Investigations on ultrastructural aspects of arbutoid mycorrhizae in Arbutus unedo showed the occurrence of bacteria between the hyphae of the mantle. Sequential cuts of mycorrhizal tissue revealed the spatial distribution of bacteria: in the outer region of the hyphal mantle many bacteria were present in the microniches formed by the hyphae interwoven around the roots; in the inner region bacteria formed microaggregates enclosed between hyphae, immersed in an electron-dense matrix. Quantitative and qualitative determinations showed the consistent occurrence of Azospirillum-like bacteria in the mycorrhizosphere and endosphere, whichever inoculum was utilized for the syntheses of arbutoid mycorrhizae. The hypothesis of a strict association between arbutoid mycorrhizae and Azospirillum-like bacteria is discussed.
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