Plants packed close enough to touch are more resilient to stress

For a plant growing on the forest floor, a beam of light from an opening canopy can be concerning. But new research suggests that plants growing together may be able to warn each other about such stressful events by touching their leaves, becoming collectively more resilient as a result.

“It’s sort of like an alarm,” says Ron Mittler, a plant biologist at the University of Missouri in Columbia. “Like, ‘Hey, something is coming our way, get ready.’”

Mittler and his colleagues grew thale cress (Arabidopsis thaliana) over several weeks, some in solitude and some in dense groups where the plants could touch one another’s leaves. When exposed to excess light, individual plants showed more signs of stress and damage than those in groups, the scientists report December 12 at biorXiv.org. “They seem to be more primed to deal with the stress if they touch each other,” Mittler says.

Plants communicate underground through their roots, via microbes or by forming networks with fungi. Research also suggests that aboveground communication may happen through several channels, including airborne chemicals that alert other plants to herbivore attacks or sounds that communicate stress. Plants can also pass electrical signals to each other through their leaves, forming a network connected by touch, though the effects of this on their health had previously been unknown.

Mittler and his team ran a series of experiments on wild thale cress plants grown from seedlings in a lab. They analyzed changes in gene expression in isolated plants and those whose leaves touched another’s, monitored signals passed between them and measured resilience to stress by shining a strong light on the plants. By using genetically altered plants unable to transfer certain chemical signals, the scientists teased apart which signals were responsible for any stress acclimatization.

Just one hour after making contact, plants whose leaves touched had activated over 2,000 stress-response genes, including those that help them cope with light, cold, waterlogging, salt and wounding. Compared with plants that touched each other, isolated plants exposed to light exhibited higher levels of cell damage and accumulated more stress-related pigments.

The experiments with genetically altered plants also revealed that the transfer of hydrogen peroxide was crucial for inducing resilience in neighboring plants. Plants produce hydrogen peroxide when triggered by a range of stresses, Mittler says. But this is the first time it’s been identified as a signal passed from plant to plant.

“What we’re looking at is a really important general signaling mechanism,” says Christine Foyer, a plant scientist at the University of Birmingham in England, who was not involved in the study. “If you think about it, plants have to have it because they don’t move. They have to be alarmed by what’s happening in the environment.”

Mittler says the findings explain why growing crops together often helps them survive in difficult conditions and could one day be used to design mixed plant communities that are more resilient to overlapping threats from climate change, such as flooding and heat.

“I can put three different species there together that I know will communicate above- and belowground the best way,” he says. “That’s what my hope is.”

Even Darwin said that mixtures of plants grow better than those that are alone, Foyer says, and this could be one of the reasons why. “This is just saying that plants of similar type will communicate signals,” she says. “It might be that plants of different types would use this but do it better.”