"Plants are not as defenseless against their predators as is often assumed," explained Steppuhn. Some species protect themselves with thorns or poisons, or by attracting predators, which in turn attack the pests. Some plants have glands (nectaries) that produce nectar and thus attract ants as "bodyguards." The bittersweet nightshade does not have this type of gland, but it can defend itself with nectar.
The bittersweet nightshade is a native European relative of the potato and the tomato. It is widely distributed and occurs in various, mostly light and wet, habitats. Nevertheless, scientists have only now discovered that this plant attracts ants to defend against predators, in the same manner as only known for plants with nectaries up to now.
In the greenhouse, the scientists observed droplets that came out of the edges of wounds on leaves, petioles, or stems of the bittersweet nightshade, after the plant had been damaged by various herbivores. Chemical analysis showed that these droplets are clearly distinct from the sap in the vascular tissues of the plants and almost exclusively contain sugar. Initial experiments demonstrated that these sweet droplets attract ants. In a field study and greenhouse experiments the scientists were able to prove that the ants – in return for the sweet meal – defend the plant from two of their biggest predators: flee beetle larvae and snails.
When the flea beetles end their hibernation in the spring and begin to feed on the young leaves, the bittersweet nightshade begins to attract its protectors with the wound nectar. With their hard armor the adult beetles are well-protected against the ants and have nothing to fear. But they lay their eggs in the soil, and the young beetle larvae have to climb up the twining plant to reach the young stems, in which they feed. They are easy prey for the ants, which ensure that substantially fewer larvae survive to bore into the stems. In addition, the Berlin scientists found that the ants also attack voracious slugs.
Such a plant defense against herbivores by attracting predatory insects is well known for plants with prominent nectaries outside the blossom. The example of bittersweet nightshade now indicates that plants can use this form of defense even without such nectaries. "The bittersweet nightshade controls the amount and composition of the wound nectar in a similar manner to plants with nectaries," said Steppuhn. This finding may help to explain the huge variety of nectaries and their distribution in different plant families. "The prevailing view that nectar needs to be produced by special organs was difficult to reconcile with the realization that these nectaries evolved numerous times in various plants," explained Steppuhn. Perhaps the apparently very simple mechanism of wound nectar could mark a transition between plants that produce nectar and those that produce none. The scientists suggest that this type of wound nectar could be widespread in the plant kingdom and may heve been simply been overlooked so far.
Black garden ants (Lasius niger) eating the wound nectar of bittersweet nightshade (Solanum dulcamara)
Credit: Tobias Lortzing
Red garden ants (Myrmica rubra) collecting the wound nectar on a leaf of the bittersweet nightshade (Solanum dulcamara)
Credit: Tobias Lortzing
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Lortzing, T., Calf, O.W., Böhlke, M., Schwachtje, J., Kopka, J., Geuss D., Kosanke, S. van Dam, N.M., and Steppuhn*, A. (2016): Extrafloral nectar secretion from wounds of Solanum dulcamara. In: Nature Plants. Doi: 10.1038/NPLANTS.2016.56
Prof. Dr. Anke Steppuhn, Institute of Biology, Freie Universität Berlin, Tel.: +49 30 838-56586, Email: firstname.lastname@example.org