Elsevier

Phytochemistry

Volume 68, Issue 1, January 2007, Pages 33-40
Phytochemistry

Review
Plant signals and fungal perception during arbuscular mycorrhiza establishment

https://doi.org/10.1016/j.phytochem.2006.09.036Get rights and content

Abstract

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that need their plant hosts to complete their life cycle. In the absence of the plant, germlings arrest growth after a few days and retract most of their cytoplasm back into the multinuclear spores. The spores can germinate again during more favorable conditions. How AM fungi recognize compatible host roots and activate their symbiotic program is not yet understood. However, research in this field in the last years has shed light into this topic. We, and others, have approached some of these aspects by studying changes in fungal gene expression observed at early stages of development, before and at the plant recognition stage in an attempt to identify genes and proteins featuring as key regulators in the switch between the asymbiotic and symbiotic style of life. The molecular bases of this recognition process are now starting to be understood and point to common signaling pathways shared with other microbe-plant associations and to arbuscular mycorrhiza specific signaling pathways.

Graphical abstract

Arbuscular mycorrhizal fungi and plant roots talk to each other in a language that has not been deciphered yet. Plant signals induce in the fungus the activation of the symbiotic program to overcome its preset growth arrest. Molecular components have been identified that could link plant signals with this fungal checkpoint.

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Section snippets

The arbuscular mycorrhiza symbiosis

Arbuscular mycorrhizal fungi are soil inhabitants belonging to a recently new ascribed phylum, Glomeromycota with a presumed origin at least 460 million years ago (Redecker et al., 2000, Schüßler et al., 2001). Glomeromycota fungi live in permanent association with roots of the majority of the plants on this planet among Angiosperms, Gymnosperms, Pteridophytes and some Bryophytes (Smith and Read, 1997). Perhaps due to this ancient association with plants, AM fungi have lost their ability to

Plant-induced morphogenesis of AM fungi

There is increasing evidence showing that fungus and plant start to recognize each other long before the first colonization structures on the root epidermis appear. It is known since a long time that germinating hyphae from spores respond to the presence of roots in their vicinity (Mosse and Hepper, 1975). Although no directional growth has been observed towards the root, several experiments showed that exudates from host root elicit growth stimulation in contrast to non-host exudates (Bécard

Where to go now?

Molecular results show that in the mycorrhiza symbiosis novel signaling components as well as conserved ones are directing the molecular dialogue between the fungus and plant. Major goals in the future will be to identify all players of these signaling networks, particularly the signals and receptors that open the door to symbiosis formation.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft in projects Re-1556/1-1, 1556/1-2 and 1556/1-3 (all SPP 1084 “Mykorrhiza”). We acknowledge financial support from the Landes Baden-Württemberg for the Habilitation Stipendium to N.R. We are very grateful to M. Krebs for her help during expression analyses and isolation of Gin1 orthologue.

Natalia Requena is the group leader of the Plant–Fungal Interactions Group in the Institute of Applied Biosciences at the University of Karlsruhe. She studied at the University of Granada (Spain) where she obtained her PhD in 1996 working on the role of arbuscular mycorrhizal fungi in restoration of semi-desertified ecosystems. In 1997 she joined the group of Dr. Philipp Franken at the Max Planck Institute for terrestrial Microbiology in Marburg (Germany) with a Marie Curie fellowship from the

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    Natalia Requena is the group leader of the Plant–Fungal Interactions Group in the Institute of Applied Biosciences at the University of Karlsruhe. She studied at the University of Granada (Spain) where she obtained her PhD in 1996 working on the role of arbuscular mycorrhizal fungi in restoration of semi-desertified ecosystems. In 1997 she joined the group of Dr. Philipp Franken at the Max Planck Institute for terrestrial Microbiology in Marburg (Germany) with a Marie Curie fellowship from the European Community to study molecular aspects of the mycorrhiza symbiosis. With this subject she obtained her Habilitation in 2005 at the University of Tübingen where she worked at the Physiological Ecology of Plants Department supported with a Wrangell Habilitation fellowship from Baden-Württemberg. She recently moved to Karlsruhe with the Heisenberg Stipendium to ascertain the molecular aspects of early signaling during arbuscular mycorrhiza formation.

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