An arbuscular mycorrhizal fungus and a root pathogen induce different volatiles emitted by Medicago truncatula roots

Using non-targeted metabolite profiling, we show that Medicago truncatula roots grown in an aeroponic cultivation system emit specific volatiles in response to two soil-born microorganisms, the symbiont Rhizophagus irregularis and the pathogen Aphanomyces euteiches. [Display omitted] •Medicago roots emit volatiles as an early response to microorganisms (MOs).•More than 90 compounds were released from roots as detected by untargeted GC–MS.•Principal component analyses clearly distinguished untreated roots from treated roots.•Several volatiles were found to be emitted specifically in response to symbiotic or pathogenic MOs.•Plants discriminate between friend and foe at very early stages of their interaction. Plants are in permanent contact with various microorganisms and are always impacted by them. To better understand the first steps of a plant’s recognition of soil-borne microorganisms, the early release of volatile organic compounds (VOCs) emitted from roots of Medicago truncatula in response to the symbiont Rhizophagus irregularis or the pathogenic oomycete Aphanomyces euteiches was analysed. More than 90 compounds were released from roots as detected by an untargeted gas chromatography-mass spectrometry approach. Principal component analyses clearly distinguished untreated roots from roots treated with either R. irregularis or A. euteiches. Several VOCs were found to be emitted specifically in response to each of the microorganisms. Limonene was specifically emitted from wild-type roots after contact with R. irregularis spores but not from roots of the mycorrhiza-deficient mutant does not make infections3. The application of limonene to mycorrhizal roots, however, did not affect the mycorrhization rate. Inoculation of roots with A. euteiches zoospores resulted in the specific emission of several sesquiterpenes, such as nerolidol, viridiflorol and nerolidol-epoxyacetate but application of nerolidol to zoospores of A. euteiches did not affect their vitality. Therefore, plants discriminate between different microorganisms at early stages of their interaction and respond differently to the level of root-emitted volatiles.