Plants are quietly communicating all around us. Some send out chemical signals by air, for example, and many rely on an underground internet built by soil fungi.
And some, a new study finds, can use parasitic vines as communication cables. The parasites may be harmful, but they also link multiple plants into a network, and these "bridge-connected hosts" seem to capitalize by communicating through the vines.
The parasites in this study are dodder vines, aka Cuscuta, a genus of about 200 species in the morning glory family. They don't look like much at first, initially rising from the soil as a thin tendril with no roots or leaves. Their growth depends on finding a host, which they do by sniffing out odors from nearby plants. (They can even use scent to track down their favorite hosts, such as tomatoes instead of wheat.)
"It's really amazing to watch this plant having this almost animal-like behavior," biocommunication researcher Consuelo M. De Moraes told NPR in 2006.
Once it finds a suitable host, a dodder wraps around the stem and inserts fang-like "haustoria" into the plant's vascular system. With little or no chlorophyll of its own, a dodder must drink nutrients from its host like a vampire. This lets the tiny tendril grow into a sprawling tangle of vines (pictured below), which has earned it ominous nicknames like devil's guts, strangleweed, hellbine and witch's hair.
A dodder can end up with its fangs in many hosts, forming clusters of connected plants that may include multiple species. As Ed Yong reports in the Atlantic, a single dodder vine is capable of linking dozens of hosts together. "In our lab, we could connect at least 100 soybean plants with a dodder seedling," study co-author Jianqiang Wu, a botany professor at the Chinese Academy of Sciences, tells Yong.
The parasites are known to take water, nutrients, metabolites and mRNA from their hosts, and their bridges "even facilitate host-to-host virus movement," the study's authors point out. But, as they report in the Proceedings of the National Academy of Sciences, those bridges also seem to boost the hosts' communication abilities.
And they aren't just enabling idle chatter: A dodder's network of "bridge-connected hosts," as the researchers call them, can perform valuable community services, such as warning each other about an attack from leaf-eating caterpillars.
Many plants are able to resist herbivorous insects, using a variety of tactics to warn their neighbors as well as defend themselves. They may produce defensive toxins, for example, rallying various parts of the plant to coordinate a systemic response.
"Insect herbivory not only activates defenses at the site of feeding," the researchers write, "but also induces unknown mobile signals that travel through vasculatures" to other parts of the damaged leaf as well as undamaged leaves and roots.
Since plants send these signals through their vascular systems, the researchers wondered if a dodder vine can inadvertently share them among its hosts, creating another channel for communication. To find out, they placed two soybean plants near each other and allowed both to be parasitized by the Australian dodder (Cuscuta australis), which soon formed a bridge between the two hosts.
Larvae and war
Next, they infested one of the soybean plants with caterpillars, while keeping its partner pest-free. The second plant hadn't suffered any bites, yet when the researchers examined its leaves, they found it had regulated hundreds of genes — many of which encode anti-insect proteins often used when under attack.
When the researchers did let caterpillars attack the second soybean, it "consistently exhibited elevated resistance to insects," they write, suggesting its pre-emptive defenses paid off. But what triggered those defenses? To see if its fellow host had really sent a warning via parasitic vine, they conducted similar experiments without the dodder bridge — and found no anti-insect proteins or increased resistance in the second host. They also tested for airborne signals between two unconnected soybean plants, finding no warning like the one between bridge-connected hosts.
Dodder vines may not rival high-speed data cables, but they do transmit their hosts' signals in as little as 30 minutes, the researchers report. The vines can also carry the signals over long distances — at least 10 meters (33 feet) — and even between hosts from different species, such as rockcress and tobacco.
Take your dodder to work
Thin strands of chaparral dodder grow on vegetation in Costa Mesa, California. (Photo: Emilie Chen/Flickr)
Since caterpillars could spell disaster for a soybean plant, this kind of alert seems like a pretty big benefit. Dodder vines are still parasites, though, a term for organisms that sustain themselves at the expense of their hosts. According to the study's authors, a dodder likely harms its victims more than it helps them.
Yet parasites also have an incentive to keep their hosts alive and viable, since they rely on them for long-term support. And even if the net impact is negative, the authors note that some parasites offer benefits beyond not killing their hosts. Roundworms have been shown to increase human fertility, for example, while other helminths can reduce autoimmunity and allergies in human hosts.
Being wrapped up by a dodder definitely takes a toll, but the vines "could alleviate resource-based fitness costs by providing information-based benefits to their hosts," the researchers write. And the parasite might benefit, too, "given that better defended and prepared hosts could provide Cuscuta with more nutrients than undefended or naïve hosts in the face of a rapidly dispersing herbivore."
Still, they add, dodder vines are generalists that can target a wide range of plants, and their networking services are probably a coincidence, not a co-evolved response. More research is needed to really understand this relationship, the researchers say, including how exactly the hosts' signals are spread, how much a dodder's perks offset its costs, and whether those benefits are "ecologically meaningful."
In the meantime, research like this can help illustrate how the ecosystems around us — including apparently passive plants — are more sophisticated than they seem.