Your Plants Probably Hate Being Touched

We will never stop carrying a torch for plants — they are way more amazing than most people give them credit for. They can hear oncoming attackers, and many send noxious chemicals into their leaves to fight back. Some even have a primitive sense of sight. So believe us when we say this: They don’t like it when you pet them. According to research, they have a very strong reaction to touch.

Leaf Me Alone

Plants’ hatred of cuddles is nothing new to scientists. In the early 1960s, for instance, scientist Frank Salisbury was studying how cocklebur plants grew by measuring their leaves with a ruler every day. Weirdly, he noticed that the plants he was measuring didn’t grow as much as their neighbors, and they eventually shriveled up and died. He concluded that it was simply the act of touching the plants that killed them.

A decade later, a plant physiologist named Mark Jaffe published the first workon this phenomenon — and coined the first word for it: thigmomorphogenesis (in Greek, thigmo means “touch,” morpho means “shape,” and genesis means “origin.”). Of the dozen or so plant species he used in his study, six had slowed growth after being touched daily. After a few more days of no touching, however, they resumed their regular growth rate.

In 1990, plant biochemist Dr. Janet Braam discovered that this stunted growth happened because of a genetic change. Touching a plant led to a specific handful of its genes being activated, which she named the “touch” (TCH) genes.

In December, researchers at La Trobe University in Australia took a closer look at this phenomenon to uncover exactly what was going on inside of the plant to activate these genes. For their study, the researchers used a plant called thale cress (Arabidopsis thaliana), a plant that’s known to pump its leaves with toxic mustard oil when insects attack and the same genus as the plants used in Braam’s original study. The scientists stroked the leaves of the plants with a soft paintbrush every 12 hours, then measured their biological response at varying periods of time after each stroke.

They found that within 30 minutes of being touched, 10 percent of the plant’s genome had been altered. At the site where the plants had been stroked, their mitochondria had ramped up their activation of genes known to suppress the touch response. Even more interesting, the same thing had happened at other places on the plant that hadn’t been touched, though to a lesser degree. Specifically, the mitochondria altered the genome by tweaking the plant’s immune system and hormone levels.

“This involves a huge expenditure of energy which is taken away from plant growth. If the touching is repeated, then plant growth is reduced by up to 30 percent,” said professor Jim Whelan, the lead researcher on the study and research director of the La Trobe Institute for Agriculture and Food, in a statement.

Antisocial Arabidopsis

Why would this be? It makes sense for a plant to send out toxic chemicals when it feels the brush of a caterpillar since that could help convince the predator that it’s not a tasty snack. But to inhibit growth after too much touching? That seems like cutting off the nose to spite the face.

But there’s some logic there. For example, if plants grow too close together, they’ll get less light and fewer nutrients. Growing smaller could be a way to ensure that there’s enough to go around, co-author Dr. Yan Wang explained.

This new research might help farmers know exactly how far to space their plants to ensure they grow as big as possible. Knowing the genetic mechanisms at play in a plant’s touch defenses might even help scientists engineer plants that aren’t as touch sensitive. For that, though, they have to be careful, since it could be easy to knock out some other important senses in the process, like sensitivity to cold and heat and disease defense.

But as for houseplants, the message is clear: water them, give them sunlight, even play them music if you’d like to. But don’t pet them. They don’t like it when you pet them.

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