Ethylene production by ectomycorrhizal fungi, Fusarium oxysporum f. sp. pini, and by aseptically synthesized ectomycorrhizae and Fusarium-infected Douglas-fir roots.

The ectomycorrhizal fungi Cenococcum geophilum, Hebeloma crustuliniforme, and Laccaria laccata produced ethylene in vitro in modified Melin-Norkrans liquid medium only if amended with 2.5-10mM methionine; Pisolithus tinctorius failed to produce ethylene unless the cultures were renewed with fresh methionine-amended medium before ethylene assay. An additional 19 ectomycorrhizal fungi, plus five isolates of Fusarium oxysporum f. sp. pini, all produced ethylene in renewed and (or) nonrenewed media. Although the rates varied, ethylene production by many ectomycorrhizal fungi equaled that of Fusarium. Culture filtrates of H. crustuliniforme and L. laccata also evolved ethylene that was apparently of nonenzymatic origin. Ethylene was produced by aseptically grown Douglas-fir seedlings inoculated with C. geophilum, H. crustuliniforme, and L. laccata and appearance of ethylene coincided with the formation of mycorrhizae; production by P. tinctorius inoculated seedlings was inconsistent. Lateral root formation of Douglas-fir was stimulated by inoculation with C. geophilum, H. crustuliniforme, and L. laccata but was inhibited by P. tinctorius. Fusarium-inoculated seedlings produced more ethylene sooner than seedlings inoculated with mycorrhizal fungi. The disparity in the levels of ethylene associated with ectomycorrhiza formation compared with Fusarium infection suggests a possible differential role for ethylene in symbiotic and pathogenic fungus-host interactions.