Soil is one of the major habitats of bacteria and fungi. In this arena their interactions are part of a communication network that keeps microhabitats in balance. Prominent mediator molecules of these inter- and intraorganismic relationships are inorganic and organic microbial volatile compounds (mVOCs). In this review the state of the art regarding the wealth of mVOC emission is presented. To date, ca. 300 bacteria and fungi were described as VOC producers and approximately 800 mVOCs were compiled in DOVE-MO (database of volatiles emitted by microorganisms). Furthermore, this paper summarizes morphological and phenotypical alterations and reactions that occur in the organisms due to the presence of mVOCs. These effects might provide clues for elucidating the biological and ecological significance of mVOC emissions and will help to unravel the entirety of belowground' volatile-wired' interactions.

At present, more than 400 volatiles are known to appear in bacterial headspace samples, but more are expected as more bacteria will be investigated and several identification technologies will be applied. A comprehensive list of bacteria and their respective effects on plants were presented. The volatiles emitted from Serratia plymuthica HRO-C48 and Stenotrophomonas maltophilia R3089 retarded leaf and root development of Arabidopsis thaliana starting at day 2 of cocultivation, while first signs of activation of stress promoters appeared already after 18 h. Most A. thaliana ecotypes reacted similar to the volatiles of S. plymuthica, but a stronger root growth inhibition was observed for the accession C24. b-Phenyl-ethanol was identified as one compound of the S. plymuthica volatile mixture inhibiting the growth of Arabidopsis thaliana.