Washington, October 18 : Researchers at the University of Delaware (UD) in the US have discovered that when the leaf of a plant is under attack by a pathogen, it can send out an S.O.S. to the roots for help, and the roots will respond by secreting an acid that brings beneficial bacteria to the rescue.
The finding quashes the misperception that plants are "sitting ducks" - at the mercy of passing pathogens - and sheds new light on a sophisticated signaling system inside plants that rivals the nervous system in humans and animals.
The research was led by Harsh Bais, assistant professor of plant and soil sciences at UD, former postdoctoral researcher Thimmaraju Rudrappa, Kirk Czymmek, associate professor of biological sciences and director of UD's Bio-Imaging Center, and Paul Pare, a biochemist at Texas Tech University.
"Plants are a lot smarter than we give them credit for," said Bais. "People think that plants, rooted in the ground, are just sitting ducks when it comes to attack by harmful fungi or bacteria, but we've found that plants have ways of seeking external help," he added.
In a series of laboratory experiments, 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.
However, the infected plants whose roots had been inoculated with the beneficial microbe Bacillus subtilis were perfectly healthy.
Farmers often add B. subtilis to the soil to boost plant immunity. It forms a protective biofilm around plant roots and also has antimicrobial properties, according to Bais.
Using molecular biological tools, the scientists detected the transmission of a long-distance signal, a "call for help," from the leaves to the roots in the plants that had Bacillus in the soil.
The roots responded by secreting a carbon-rich chemical - malic acid.
According to Bais, all plants biosynthesize malic acid, but only under specific conditions and for a specific purpose. In this case, the chemical was actively secreted to attract Bacillus.
Magnified images of the roots and leaves showed the ratcheted-up defense response provided by the beneficial microorganisms.
Bais and his colleagues are now working to determine what the aerial signal is from the infected leaf to the root using different pathogen-associated molecular markers (PAMPs).