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Plants Talk—Just Not the Way You Think

Last year my mum got two chili pepper plants, planted them right next to each other. Then one day she noticed that from one plant most of its leaves where gone, chomped up by some hungry insect. A caterpillar or a hungry slug. She did not know. But she did notice that the other chili pepper plant, the one standing right next to it, was unharmed, as if it just wasn’t that tasty as its almost devoured neighbour.

So, what happened here? Did the attacked plant ‘warn’ its neighbour. Do plants really talk among themselves or is this all just chemistry with a bit of humanising sauce put in the mix.

What is communication

But first what do we mean with plants talking. Talking among humans seems pretty clear. Someone says something, whether this is by speech, written word, or sometimes even using images, to get a message across to which their audience can reply or act. There is some form of intent behind the message that is conveyed. Someone just muttering to themselves is not seen as talking in a communicative sense.

So how does that translate to plants. The simplest definition can be that one plant produces a signal and another organism, plant or otherwise, acts on it. It changes its behaviour, like the chili pepper plant, who, upon sensing that its neighbour was being eaten, produced some compounds that made its leaves less tasty.

Obviously, plants don’t speak. Neither can those signals plants produce for communication be translated into the speech-like communication we have. For talking, plants use molecules which they kind of let loose in a ‘noisy smell-based groups chat’. But that is how a human would describe it to other humans so they can get a feel for the world plants live in.

It is just as if we can’t help ourselves but to make plants appear a little bit more human when we talk about them. We project on them human capabilities. Which is just the reason why scientists are hesitant to use terms like ‘language’, ‘pain’, and ‘intelligence’ when talking about plants.

Writing this makes me remember my PhD supervisor who reprimanded me for saying that cells ‘decided’ something. He was completely right on this of course. But by taking the emotion out of the message, the message gets not only clearer but also less relatable.

Now we have that out of the way, I tell you the real non-humanise story of talking plants. Because talking they do, they can be real chatterboxes. Oke that was the last of the human metaphors, here we go.

Attacked chili peppers

So, what happened to these two chilli pepper plants between the moment the first one got munched on and the second one became unpalatable. The moment an insect starts eating or sucking plant sap it creates a breach in one or more plant cells. This is sensed by one of the many sensors present in the cell membrane. Plants have also learned to recognise specific molecules present in insect saliva. Detection of either one or both of them results in the sounding of the alarm.

You can visualize this sounding of the alarm as a current moving from cell to cell. From the wound side it is spreading throughout the whole plant. Each cell the current passes gets to work to startup their defence program, consisting among others of producing volatile organic compounds, a.k.a. scent molecules.

These scent molecules are small molecules that can become airborne and travel through the air. When we smell freshly cut grass or crushed mint leaves, we smell a mixture of released volatile organic compounds. Plants use them for communication.

The scents that a plant releases after an herbivore attack can be grouped into two groups. Green leave volatiles, consisting of six carbon alcohols, sesquiterpenes and homoterpenes, are the first, within seconds after the damage they get released. Their fast production is due to the plant storing the precursors of those green leave volatiles in their cell membranes. But it doesn’t stop there. After several hours the plant releases a second salvo of scents. This time consisting of terpenoids and aromatic compounds.

Neighbouring plants

These scents, both the first and second round, neighbouring plants pick up. The scent molecules drift via pores, stomata, into their leaves. There, like in our nose, leaf cells are full of receptors that can pick up those scent molecules. Although, here I have to be honest, most of those receptors have not been identified so far, partly because they in no-way resemble the receptors we have in our nose. But researchers know that there must be receptors of some sort because what happens next.

Only when plants perceive the volatile organic compounds attacked plants release, do they put up their own defences. So, upon perceiving the stress signal from its neighbour that is being eaten, the non-attacked chili pepper plant in my mum’s garden sounds its own alarm, likewise by sending a current through the plant.

But unlike its neighbour that is being eaten, the non-harmed plant doesn’t go all out yet. They ready themselves, a process scientists call priming, so that when herbivores show up, they can attack twice as strong.

This priming involves converting the molecules in these scent bouquets from their attacked neighbours into non-volatile toxins and nasty tasting molecules that stop the herbivory insects from wanting to eat more of that particular leaf. Primed plants also produce their own toxic and nasty tasting molecules as well as enzymes that inhibit the insect’s digestion of plant material.

The last thing that primed plants do, is producing extra floral nectar to attract predatory insects. These insects lay eggs in herbivory insects, hunt them for their young, or eat the herbivory insects themselves, but predatory insects also feed on the extra floral nectar.

What is in it for the wounded plants

Now you might be wondering are those wounded plants just releasing those scents to warn their neighbours or is there something in it for them too.

