The mycorrhiza helper bacteria revisited

Abstract

Summary: 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. Contents Summary 22 I. Introduction 23 II. Evidence for the occurrence of MHB 23 III. Ecological and evolutionary implications of MHB 23 IV. The question of MHB specificity 27 V. Mechanisms of the MHB effect 28 VI. Role of MHB in mycorrhizal functions 31 VII. Conclusions and research priorities 32 References 33