SUMMARY: The establishment of vesicular-arbuscular mycorrhizal infections by inoculation with germinated resting spores of an sp. was investigated under microbiologically controlled conditions; pure two-membered cultures were obtained for the first time.

Seedlings were grown in a nitrogen-deficient inorganic salt medium; in these conditions the fungus failed to form an appressorium and to penetrate the plant roots unless a sp. was also added. Adding soluble nitrogen to the medium completely inhibited root penetration, even in the presence of the bacteria. Various sterile filtrates could be used to replace the bacterial inoculum but these substitutes induced only few infections per plant.

Mycorrhizal roots grew more vigorously than non-mycorrhizal roots of the same seedling. They were longer and more profusely branched. At first mycorrhizal infections were predominantly arbuscular, but many prominent vesicles developed as the seedlings declined, and then the fungus grew out of infected roots and colonized the agar. The fungus could not be subcultured without a living host.

The possible interpretation of these results is considered with reference to the specialized nutritional conditions under which test plants were grown.


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