Some time ago, Stevie Wonder put together an adventurous album called Journey Through the Secret Life of Plants.
It was an interesting listen, but nothing quite like this. A new study arrived at the fairly gruesome conclusion that plants can feel insects eat them alive. And a specific type of plant, the Arabidopsis in the cabbage family, can actually distinguish between insects munching on it based on the way they chew and drool.
“It was no surprise that plants responded differently to having their leaves chewed by a caterpillar or sucked by an aphid,” said Heidi Appel, senior research scientist in the Division of Plant Sciences at the University of Missouri and a lead author on the paper. “What surprised us was how different plant responses were to each of the caterpillars and aphids. The plants could clearly tell insects apart—they really seem to ‘know’ who’s attacking.”
Plants don’t bleed red and scream the way wildebeest do when lions bite into their hind parts on the Serengeti, but they often die all the same if they can’t defend themselves. This study by researchers at Missouri and the University of British Columbia set out to prove a hypothesis that plants not only know when they’re being eaten, but that they tell several different insects apart even when they eat at the same time.
This could allow agriculturalists who breed and design cash crops to give them the skills to fend off farm pests. “If we are breeding plants to be more insect resistant,” Appel said, it would be helpful to know how they respond to different insects that prey on them.
Like other plants, Arabidopsis are able to handle predators one-on-one, but fights are rarely that fair in the wild. Pests often come in waves, and have horrible manners.
Before developing into colorful butterflies, caterpillars are as plump and sloppy as Homer Simpson. They don’t close their mouths when they chew and they slobber. As the insects dine, the plant’s genes spring into action, hashing out the chewing motion and the drool to say, for example, that this one here is a cabbage butterfly caterpillar and that one is a beet army worm.
To prove this, Appel and her co-authors — Jack Schultz, a University of Missouri researcher who led the study, and Joerg Bohlmann, a University of British Columbia researcher who performed genetic studies, along with six others — planted Arabidopsis seeds in a lab and allowed them to grow to a size that caterpillars and worms love.
Then they sacrificed them. They herded up a bunch of cabbage butterfly caterpillars and beet army worms and let them go to town on the plants. They took picture of the insects having dinner and even posted them online the way people do on Facebook. The experiment took a year.
Plants lose water while being chewed on by some insects that bite and poked by other insects with a needle for a mouth that suck out their innards. In the process, they lose tissue and, worse, might get infected with some sort of ghastly bacteria, but generally they live to see another day.
It’s hard to tell with the naked eye how they respond to prey, but they are known to react to pests with a suite of defenses — pretty much chemicals that give them the spicy and pungent flavors humans love (and hate) — to repel them. In order to know for sure, researchers harvested the chewed up Arabidopsis, extracted the genes, froze it in a Missouri lab, then shipped it to British Columbia, where Bohlmann took over.
There is detailed technical information in the paper showing what he did there. But it all comes down to taking microscope slides with the plant’s genes printed on them and using florescent dye to show how the expression of genes were influenced by the insects.
The study, funded by the National Science Foundation and published in the journal Frontiers in Plant Science, wasn’t conducted for charity. There’s money in Arabidopsis, which flowers into major food crops kale, broccoli, cauliflower and brussel sprouts.
Rather than soaking plants with insecticides, agricultural breeders are keenly interested in designing them in a way that allows them to naturally resist pests such as two the researchers chose for their experiment. They want to pack martial arts insect fighting capability in the seed.
Whatever benefits the plant is secondary to the boon to agriculture. The cabbage butterfly caterpillar is a fussy eater that prefers to prey in the Arabidopsis family, and the beet army worm will eat just about anything. Both eat like slobs.
“Caterpillars drool as they eat,” Appel said. Anyone who’s watched a kid at the dinner table knows that drool normally slimes straight down. Based on the genetic study of the Arabidopsis tissue, the plant feels the icky sensation, and even more so, the gnawing into the leaf. There were different genetic responses to each worm, showing that the plant actually identified what was eating them.
“It’s a signal that the plant could be responding to insects differently,” she said. “Their saliva may be different.”
“Among the genes changed when insects bite are ones that regulate processes like root growth, water use and other ecologically significant processes that plants carefully monitor and control,” Schultz said. “Questions about the cost to the plant if the insect continues to eat would be an interesting follow-up study to explore these deeper genetic interactions.”
The findings have already been incorporated into research that was published at the same time in the same journal, Appel said. The value of the study is the revelation that plants respond with different chemicals to different insects, and the more defenses, the better. With Arabidopsis, for example, breeders who want to fight off farm pests have to take each insect into account, and not grow plants that can defend against only one.
“Breeders have to do that. This is a really interesting new frontier in plant biology,” Appel said. “How to deal with multiple stressers,” giving plants the ability to not only fend off one attacker, but many.