People tend to think that plants don’t do much from day to day – certainly when I asked my first-year students at the start of the course, they were far more interested in animals than in plants.
Poor plants! But then, to the casual eye, I guess they’re fairly static creatures
However, it turns out that their responses to the environment are probably just as nuanced as the behaviours that we see in animals. For example, not only can plants detect that a herbivore is munching on them, some can also warn other plants that this is happening, giving them time to prepare their own defences. And some can also call in the cops: they can signal to predatory insects that a caterpillar or beetle is there, ready to be attacked in turn. (You can read more about other forms of plant defences against herbivory here.)
The combination of the physical damage due to grazing, plus detecting saliva from the herbivore, initiates what’s called a signal transduction pathway. One result may be the manufacture and release of a range of volatile chemicals into the air – these are the signals aimed at detected by other plants and, sometimes, predators on those herbivores. (A second outcome is the production of aversive chemicals by the damaged plant itself, including in those parts that have yet to be actually nibbled on. There’s a nice graphic here.)
Over the years there’s been quite a lot of research done on this. For example, Karban et al. (2000) found that sagebrush plants emitted more of two airborne chemicals (VOCs, or volatile organic chemicals) after they were grazed on by grasshoppers, than before. The researchers simulated grazing by clipping the plants with scissors, and got the same result: more VOCs emitted after the plants were damaged.
They hypothesised that these chemicals were alarm signals that other plants might a) detect and b) respond to by preparing their own defences before being grazed on themselves. To test this, they looked at what chemicals were produced by undamaged plants grown near those that were harmed.
They found that tobacco plants grown next to unclipped sagebrush plants produced only low levels of defensive chemicals – and that production shot up if the sagebrush plants were damaged by clipping them. They also found that grasshoppers did a lot more harm to the first group of tobacco plants. This not only confirmed that the VOCs were indeed carrying a signal – but also that these signals could be detected and responded to by other species of plants.
Then, there’s that ability in some plants to call on other insects for help. How does that work? As Ed Yong describes in this article:
When hornworm caterpillars eat tobacco plants, they doom themselves with their own spit. As they chew away, a chemical in their saliva reacts with airborne substances that are released by the beleaguered plants. This chemical reaction sends out a distress signal that is heard and answered by the predatory big-eyed bug. The bug eats hornworm caterpillars. Drawn by the chemical SOS of plants under distress, it finds plenty to devour.
And that’s a very specific message indeed – it only works when there’s the right mix of caterpillar spit and plant signalling chemicals, and calls in one particular predator.
As War et al (2018) point out here, this “silent war” between plants and the insects that consume them has been going on for pretty much as long as plants and insects have coexisted. (In fact, I remember using a video called “The 300-million year war” with my senior biology high-school classes, back in the 1980s.) It’s an example of coevolution: “Both plants and insects have developed morphological and biochemical defensive traits to dodge each other’s strategies.”
So it shouldn’t come as a surprise that natural selection has driven the evolution of the ability to mess with plants’ alarm systems. As Ed Yong reports,
the silverleaf whitefly—can hack this communication system. When the whiteflies bite, they somehow change a plant’s airborne warnings so that they convey information about the wrong threat. Deceived, the neighboring plants invest in the wrong defenses and become more susceptible to the whiteflies. These pests, by seeding entire fields with faulty intelligence, can prime the plants ahead of them for their arrival.
300-million years, and counting…
R.Karban, I.T.Baldwin, K.J.Baxter, G.Lane, & G.W.Felton (2000) Communication between plants: induced resistance in wild tobacco plants following clipping of neighbouring sagebrush. Oecologia 125: 66-71
A.R.War, G.K.Taggar, B.Hussain, M.S.Taggar, R.M.Nair & H.C.Sharma (2018) Plant defence against herbivory and insect adaptations. AoB PLANTS, 10 (4): ply037
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