We all can relate to the smell of a freshly mowed lawn, what we can’t relate to is plants crying for help. When attacked, plants release airborne chemical compounds which is used almost like a language, in notifying nearby creatures who can rescue them from the attacks.
“Plants are a lot smarter than we give them credit for,” said Harsh Bais, assistant professor of plant and soil sciences at the University of Delaware. “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 notes.
To figure this out, Bais and colleagues infected the leaves of the small flowering plant Arabidopsis thaliana with a pathogenic bacterium, Pseudomonas syringae. The plants started to look sickly. 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, Bais said.
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.
All plants biosynthesize malic acid, Bais explains, but only under specific conditions and for a specific purpose.
In the lab tests, 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.
Scientists are now trying to figure out what exactly is the signal sent down to the roots.