Graduate Thesis Or Dissertation
 

The role of ethylene in ectomycorrhiza formation and Fusarium infection of Douglas fir roots

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  • 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 to 10 mM methionine; Pisolithus tinctorius failed to produce ethylene unless the cultures were renewed with fresh methionine-amended medium prior to ethylene assay. An additional 19 ectomycorrhizal fungi plus 5 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. Fusariurn-inoculated seedlings produced more ethylene sooner than seedlings inoculated with mycorrhizal fungi. Two-month-old Douglas-fir seedlings were exposed to six ethylene concentrations, ranging from 0.006 (soil ambient control) up to 0.5 ppm, established by adding Ethrel, an ethylene-releasing compound, as a soil drench to the root zone. Exposure of roots to the different ethylene concentrations for 2 months either stimulated (0.01-0.05 ppm), had no effect on (0.05-0.15 ppm), or inhibited (> 0.15 ppm) lateral root formation. Root dry weight increased and shoot dry weight decreased as the ethylene concentration was increased. Mycorrhiza formation by Hebeloma crustuliniforme was not increased by exposing inoculated 4-month-old seedlings to 0.1 ppm ethylene for 3 months whether or not the seedlings had been exposed to ethylene for two months prior to inoculation. By contrast, when 2-month-old seedlings pre-inoculated with Fusarium oxysporum f. sp. pini were exposed to 0.1 ppm ethylene for 2 months, disease was significantly increased. These studies suggest that there is a fundamental difference between pathogenic Fusarium and symbiotic mycorrhiza.1 fungi in the levels of ethylene produced by the host-fungus interaction and in the function of ethylene in disease development versus mycorrhiza establishment and maintenance.
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