Diffusible signals from arbuscular mycorrhizal (AM) spores lead to transcriptional changes in host but not in non-host plants.

It has been demonstrated previously that germinating spores of AM fungi (AMF) release one or more small (< 3kDa), heat-stable, partially lipophilic and diffusible compounds that elicit a transient cytosolic calcium elevation in soybean cells. Because the molecule(s) is (are) of exclusively fungal origin, we wanted to investigate whether the presence of the plant could act as an enhancing or stimulating factor, and if, on the other hand, the diffusible signals released by the fungus could lead to transcriptional changes in root cells. For that, two different experimental systems were used. In the first one, the water in which spores of Gigaspora margarita were germinated (“myc-water”) was applied to 3-week old, P-starved plantlets of Lotus japonicus or Arabidopsis (negative control). In the second one, a modified sandwich system was used, with an additional nitrocellulose membrane preventing physical contact between the AMF spores and the roots, but allowing the exchange of diffusible signals from both partners. Microarray results are currently being validated by RT-qPCR on Lotus and Arabidopsis. The emerging picture, though, already suggests that i) host plants respond to the “myc-water” also at the transcriptional level; ii) non-host plants do not; and iii) the diffusible signals produced by germinating AMF spores are influenced by the presence of the host plant.