Plants can also experience stress. Due to drought, mold or damage – or simply by a mowing machine. Because that wonderful smell of freshly cut grass is, as biologists discovered years ago, actually a distress signal. Plants that are attacked produce odorants that can attract beneficial insects as henchmen in battle.
Such allies can do little against a lawnmower, but against nibbling caterpillars or sucking aphids they can. “Take parasitic wasps,” says biologist Erik Poelman (42) in his study at Wageningen University. “When certain plants are eaten by caterpillars, they produce substances that attract those parasitic wasps. As soon as they catch sight of the caterpillars, they lay eggs in them, which means that the caterpillar is doomed and the plant is freed from its attacker.” A few meters away from him is a display case with a giant fake parasitic wasp, in which the situation is visually explained. “We regularly have primary school classes here. These students are very interested and always want to know everything about insects.” A few years ago, even children’s TV program The core an episode to plant stress.
Poelman’s research is not only popular with children. Since he obtained his doctorate in 2008 on interactions between plants and insects, he has been awarded numerous grants – including the Veni grant from NWO. This spring, he received the prestigious Vici grant from the same science financier, with which he can further expand his research into plant stress management over the next five years. “And to think that I wasn’t really interested in plants at all in the first place.”
As a plant, you cannot afford to invest all your energy in defending against one attacker
Did you find plants uninteresting?
“That’s what I thought at first. During my studies I was researching evolutionary adaptations in poison frogs in South America, and I hoped to continue in that direction. But there was no budget for that. Marcel Dicke, professor of entomology here in Wageningen, asked me whether I wanted to do research into plant-insect interactions. That was an idea that took me a while to get used to… But then I realized it wasn’t so much the poisonous frogs that fascinated me, but that evolutionary development. And that is just as present in the race and the cooperation between plants and insects.”
What kind of interactions are you investigating?
“For plants, it is always a question of when they will be attacked by which insects. That brings uncertainty, and it requires flexibility. Because as a plant you cannot afford to invest all your energy in the defense against one attacker, because you do not yet know which future attackers you will have to deal with. Compare it to a game like Stratego: you shouldn’t play all your trumps right away, because then you will be left with nothing. At the same time, many plants also depend on insects for pollination and protection, such as the parasitic wasp. A defensive response – for example in the form of toxins, stiff hair or extra thorns – should not ensure that these beneficial insects also stay away.”
First of all, we want to unravel patterns: which insects come when, and how long do they stay?
How does a plant recognize what kind of attacker it is dealing with?
“For example, the saliva composition, or the eating pattern.”
We walk to a field behind the university building, where Poelman’s group has run various experiments. PhD candidate Hanneke Suijkerbuijk, who studies how caterpillar feeding affects pollinators, stands with a stopwatch and notepad in a field full of Chinese cabbage with bright yellow flowers. She keeps track of which pollinating insects are attracted to them, and how long they last. Poelman: „In the coming years we will focus specifically on the Brassicaceae, the cabbage family. It consists of very different types. There is the thale cress, for example, which is often used as a ‘model plant’ for biological research and whose strategy is to flower very quickly, when there are no insects yet, so that it is ahead of the game. But you also have larger varieties that flower for more than one growing season, and can therefore also have to deal with insects more often.”
What do you want to discover about those cabbage plants?
“First of all, we want to unravel patterns: which insects come when, and how long do they stay? Then we will investigate the physiology of the plants in the lab. Which plants react to what, and in what way? Do plants match their defense strategy with insect attack patterns? Biology often looks at co-evolution between certain species—that is, interactive evolution, where one species reacts to another and vice versa. In fact, we want to find out how an entire community of life shapes the evolution between species.”
If you are not resilient as a plant, it will lead to you losing from the pest insects
Poelman moves on to an experiment with black mustard – also a species from the cabbage family. Blue nets have been stretched around the fields, with meshes of different widths. “Because of the large meshes, butterflies such as cabbage whites can still fly to the plants, but not anymore because of the small meshes. In the latter case, no eggs are laid on the leaves, and the leaves will be gnawed less. We want to measure the extent to which this affects seed production: does the absence of caterpillars lead to more seed of better quality? We still put caterpillars on some plants that cannot be visited by cabbage whites, to see to what extent those caterpillars really affect the seed.”
Does your research also have consequences for agriculture?
“Our research is primarily driven by curiosity. But you could see it as a precursor to actual applications. According to the Ministry of LNV, sustainable crop protection must be in place in the Netherlands by 2030. If those pesticides disappear, plants will have to deal with more pests. And then from a plant breeding perspective it is important to know how plants deal with attacks from entire insect communities.”
Can a plant burn out?
“If you are not resilient as a plant, it means that you lose to the pest insects – then you are often doomed. Plants often have a harder time when various stressors build up, for example when they have to deal with drought and aphids at the same time. The same goes for humans: it’s hard to keep several balls in the air at the same time, so you have to learn what to prioritize along the way. In that sense, we are like plants.”