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

The structure and functioning of mycorrhizosphere bacterial communities fro m the soil organic fermentation horizon (FH) of a Pinus sylvestris stand wa s investigated. Bacteria were isolated from A sylvestris-Lactarius rufus mycorrhizas, and t heir effect on ectomycorrhiza formation, localization within the mycorrhizo sphere, and phenotypic and genotypic characteristics determined. The isolates could be divided into seven distinct phenotypic and physiologi cal groups. Two Paenibacillus isolates stimulated infection of both first a nd second order lateral roots, resulting in a doubling of the percentage ro ot conversion to mycorrhizas, relative to the control treatment, after 8 wk . Two Burkholderia. isolates, and a Rhodococcus sp., had no effect on numbe rs of first order mycorrhizal lateral roots, but increased formation of sec ondary mycorrhizal lateral roots. There was evidence that the Burkholderia, but not the Paenibacillus or Rhodococcus isolates, associated preferential ly 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 mea ns of a variety of mechanisms, and there are similarities in the nature of bacteria inhabiting ecto- and arbuscular mycorrhizospheres.