The bittersweet nightshade plant produces sugary nectar directly from wounds inflicted by herbivores chewing on its leaves, which acts as an indirect defence mechanism as it attracts ants that attack the herbivores, reports a paper published online this week in Nature Plants.
Plants are not passive organisms; many highly diverse plants use specialized glands called extrafloral nectaries, not involved in pollination, to secrete nectar and attract natural enemies of herbivores. However, the attraction of predators to nectar produced directly from wounds, without the need for any specialized structure or gland, has not been observed - until now.
Anke Steppuhn and colleagues found that, when chewed on by herbivores, the bittersweet nightshade (Solanum dulcamara) secreted small droplets from the edges of the fresh wounds. These droplets, which are more chemically similar to nectar than plant sap and can be mimicked with a sugar solution, attracted at least three species of ants often observed foraging around the plants in the field. In greenhouse experiments, the ants defended the plant by strongly attacking flea beetle larvae (though not adult beetles) and adult slugs. The authors show that the wound secretions of the bittersweet nightshade, unlike other plants, are not associated with any specific structure or restricted to any specific location. They conclude that the direct production of nectar from wounds has a clear ecological and adaptive role, and that it could be an important step in the evolution of the structured extrafloral nectaries observed in other plants.