After decades of seeing plants as passive recipients of fate, scientists have found them capable of behaviors once thought unique to animals. Some plants even appear to be social, favoring family while pushing strangers from the neighborhood.
Research into plant sociality is still young, with many questions unanswered. But it may change how people conceive of the floral world, and provide new ways of raising productivity on Earth’s maxed-out farmlands.
“When I was in school, researchers assumed that some plants were better or worse than others at getting resources, but they were blind to the whole social situation,” said Susan Dudley, a McMaster University biologist. “I went looking for it, and to my shock, found it. And we’ve found more of it since.”
In a paper published in the November American Journal of Botany, Dudley describes how Impatiens pallida, a common flowering plant, devotes less energy than usual to growing roots when surrounded by relatives. In the presence of genetically unrelated Impatiens, individuals grow their roots as fast as they can.
Acknowledging relatives in this way is an example of kin recognition. It’s common in the animal world, and is a precursor to kin selection, in which animals help their familial group, not just themselves. Dudley thinks plants have kin selection, too. It’s a controversial idea, but that it’s even being debated shows how far research into plant sociality has come.
When Dudley was in school in the 1980s, the very idea of plant sociality was practically taboo among scientists. It had burst into popular consciousness a decade earlier with the publication of The Secret Life of Plants, a New Age classic which also discussed orgones and dowsing. Later studies on “talking trees” went unreplicated, and the idea fell into disrepute.
But even if full-blown sentience was a silly idea, research on plant communication gathered. Much of it described how plants defended themselves, producing toxins and concentrating resources on their immune systems when unrelated neighboring plants were eaten. That clearly involved some sort of chemical signaling. Further studies conclusively showed plants were able to recognize themselves. Whether plants might respond to their relatives became a legitimate and intriguing question.
The answer isn’t only of concern to people with imaginations stirred by thoughts of chatting flora. It could provide a whole new perspective on plant behavior and evolution. By providing insights that improve agricultural productivity, studies of kin recognition could literally bear fruit.
“We know that in the animal world, kin recognition and selection plays a very important role for family structure, altruistic behavior and those kinds of things,” said Hans de Kroon, a plant ecologist at Radboud University in the Netherlands. “It’s so prominent in the animal literature. Once we start to discover that plants can recognize their kin, there’s a whole set of hypotheses we can apply to studying plants, that nobody ever thought to.”
The field’s landmark paper came from Dudley’s laboratory in 2007, when she showed how American searocket plants accelerated their root growth when placed in pots of strangers, but slowed it down when potted with siblings. Were they animals, they’d be described as sharing water and food.
In a Communicative and Integrative Biology paper published in October, University of Delaware biologists Harsh Bais and Meredith Biedrzycki tried to isolate the means of recognition by exposing Arabidopsis thaliana seedlings, each in its own pot, to root secretions from other Arabidopsis plants. The signal indeed proved to be in the roots — and just as Dudley had seen, growth patterns varied according to whether secretions came from genetically unrelated plants, or family.
Intriguingly, the plants in Dudley’s latest study were potted separately and unexposed to each others’ secretions, suggesting that their leaves emit chemical signals, as well as their roots. That’s supported by the research of University of California, Davis ecologist Richard Karban, who in a June Ecology Letters study showed that sagebrush boosts its immune system when exposed to the damaged cuttings of a related plant [pdf]. It seems to hear warnings from its kin.
More studies are needed to show exactly what sort of benefits are provided by these signaling and response systems. De Kroon said kin recognition doesn’t necessarily mean kin selection: maybe the plants are communicating, but it doesn’t do them much good in practice.
One of Dudley’s students, Amanda File, is now studying whether some trees favor their own progeny, which might grow best near their parents. Dudley and graduate student Guillermo Murphy, a co-author of the American Journal of Botany paper, are looking for for kin selection in invasive plants.
“We’re testing the hypothesis that invasive plants evolve greater altruism within their populations, allowing them to be better invaders of their new habitats,” said Dudley.
For plants used in agriculture, Dudley recommends kin recognition studies to see whether certain arrangements of relatives and strangers would be especially productive. De Kroon is looking at multi-species mixes. Karban hopes to use communication insights to engineer natural defense systems against pests.
“Maybe we thought before that only humans could do certain things, or vertebrates, or animals,” said Karban. “Plants are capable of much more sophisticated behavior than we assumed.”
Plants Know their relatives and Like Them
Unlike many human brothers and sisters, plant siblings appear to do their best to get along, sharing resources and avoiding competition.
In a study of more than 3,000 mustard seedlings, scientists discovered that the young plants recognize their siblings — other plants grown from the seeds of the same momma plant — using chemical cues given off during root growth. And it turns out mustard plants won’t compete with their brethren the way they will with strangers: Instead of rapidly growing roots to suck up as much water and minerals as possible, plants who sensed nearby siblings developed a shallower root system and more intertwined leaves.
“It’s possible that when kin are grown together, they may balance their nutrient uptake and not be greedy,” plant biologist Harsh Bais of the University of Delaware said in a press release. The work will be published in an upcoming issue of Communicative and Integrative Biology.
Two years ago, co-author Susan Dudley of McMaster University in Canada observed a similar pattern in the sea rocket, a common seashore plant that also appears to favor its siblings. But the initial studies of kin recognition have been criticized for failing to control for complicating factors, such as resource depletion caused by competition between the unrelated plants. And until now, the researchers didn’t know how plants managed to identify their kin.
As seedlings grow, their developing root system gives off a variety of chemical signals, and the researchers guessed that these secretions might play a role in sibling recognition. To test their theory, the scientists grew wild Arabidopsis thaliana in a sterile liquid containing root extracts from sibling plants, unrelated plants or their own roots. Because each plant was grown in a highly controlled setup, the researchers could be sure any changes in growth were due to differences in the root extracts.
As shown in the time-lapse videos below, the seedlings exposed to root secretions from unrelated plants grew significantly longer and more elaborate root systems than those grown in secretions from their siblings. The top video shows unrelated plants, while the bottom one shows siblings.
However, when the scientists blocked root secretions using a chemical called sodium orthovanadate, the differences disappeared, suggesting that the sibling identification system indeed depends on chemicals released by growing roots.
The researchers say their results may have significant implications for farming and agriculture. Although no one knows for sure how sibling recognition would affect crops grown in large monocultures, some researchers think that decreased competition among plants from identical seeds may make monocultures more susceptible to insects and disease.
However, Bais says that the effect of growing a plant with its siblings is likely to be species-dependent, as initial studies have been contradictory. “There is a possibility that the explanation of the trade-offs is not that simple,” he wrote in an e-mail. “We have found that plants could resist pathogens better when grown with siblings compared to strangers, so I would take this with caution and not stretch it to all the plant species.”
Regardless of how sibling recognition affects agriculture, it may be an important consideration for the home gardener.
“Often we’ll put plants in the ground next to each other and when they don’t do well, we blame the local garden center where we bought them or we attribute their failure to a pathogen,” Bais said in the press release. “But maybe there’s more to it than that.”