There is, quite a bit actually. Remember how those perceptions of the stress scents triggered the defence response in neighbouring plants? It does the same in the distant far from the wound site located tissues of the attacked plant. Volatile molecules reach those sites faster than the current, that is initiated upon perception of herbivory, can.

But those released scents also help to repel, deterrent, or even kill the attacking herbivore. In addition, they are a sign for predatory wasps, mites or ants, that there is prey to be found. These predatory insects subsequently help to reduce the number of plant munching insects.

Released volatile organic compounds also help keeping the damage that those munching insects cause to a minimum. They kill potential plant pathogens that these hungry insects bring along, and seal off the wounds that the insects create.

So, plants chatter, with help of scents, when they are wounded. To themselves and to predatory insects. Not so much directly towards their neighbour though. That neighbour is just ‘eavesdropping’ in on the conversation that is going on in within its hearing, or better said, sensing distance.

Plants not only ‘talk’ when they are wounded. Enticing insects to visit their flowers with tempting smells, whether it is the sweet smell of roses, or the pungent smell of rotting flesh of the carrion flower, is probably the best know example of plant communication.

Belowground

But communication doesn’t restrict itself to life above ground. Underground plants communicate just as much if not more. Volatiles, but also non-volatile excretions are used belowground to communicate with bacteria, fungi, and archaea, and if necessary to tell roots of other plants to stay of their turf.

Firstly, by releasing small molecules like sugars and amino acids plants recruit microbes. But to make sure that only those that are welcome stay, they mix this with toxins that keep the unwanted ones, like pathogens, away. Plants also use those toxins to keep encroaching neighbouring roots out of the way. And lastly like aboveground, belowground plants also release specific compounds to attract microbes that help with things like nutrient and water uptake.

Mycorrhiza, a group of fungi that intimately interact, forming a symbiosis, with plants by literarily nestling themselves in the cells of roots, are well known for giving a helping hand. Although, they do like to get sugars in exchange for the nutrients and water they deliver.

Now reading this you might be thinking that those mycorrhizae are connecting all plants with each other, like the wood wide web narrative suggests. But while those fungi can have connections with multiple plants at the same time, a better analogy for the connectedness would be the closed-off intranet networks within companies and organisations. Both plant and mycorrhiza have preferences with which species they connect, and the fungi often connect to a limited number of plants. And yes, some of those networks can become large, just as some companies do.

However, and this is important to mention, as this is where the intranet analogy stops. Although it is clear that communication between the fungus and each individual plant occurs. If the fungus also functions as a go between for the plants that are connected to it, that is unclear.

Take for example tomato plants whose roots are all connected by the same mycorrhiza. If one of those plants is attacked by let’s say a hungry caterpillar, then those other plants prepare their defences. Even if they do not receive the aboveground emitted scents, due to a scientist placing containers around the aboveground tissue of each plant. Now this clearly shows that those non-attacked plants get some kind of signal that triggers them to prepare their defence. But if this signal is coming directly from that attacked plant, or from the fungus who wants to protect its carbon sources, that, is not yet clear.

Benefits of all that talk

So, plants talk a lot, or better said they release lots of volatile organic molecules and other small molecules that function as messengers and signals for other organisms, be it plants, insects or microbes. These signals can be intended for a specific receiver, like a pollinator or mycorrhiza fungus, or they can be more general. But they all have a function, a benefit for the plant producing them. Again, the clearest example here are the volatiles to attract pollinators who bring pollen from one flower to the next to fertilize those flowers.

But, attacked plants don’t ‘warn’ neighbouring plants on purpose, as, like you just read, even the release of volatiles during an herbivory attack is mainly for the benefit of the attacked plant. That doesn’t mean neighbouring plants can’t benefit from ‘eavesdropping’.

Talking with sound?

Now so far, I have talked about how plants communicate using chemistry. For this there is solid evidence. Even if there is also still a lot unknown, take those scent receptors for example.

But some other forms of plant communication have been making headlines. Communication via sounds. That plants produce ultrasonic sounds that others react to for example. These claims, even though they have been around for some time, are still contested and tentative. In science claims that go beyond mapped territory receive extra scrutiny. And as annoying this might be for exciting claims, it is with good reason, as it would not be the first time that the ground below a scientific claim is less solid than first thought.

That is not to say that these claims don’t deserve attention, they do and preferably with scrutiny and all. But that is for another post not to make this one twice as long.

Where are we standing

Plants are obviously communicating with the outside world, not by using words or any equivalent to those, but by scents that mix and diffuse. Surprisingly they mainly communicate with non-plants. A bit like if a human only communicates with say dogs or insects and not with any other humans. Which can understandable feel alien to us. I think that is why we, science communicators and journalists, like to humanise them a bit with saying that they ‘cry for help’ and ‘warn’ their neighbours. Not that plants need it, they communicate quite well on their own.

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