Six years ago, Anke Steppuhn noticed that the bittersweet nightshade, when attacked by slugs and insects in a greenhouse, would bleed. Small droplets would exude from the wounds of its part-eaten leaves. At the same time, Steppuhn and her colleagues saw that the wild plants were often covered in ants.
These facts are connected. Steppuhn’s team from the Free University of Berlin, including student Tobias Lortzing, have since discovered that the droplets are a kind of sugary nectar, which the beleagured nightshade uses to summon ants. The ants, in return for their sweet meals, attack the pests that are destroying the plant. And this discovery provides important clues about the evolution of more intimate partnerships between ants and plants.
Acacia trees, for example, are masters at recruiting ant bodyguards. The insects protect the trees from plant-eaters and even prune back invading vines. In return the trees provide them with shelter in the form of swollen thorns, snack stations that look like orange berries, and drinks in the form of nectar. The latter come from small green lumps called extrafloral nectaries, which the ants sip from.
Some 4,000 species of plants have extrafloral nectaries, which vary considerably in their shape. Some are obvious structures, like those of the acacia. Others are mere pits or hollows. But whatever their form, their benefits are invaluable. They are not only ant rewards, but also ant concentrators.
“Ants often appear to be whimsically inefficient plant defence agents,” says Elizabeth Pringle from the Max Planck Institute for Chemical Ecology. “They wander to and fro, haphazardly nipping at anything that happens to be in their way, which gives plenty of time for something with a hard exoskeleton and wings, like an adult flea beetle, to escape and happily land somewhere else to feed. But concentrate lots of ants around a sugar source, and pretty soon nothing soft and slow stands a chance. This is the value of extrafloral nectaries.”
The bittersweet nightshade’s oozing droplets have almost all the characteristics of extrafloral nectaries. It’s a sweet liquid, obviously. But Lortzing showed that it’s not just fluid that passively leaks from a damaged leaf. When he cut the nightshade with a clean scalpel, the nectar droplets didn’t appear. They did emerge, however, if Lortzing first coated his scalpel in jasmonic acid—a hormone that plants release upon insect attack.
He also showed that the nectar is chemically distinct from the plant’s actual sap—full of sweet sucrose, and deficient in almost everything else. Clearly, it is actively produced and secreted by the plant.
To find out, the team added droplets of either sucrose or water to wild undamaged nightshades. After a month, they saw that the sucrose-treated plants were patrolled by more ants, and had suffered half as much damage to their leaves. To their surprise, the ants even seemed to protect the nightshades against slugs. That’s new. Ants have been known to defend plants against other insects and mammals, but never before slugs or snails.
More bizarrely, the ants didn’t seem to attack adult flea beetles—the nightshade’s greatest enemies. They seemed like poor defenders, until Steppuhn’s team realised that the ants were focused not on the beetle adults, but on their larvae. The larvae hatch from eggs in the soil, climb up the nightshade’s shoots, and bury themselves in its stem.
Ants will pick up the larvae and carry them into their nests, never to be seen again. The ants might ignore the adults, but they stop the next beetle generation from causing even greater harm.
So, the nectar droplets, being actively produced, chemically distinct, and efficient at summoning guardian ants, are very much like the extrafloral nectaries of other plants. The only difference is that they’re not associated with any specific structure—no obvious lump or pit. It’s the “most primitive extrafloral nectary that has been discovered so far and shows how little is needed to make a functioning nectary,” says Martin Heil from CINESTAV in Mexico.
Although such structures are common throughout the plant world, every group with nectary-bearing species also has nectary-less members. “This means that extrafloral nectaries appear and disappear quickly, in evolutionary terms,” says Heil. And Steppuhn’s discovery “helps us understand why and how these nectaries can evolve out of nowhere.”
Perhaps, at first, fluids that passively leak from wounds are visited by ants. Gradually, plants evolve to recruit the ants more effectively by controlling those leaks and tweaking the liquids that emerge, as the nightshade has done. Eventually, they develop specialised structures.
But nectaries are lost so frequently among plant families that they clearly incur some cost, says Pringle. It takes a lot of up-front investment to build the dedicated structures and to keep them constantly brimming with nectar. By contrast, the nightshade’s droplets show that plants can summon ants in a more ad hoc and less effortful way.
Whether plants go for that cheaper option, or head towards full-blown nectaries, probably depends on how badly they’re threatened by plant-eaters, how effective ants are, and how much energy it takes to summon and reward them. Nothing in nature comes for free, and evolution is the ultimate arbiter of costs and benefits.