title: The mycorrhiza helper bacteria revisited.
abstract: In natural conditions, mycorrhizal fungi are surrounded by complex microbial communities, which modulate the mycorrhizal symbiosis. Here, the focus is on the so-called mycorrhiza helper bacteria (MHB). This concept is revisited, and the distinction is made between the helper bacteria, which assist mycorrhiza formation, and those that interact positively with the functioning of the symbiosis. After considering some examples of MHB from the literature, the ecological and evolutionary implications of the relationships of MHB with mycorrhizal fungi are discussed. The question of the specificity of the MHB effect is addressed, and an assessment is made of progress in understanding the mechanisms of the MHB effect, which has been made possible through the development of genomics. Finally, clear evidence is presented suggesting that some MHB promote the functioning of the mycorrhizal symbiosis. This is illustrated for three critical functions of practical significance: nutrient mobilization from soil minerals, fixation of atmospheric nitrogen, and protection of plants against root pathogens. The review concludes with discussion of future research priorities regarding the potentially very fruitful concept of MHB.
authors.affiliation: INRA, UMR1136 INRA-UHP 'Interactions Arbres/Micro-organismes', IFR 110, Centre de Nancy, 54280 Champenoux, France.
authors.forename: P
authors.initials: P
authors.lastname: Frey-Klett
authors.affiliation: INRA, UMR1136 INRA-UHP 'Interactions Arbres/Micro-organismes', IFR 110, Centre de Nancy, 54280 Champenoux, France.
authors.forename: J
authors.initials: J
authors.lastname: Garbaye
authors.affiliation: UFZ-Department of Soil Ecology, Helmholz Centre for Environmental Research, Theodor-Lieser-Strasse 4, 06120 Halle, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Tarkka
mesh_terms: D018407:Bacterial Physiological Phenomena; D005075:Biological Evolution; D025282:Mycelium / Q000254:growth & development; D038821:Mycorrhizae / Q000502:physiology*; D010944:Plants / Q000382:microbiology*; D012988:Soil Microbiology; D013172:Spores, Fungal; D013559:Symbiosis