Long-distance love: plants seduce bacteria with smellsNetherlands Institute of Ecology (NIOO-KNAW)
"Plants are exposed during their entire life to various environmental stresses", writes a team of NIOO-researchers led by Kristin Schulz-Bohm and Paolina Garbeva. Interactions with the right bacteria can help plants 'de-stress'. But those bacteria may not always be around.
Volatile organic compounds emitted by plant roots are known to serve as a defence against enemies. Maize plants, for instance, which are attacked by root feeding beetle larvae, release a compound that makes them more attractive to nematodes. The nematodes then hunt and feed on the larvae.
So will the same trick attract bacteria that can promote plant growth and relieve stresses? Even if those bacteria are not found in the plants' rhizosphere, the narrow region of soil directly around the plant roots? Until now, write the researchers, little knowledge was available about this.
Migrating across distances
Due to their physicochemical properties, volatiles can easily diffuse through gas and water-filled pores and can, therefore, have a wide effective range far beyond the rhizosphere.
The researchers set up a system to test whether sand sedge (Carex arenaria) - a common plant known for its long runners or 'stolons' - could attract soil bacteria from a long distance by releasing volatile organic compounds into the rhizosphere.
They also tested how infestation with Fusarium culmorum, a soil-borne fungal pathogen, would affect the process. The fungus also releases volatiles into the soil, but most of those did not seem to have much impact on the soil bacteria, which were kept apart from the plants by a glass tube.
The 'messages' sent by the plant roots, on the other hand, proved to be quite effective, causing bacteria to migrate across distances of up to 12 centimeters - an enormous stretch for such tiny organisms. And some beneficial bacteria were in fact attracted even more strongly by plants infected with F. culmorum. The mix of compounds released by those plants differed from that released by healthy sedge roots.
Through the stomach
Another significant factor in all these cases turned out to be the availability of nutrients. A number of key bacteria were less attracted by the volatiles that the plant roots had emitted in the presence of nutrients in the soil.
So the way to a bacteria's heart, too, may be through the stomach. Volatiles may be sending a strong signal that food is within easy reach: all the bacteria will have to do to come and get it is migrate to the plant roots.
The experiments have certainly demonstrated that when it comes to 'stress control', plants can be much more than passive bystanders. And that's good news for agriculture, too, where reducing the use of conventional, chemical pesticides and fungicides, is becoming more and more of a priority.
This study is published in the scientific ISME journal.