PITY the poor Christmas tree! Cut down as a mere youngster and taken away from family and friends to be dumped in stifling warmth, daubed with ridiculous baubles and left to a lonely, lingering death. OK, so maybe that’s pushing the anthropomorphism too far. Of course trees don’t feel sadness, indignity or pain, but neither should we think of them as dumb lumps of lignin and chlorophyll. It is becoming clear that plants have much richer social relationships than anyone imagined. Many are in contact with one another by direct line or mysterious underground signals. Others recognise and tolerate neighbours. Some are even generous and self-sacrificing towards their kin.
One of the foundations of any relationship is communication. In the 1980s, when plant biologists first proposed the phenomenon of “talking trees”, it was controversial. Today everyone accepts that many plant species release volatile chemicals into the air when they are attacked by herbivores, and that these chemicals are detected by other plants, which then prepare for the threat. What’s still unclear is whether these interactions constitute two-way communication. Plants may have no choice but to emit volatiles when their leaves are damaged, and once broadcast, the signals are undirected and available to anyone. So it is far from clear that these are messages intended to warn other plants. Recently, however, a more convincing form of plant communication has come to light.
Some plants reproduce by throwing off plantlets and stay connected to these offspring with horizontal stems above or below ground. Strawberries form such networks, as do bracken, clover, buttercups, reeds and bamboo. “In temperate ecosystems, 50 to 70 per cent of the herbaceous vegetation grows in this way,” says Josef Stuefer at Radboud University in Nijmegen, the Netherlands. “We have known for decades that network-forming plants exchange resources like water and carbohydrates.” He wanted to see if they share information too.
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To test the idea, his group let a caterpillar feed on one part of a network of white clover for two days, then looked for a defensive response in plants some way down the line. The results were clear. “When there is damage elsewhere in the network, the chemistry of the leaves is quite dramatically changed,” Stuefer says. Leaves throughout the network become tougher and produce chemicals that make them distasteful to other caterpillars. He is in no doubt that this effect is in response to a signal. Although he does not yet know what form it takes, he does know that it travels in the phloem, the transport vessel that actively moves sugars and hormones around a plant. “If you block phloem transport, there is no effect on the network beyond the block,” he says (Evolutionary Ecology, ).
All the plants in a network are genetically identical, so in evolutionary terms there is a clear advantage to the sender of the message as well as the recipients. Channelling information around a network is really not much different to sending messages from one branch to another within the same plant. It would be far more impressive if plants could distinguish their relatives from other plants nearby and selectively help them out. Earlier this year, Susan Dudley at McMaster University in Ontario, Canada, published work indicating that at least one plant can do this.
Dudley had long thought that plants should have evolved to favour their relatives. “Plants can be so much more closely related to each other than most animals can,” she says. Unable to change their location and with fairly limited options for dispersing seeds, plants are often surrounded by offspring, siblings and half-siblings. In a family crowd like this, evolutionary theory tells us there is selection for cooperative behaviour, rather than for selfish individuals. However, to cooperate with your kin, you have first to be able to recognise them.
“People have started to show that the roots of plants respond differently to self and non-self,” says Dudley, “so this seemed the perfect place to go looking for kin recognition.” Working with a small plant called sea rocket (Cakile edentula), which grows around the shores of the Great Lakes of North America, she found that when a group of unrelated plants shared a pot, they put more resources into growing roots than those sharing a pot with members of the same family (Biology Letters, ). Plants gather water and nutrients through their roots, so Dudley’s results mean that in the presence of kin, the plants are competing less, allowing their siblings to have more of a share. In work not yet published, she has found the same result in two other plant species.
Friend or foe?
The discovery that plants can recognise family is a bombshell. “This is extremely surprising,” says Ariel Novoplansky of Ben-Gurion University of the Negev in Israel, whose own findings seem to contradict Dudley’s. His studies show that buffalo grass plants can recognise themselves, in that roots from the same plant grow less when in close proximity. However, when he took a cutting from one plant and grew it separately for a few days, then replanted it next to its parent, the genetically identical plants treated each other as aliens, their roots competing vigorously for the available nutrients. The longer the plants were separated, the more they competed when they met again. So these plants can’t tell who their relatives are (Proceedings of the National Academy of Sciences, ).
One possible explanation for the discrepancy is that different plants have different recognition systems. In a third study published this year, Marina Semchenko of the University of Tartu in Estonia reported that strawberry root growth is unaffected by proximity to the roots of other strawberry plants – self, kin and non-kin alike – but the roots grow aggressively towards the roots of plants of other species. Ground ivy roots, by contrast, happily grow in close contact with others from the same plant but avoid growing near the roots of any other plants, whether of the same species or not (New Phytologist, ). It would appear that the ability to recognise self, neighbours and family varies from one species of plant to another – as do the resulting responses.
How these interactions between plants are mediated is anyone’s guess. “At the moment, we have no evidence as to what the signal could be,” says Semchenko. The findings imply that the messages pass secretly underground between roots, so Dudley suggests there might be a genetic tag on the roots, or chemicals oozing out into the soil.
Novoplansky has another idea. In the 1980s, he says, Russian scientists showed that when two seedlings are planted close together, their internal electrical oscillations begin to coincide. If electrical coordination is more effective among kin, it could explain what Dudley has observed, Novoplansky says. Related individuals might treat one another more like they treat themselves because their oscillations are more in tune.
“The messages might pass secretly underground, perhaps though chemicals oozing out into the soil”
Whatever the mechanism, the discovery that roots communicate opens a new chapter in plant ecology. “The default picture so far is that plants are just sitting there, fighting everything they meet,” says Novoplansky. “But if they are responsive to particular signals, and know who is who in some sense, it changes the whole picture.”
So if you have a real fir tree in your home, you could be forgiven for feeling a modicum of sympathy for it. It may not mind being dressed like a carnival queen, but it is possible that the poor thing might be lonely.
