Vierheilig H, Piché Y (2002) Signalling in arbuscular mycorrhiza: Facts and hypotheses, In: Flavonoids in Cell Functions, Eds. Buslig B, and Manthey J; Kluwer Academic/Plenum Publishers, New York; p.23-39. ISBN 0-306-47254-6

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The arbuscular mycorrhizal symbiosis is an association between plant roots and fungi. Arbuscular mycorrhizal fungi (AMF) colonize roots improving plant nutrition mainly by transferring phosphate (P) from the soil to the plant, whereas plants provide the fungi with carbohydrates. In contrast to the rhizobial symbiosis with a host range limited to the leguminoseae, AMF form symbiotic associations with a wide range of plant species. Interestingly, there seem to be some striking similarities between the signalling in the rhizobial and the arbuscular mycorrhizal symbiosis. Apart from the effect of plant derived secondary plant compounds (SPC) on the bacterial and the fungal symbiont, SPC (e.g. flavonoids) are accumulated in the roots of the respective host plants during the establishment of both symbioses. Whereas there is some information on the role of SPC in the rhizobial symbiosis, the exact role of SPC during the establishment of the AM symbiosis still remains unclear.

The data about flavonoids in the arbuscular mycorrhizal symbiosis have been extensively reviewed. Although meanwhile there is some new information on the effect of various flavonoids on the growth of arbuscular mycorrhizal fungi and the induction of flavonoids during root colonization by arbuscular mycorrhizal fungi, there is still a debate on the exact role of flavonoids during the formation of the arbuscular mycorrhizal symbiosis. Based on work in progress we hypothesize in this paper that a fundamental molecular dialogue occurs that regulates not only the early development of the AM symbiosis, but also subsequent colonization which has to be balanced for a giving a genuine mycorrhizal symbiosis. In this work some aspects of the signalling and regulation in rhizobial and arbuscular mycorrhizal symbiosis are compared and possible functions of secondary plant compounds e.g. flavonoids in the mycorrhizal symbiosis are presented. Hopefully this overview gives some stimulation for further studies on signalling in AM.

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