Bacteria associated with Pinus sylvestris-Lactarius rufus ectomycorrhizas and their effects on mycorrhiza formation in vitro.

•  The structure and functioning of mycorrhizosphere bacterial communities from the soil organic fermentation horizon (FH) of a Pinus sylvestris stand was investigated. •  Bacteria were isolated from P. sylvestris-Lactarius rufus mycorrhizas, and their effect on ectomycorrhiza formation, localization within the mycorrhizosphere, and phenotypic and genotypic characteristics determined. •  The isolates could be divided into seven distinct phenotypic and physiological groups. Two Paenibacillus isolates stimulated infection of both first and second order lateral roots, resulting in a doubling of the percentage root conversion to mycorrhizas, relative to the control treatment, after 8 wk. Two Burkholderia isolates, and a Rhodococcus sp., had no effect on numbers of first order mycorrhizal lateral roots, but increased formation of secondary mycorrhizal lateral roots. There was evidence that the Burkholderia, but not the Paenibacillus or Rhodococcus isolates, associated preferentially with mycorrhizal roots, and that the Burkholderia strains were capable of more effective spread to the root tip than the other isolates. •  Ectomycorrhizosphere bacteria appear to promote mycorrhiza formation by means of a variety of mechanisms, and there are similarities in the nature of bacteria inhabiting ecto- and arbuscular mycorrhizospheres.