The bullhorn acacia trees of Mexico and whistling thorn trees of east Africa provide housing for ants in their fat, hollow thorns. Any disturbance brings out hordes of the biting insects.
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
Plants can’t run away to protect themselves from herbivores. But they are not all helpless. Some concoct a range of alkaloids in their fresh leaves, making them bitter tasting. Some grow sharp thorns. Others enlist an army of defendants. The bullhorn acacia trees of Mexico and whistling thorn trees of east Africa provide housing for ants in their fat, hollow thorns. Any disturbance brings out hordes of the biting insects, ready to join battle to protect their homes.
At least 4,000 species, such as lima bean vines, exude nectar from special glands called extrafloral nectaries. The sweet liquid, a modified plant sap, is manna for ants with a sweet tooth. By providing room and board, plants enlist these insects as their bodyguards. The European bittersweet nightshade produces toxins to protect its bright red berries, tubers, and flowers. But its green arrowhead shaped leaves are fair game for insects and slugs. The vine has a unique weapon to ward off these pests.
Plants close their wounds quickly to prevent infections and loss of valuable resources. The nightshade, however, oozes from its wounds to alert its resident ants of the attack and also indicates where the battlefield lies.
Five years ago, Anke Stepphuhn, belonging to the molecular ecology group at Freie Universität Berlin, and her PhD student Tobias Lortzing noticed ants swarming over the nightshade vines on their field trips. There was no obvious reason for the insects to lurk around the plants. “When we later let caterpillars feed on plants in the greenhouse for other research projects, we noticed the sweet and sticky wound secretions,” Lortzing and Stepphuhn told The Wire. “Only then (did we suspect) that both phenomena might be connected.”
Steppuhn and her team conducted experiments to analyse the plant’s defence mechanism. By cutting the nightshades’ leaves with a scalpel, they induced the plants to secrete a tiny bit of sweet fluid. When they damaged leaves repeatedly or coated the scalpel with jasmonic acid, the vines bled profusely. Jasmonic acid is a hormone exuded by distressed plants and which induces the secretion of sugary liquid in plants with extrafloral nectaries. These sweet beads acted like alarms, summoning three species of ants to the plants’ aid. They patrolled damaged leaves and plants longer than undamaged ones.
“This shows that the bittersweet nightshade can control the amount of secretion it releases, similar to plants with extrafloral nectaries,” say Lortzing and Stepphuhn.
To study how these golden drops discourage plant-eating creatures, the researchers sprinkled droplets of sucrose on some vines. They say that when black slugs chew up nightshade leaves, the plants don’t secrete much from the damaged edges. “They eat too fast and often the whole leaf. If they do that, there is no tissue left to secrete nectar,” according to Lortzing and Stepphuhn.
The sugar-water attracted common red ants that aggressively attacked and killed many slugs. The harried survivors lost 20% of their body mass. Plants thus protected by ants suffered only half the slug damage compared to ones on which the researchers sprayed plain water.
Both larvae and adults of a species of flea beetle are a common pest through the season. The larvae hatch from eggs in the soil, crawl up the vines and burrow into the young shoots. On the other hand, the adult insects specialise in taking small round bites of the nightshade.
Even though the nightshade didn’t secrete much nectar when larvae drilled into its stem, ants killed a third of them, while the rest dug into the pith within 30 minutes. However, nightshade vines reacted to the repeated assault of adult beetles and secreted wound nectar. Attacked vines had three times more ants than undamaged ones. When the scientists released beetle larvae on leaves damaged by adult beetles, the patrolling ants hauled off half the larvae to their nests. A fortnight later, the affected plants had grown and put forth more shoots.
“If a beetle larva manages to get through the ants and mines into the stem, the plants secrete nectar from the stem wound as well,” the duo explains. “This attracts even more ants and makes it even more difficult for the larvae that come later on. The patrolling ants deter the slugs as well, but this is probably an additional side effect of the plants’ interaction with the beetles.”
According to the researchers, the difference in the quantity of wound-nectar secretion between the single injuries caused by larvae and repeated injuries by adults is likely caused by the physiological regulation and not by the degree of damage.
Other nectar-producing plants have specialised tissues to make the liquid and specific structures in particular locations to dispense it. However, the nightshade produces nectar anywhere it’s hurt – leaves, petioles and stems. This is the first time such unusual plant behaviour was observed. The discovery has excited other researchers studying the interactions between plants and herbivores.
“This is a very cool study and a remarkably novel finding,” says Anurag Agrawal of Cornell University, Ithaca (NY), who wasn’t involved in the study. “It is one of those discoveries that plant ecologists will think ‘how could I have never observed this?’ Evolution by natural selection works in funny ways, and typically by the path of least resistance.
“As such, extrafloral nectaries are highly diverse. This research group has found perhaps the simplest mode of wound-inducible extrafloral nectaries, by letting herbivores open the tap. I look forward to seeing how common this is indeed.”
Steppuhn and her team analysed the chemical composition of plant sap and nectar. While sucrose is the main ingredient in both, the two fluids are different. And other than sucrose, nightshade nectar contains no other sugars. Instead, it has three amino acids and an antioxidant ascorbic acid, commonly found in nectars.
“The authors demonstrated that the wounds were leaking a unique substance that is distinct from the standard fluids inside the plant,” says Marjorie Weber, a postdoctoral fellow at University of California, Davis. “Using an elegant set of experiments, they then showed that this substance is sugary enough that ants are attracted to the plants and ultimately provide protection.”
How do the plants make the sweet stuff without any devoted structures? That remains unanswered. But every leaf seems to have the ability to produce nectar if it is attacked. “There are several advantages of this arrangement for the nightshade,” points out Vartika Mathur of Sri Venkateswara College, New Delhi. The plant doesn’t have to build elaborate structures for producing nectar. “Resources in producing extrafloral nectaries are not wasted in the absence of a herbivore, and they (plants) are significantly protected not only from the larvae of the damaging insect but also more destructive and larger slugs. In return for this protection, ants are rewarded with both, a source of sucrose as well as flesh. In effect, it is a win-win situation for both.”
Weber says this discovery may be a missing link in the evolution of extrafloral nectaries that are both agriculturally and ecologically important. “If this is the case, the authors may have unearthed a new clue towards solving the mystery of how cooperation between ants and plants evolves on earth,” says Weber.
Although Lortzing and Stepphuhn suspect there ought to be other wound-nectar-producing plants, they haven’t found any yet. “This wound nectar is really hard to find under natural conditions because it is either quickly consumed by animals, dries to small sugar residues, or is washed away by rain or morning dew. So it might be a largely overlooked phenomenon because nobody was looking for it. Even we were not looking for it initially.”
Several plants secrete fluids from wounds, like palm trees and Banksia flowers. “But no one has investigated if they control these fluids like the bittersweet nightshade does, or if they benefit from the release of the fluids in any way,” say Lortzing and Stepphuhn. “This is something we definitively want to look into in the future.”
The study was published on April 25, 2016, in the journal Nature Plants.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.