Spore germination of Gigaspora margarita stimulated by volatiles of soil-isolated actinomycetes

Nineteen field isolates of actinomycetes and Streptomyces orientalis were evaluated for their production of volatile compounds that stimulate spore germination of Gigaspora margarita, an arbuscular mycorrhizal fungus. Some actinomycetes enhanced spore germination, whereas others did not. When placed in a common headspace with actinomycetes, but physically separated in a divided Petri dish, spores showed germination rates up to 73%. In control treatments lacking actinomycetes, spores showed 23% germination by the final day of observation, day 11. Actinomycetes having straight spore-bearing hyphae stimulated germination more than did actinomycetes having spiral spore-bearing hyphae (P ⩽ 0.01 on day 11). Other traits of the actinomycetes that related (P ⩽ 0.05) to increased spore germination included late sporulation and possession of dark vegetative hyphae but lacking melanin or other water-soluble pigments. Amounts of the volatile 2-methylisoborneol produced by actinomycetes were well-correlated with spore germination.