Closely sown corn plants communicate to defend themselves

Corn plants whisper messages to each other to defend against their enemies. When they are very closely planted, they release a volatile substance that induces their neighbors to produce chemicals that halt their growth, but activate their defenses against plagues. And that’s not all — they also modify the soil’s microbiota, leaving a defensive legacy that preps the immune system of the next generation of plants. This discovery, published in Science, opens the door to the use of the plant’s own components as pesticide.

Using an impeccable experimental design, a group of Chinese, Swiss and Dutch researchers looked at how to improve the conditions and consequences of high-density corn cultivation. For decades, closely planting rows of it and other grains, like wheat and rice, has led to increased production of crops that are essential for the world’s population. But all that conglomeration has its risks. If a plague arrives, it spreads more easily, just as with human viruses.

Scientists experimented by planting some fields with a low density of 60,000 plants per hectare and others with double that, at 120,000 per hectare. They saw that while there were no major differences at the edges of the two types of field, the interior plants of the super-populated fields modified their root system and reduced the height of ears, chlorophyll concentration and number of kernels per ear. This confirmed that density impacts growth. But they also observed significantly less damage from pests in fields where the plants grew closer together.

“Our initial hypothesis was that, in high-density cultivations, neighboring plants are closer together, which intensifies chemical signals, while at low densities, those signals can be too weak to set off significant response,” Lingfei Hu, a researcher at Zhejiang University in China and co-author of the study, explains in an email.

To try to discern what chemical signals corn uses, the team planted hundreds of seedlings at different densities of 50, 100, 150 and even 200 seedlings per square meter. When the fourth leaf emerged, they removed the plants, but kept the soil, sowing new seeds into it.

They saw that the higher the density, the better the corn resisted four of its worst enemies, the corn earworm, which devours its leaves; the nematode Meloidogyne incognita, a parasite that feeds on roots; northern corn leaf blight, a fungus that reduces yield; and the rice black-streaked dwarf virus (RBSDV), which originated in rice and spreads through grass. Something in the air, in the soil, or both was protecting the more densely planted corn.

After ruling out that genetics by experimenting with different varietals, the team analyzed the presence of volatile organic compounds (VOC) released by corn leaves into the environment. They say that in the densely cultivated fields, the most abundant VOC was linalool, an alcohol found in many plants, particularly aromatics and citrus, whose aroma is reminiscent of lavender.

“It is a constitutive volatile compound, which is emitted under normal conditions. Isolated plants also release it,” says Hu. (“Constitutive” means that it is present in leaves on a standard basis, unlike other compounds that are produced only when a plant is under attack or stress.)

Linalool best displays its power when corn is closely planted. “We still don’t know what the exact concentration is that is necessary to set off a response from neighboring plants,” says Hu.

But upon arriving at a certain level, corn prepares for war. In less than three days, nearby plants have changed their metabolism, producing larger amounts of hormones like jasmonic acid, which reactivate their immune system. Roots begin to exude compounds called benzoxazinoids, which have pesticidal properties. One of the first things they do is affect the rhizosphere, the symbiosis between beneficial fungi and roots, and the soil microbiota as a whole. This causes the immune system to go on alert. The release of linalool, the biosynthesis of hormones, and the exudation of benzoxazinoids are all connected.

“A plant release linalool, which causes changes to the metabolism of others, changes that have an effect on soil bacteria, an effect that remains when the plant is no longer there,” says Sergio Ramos, an evolutionary ecologist at the University of Zürich and researcher in the field of corn volatiles.

As Ramos, who did not participate in the study, says, “corn ranks first or second among crops by area worldwide and has been studied so extensively that its chemical composition is known in detail.” But this is a rare unknown area. “Corn is able to identify the insect that is eating it by the proteins in its saliva,” says Ramos.

This triggers the production of induced volatiles, which only appear after the attack. But linalool is not induced; it is always present. However, as the researcher points out, it only generates a response at short distances, because “all volatiles tend to rise.”

For Lucía Martín, a researcher at the Galician Biological Mission (MBG-CSIC), the most important of many amazing aspects of this study is the legacy effect the corn has on the soil. “It works like a vaccine, preparing the immune system of the next generation,” she says. Martín did her thesis on volatiles in potatoes, and now she is studying them in other plants, such as cotton. In a few weeks, she will go to Sweden to investigate this legacy effect. She observed how the attack of moth larvae on potato plants induced the release of volatiles, which in turn activated the defenses of other plants, making them more resistant.

Martín saw few weak points in the study, but she agrees with Ramos that although the team demonstrates the triggering role of linalool, they do not explain how neighboring plants “hear,” “smell” or perceive its odor. “In other plants, several possible receptors have been identified, but research is still ongoing,” she says. In 2024, the journal Science published another study that identified a receptor in the pistil of petunias for a specific volatile compound, germacrene. Past this, not much has been learned about the subject.

Although it was not the central goal of the study, this points to the possible use of certain volatiles in agriculture. For example, in places with a high risk of plague, it could be possible to induce the release of linalool or even utilize its synthetic version, which does exist.

Claude Becker, a biologist at the University of Munich, wrote his commentary on Hu and his teammates’ study that also appeared in Science. In an email, he says “they grew barley and ryegrass in soil that had previously been used to grow high-density corn. It turned out that they showed stronger defenses against herbivores.” For Becker, “in a way, yes, linalool seems to have a general (indirect) effect on the anti-herbivore defense of plants that perceive it.” But Becker also points out that they did not compare the magnitude of the effect with that of an actual herbicide.

The University of Munich biologist sees one last issue with the study that was also noted by Ramos, Martín and even the study’s authors: “There is the disadvantage that the effects of linalool also lead to smaller plants,” says Becker. It is almost mechanical issue, resources are limited, so they are either used for growth or defense. But this points to another possibility: where there is no danger, cutting off communication, i.e. inhibiting the production of linalool, could accelerate plant development.

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