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

SUMMARYThe symbiotic establishment of mycorrhizal fungi on plant roots is affected m various ways by the other microorganisms of the rhizosphere, and more especially by bacteria. This review discusses the case of some of these bacteria which consistently promote mycorrhizal development, leadmg to the concept of ' mycorrhization' helper bacteria (MHBs). Examples of MHB evidence are given from the literature, with special reference to the Douglas fir (Pseudotsuga menzeisii Mirb. Eranco)-Lacron'o laccata Scop, ex Fr. ectomycorrhizal combination which has been more extensively studied. The fungal specificity of some MHBs and the various mechanisms underlying their effect are discussed, considering five hypotheses: effects on the receptivity of the root, effects on the root-fungus recognition, effects on the fungal growth, modification of the rhizospheric soil, and effects on the germination of the fungal propagule.MHBs are then considered for their ecological and evolutionary implications, and examples of practical applications in forest nurseries are given: when added to the fungal inoculum, MHBs can improve the success of ectomycorrhizal inoculation of planting stocks with fungi selected for their outstanding growth stimulation after outplanting.The conclusion points out a number of fundamental questions which remain unanswered about mycorrhization helper bacteria and suggests some investigation priorities in this new field of mycorrhiza research.

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Summary• Data on the diversity and distribution of enzyme activities in native ectomycorrhizal (ECM) communities are inadequate.• A microplate multiple enzymatic test was developed which makes it possible to measure eight enzyme activities on 14 individual, excised ECM root tips. Hydrolytic and oxidative enzymes are involved in the decomposition of lignocellulose, chitin and phosphorus-containing organic compounds. This test system was used to describe the functional diversity of ECM communities in two forest sites.• This set of tests proved to be accurate and sensitive enough to reveal a high diversity of activity profiles, depending on the fungal symbiont and the soil horizon. Ectomycorrhizas can be classified into specialists and generalists, and appear to complement each other in the same horizon to collectively perform all eight activities studied.• By including a higher number of different assays for more detailed analyses, ECM activity profiling will provide a valuable tool for studying the functional diversity of ECM communities.