University of Delaware researchers have discovered that when the leaf of a plant is under attack by a pathogen, it can send out a signal to other plants for help, and their roots will respond by secreting an acid that brings beneficial bacteria to the rescue.

The scientists infected the leaves of the small flowering plant Arabidopsis thaliana with a pathogenic bacterium, Pseudomonas syringae. Within a few days, the leaves of the infected plants began yellowing and showing other symptoms of disease.

The scientists detected the transmission of a long-distance airborne signal from the leaves to the roots in the other plants that had Bacillus subtilis (used by farmers to boost plant immunity) in the soil. The roots responded by secreting a carbon-rich protective chemical, malic acid.