can use. Broad-leaved Peavine, as well as most other legumes, have these root nodules. They allow the plant to direct its energy toward growing rather than fixing the nitrogen for itself.
Since the bacteria live within the roots, only the plant host benefits from this relationship, not the rest of the environment!
B. Negative Biotic Interactions
Not all interactions that plants have with other living things are good – in fact, a lot of them are bad! Here are some examples of common negative biotic interactions.
Parasites are a problem that many plants face. Plants don’t have immune systems, so they have few options if they get infected. Thankfully for us, in certain conditions, those natural parasites can be used on invasive plants as a form of biological control.
Common St. John’s Wort is a rare example of an invasive species that can be controlled using biocontrol. Certain beetles that feed only on St. John’s Wort can be released to act parasitically on this invasive; by feeding on the target invasive plant, they can help keep it to a level that is acceptable at the landscape level.
Allelopathy is when plants compete with other nearby plants by secreting compounds into the soil that alter the other plants’ gene expression and inhibits their root growth. Spotted Knapweed is infamous for its allelopathic qualities. This invasive engages in chemical warfare with our native plants, inhibiting their root growth by releasing toxic compounds into the soil.
Animals, especially herbivores can mean either good or bad news for a plant. Much of the time, being used as a snack is not an ideal outcome for a plant. In fact, some plants have chemical defenses, and release nasty-tasting compounds when they detect that they’re being chewed.
Other plants actually welcome animals, and have evolved to utilize them as a way to spread. For instance, invasive Himalayan Blackberry relies on birds and bears eating its berries to spread its seeds over great distances.
Types of Defenses
To protect themselves from predators and pests, different plants have developed a variety of defence mechanisms. A plant can have two types of defences: constitutive or inducible. Constitutive defences are always present, like sap or thorns. Inducible defences only occur in response to stressors.
Plants don’t have immune systems, but some can develop resistance throughout their system to certain pathogens. They develop inducible defences that are triggered by the pathogen! Often, when experiencing some sort of stress, plants release hormones into the air to warn their plant neighbours, which, in turn, causes other plants’ inducible defences to kick in early. You can think of this as a form of eavesdropping rather than communicating directly.
A. Constitutive defences
Constitutive defences are typically toxic compounds or mechanical barriers made by plants. Mechanical barriers are the first line of defence that plants have between them and the outside world, a bit like as armour. What we tend to call ‘thorns’ or ‘prickles’ can actually be sorted into three distinct categories, based on what plant part they’re made of:
Prickles are formed from the epidermis, or ‘skin’ layer of the stems. This is what is seen in blackberries, including Himalayan Blackberries.
B. Trichomes
While these mechanical defences are very effective against larger threats (like bears and humans), they are not effective against insects. Instead, some plants use trichomes to defend themselves against these tiny threats: these can be either little hairs or sacs, formed from the plant’s epidermis (skin) cells. However, the tiny hairs and toxic compounds that trichomes can contain can be very effective defence again insects.
Here are some examples of how trichomes benefit native and invasive species:
Hair-like trichomes make it difficult for insects to get close to the plant. Orange Hawkweed is covered in long trichomes.
Native Plant Appreciation!
Stinging Nettle has very unique trichomes that give it its intense stinging ability. The trichomes, which cover the leaves, are hollow points made of a substance that is essentially glass, sitting upon a squishy base. These glass points are filled with a mixture of acids and other compounds; when something brushes against a trichome, it breaks the off of tip the glass point, injecting the cocktail with the use of the fleshy base as a plunger.
References
- Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant Physiology and Development (6th ed.). Sinauer Associates Inc.
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Invasive species mentioned in this post
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