Root symbionts and soil microorganisms associated with actinorrhizal plants Public Deposited

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  • Symbiotic associations are established between non-leguminous (actinorrhizal) nitrogen-fixing flowering plants and two categories of microorganisms: mycorrhizal fungi and a filamentous actinomycete. The actinomydete induces nodule formation and produces nitrogenase, the enzyme responsible for the reduction of atmospheric nitrogen to a form available to higher plants. The mycorrhizal fungus is found both inter- and intracellularly in the root system, and may be found within the nodules. The two major nutrients, nitrogen and phosphorus, can be supplied to the host plant by means of these two symbiotic microorganisms. Twenty-five species of flowering plants that fix atmospheric nitrogen in actinomycete-induced nodules were sampled for mycorrhizal associates. Both mycorrhizae and nodules were present on: (1) four species of Alnus; (2) two species of Casuarina; (3) eight species of Ceanothus; (4) four species of Myrica; (5) and one species each of Shepherdia, Hippophae, Cercocarpus, Dryas, Purshia, Comptonia, and Datisca. Soil sieving revealed species of the following genera of vesicular-arbuscular (VA) mycorrhizal fungi; Gigaspora, Glomus, Acaulospora, and Entrophospora. The VA mycorrhizal fungi exhibited distinct distributional patterns when associated with actinorrhizal hosts in different habitats. Mountain mahogany (Cercocarpus ledifolius Nutt.) is an actinorrhizal shrub native to Oregon and California. Nodulated seedlings along a roadbed in central Oregon were colonized by VA mycorrhizal fungi. Greenhouse seedlings inoculated with soil from this central Oregon site became nodulated and mycorrhizal within six months. Snowbrush (Ceanothus velutinus Dougl.), an actinorrhizal shrub species native to the Pacific Northwest, is able to establish, grow, and improve infertile soil. The root system of snowbrush can be dually colonized. The possibility of a direct interaction between the endophytes in the symbiosis was investigated. Dually infected plants showed greater increases in total dry weight, number of nodules, nodule dry weight, increases in nitrogenase activity as measured by acetylene reduction, as well as higher levels of tissue nitrogen, phosphorus, and calcium than nodulated plants without mycorrhizae. In assessing mycorrhizal associations of actinorrhizal plants, soil was sampled for Endogonaceae by wet sieving and decanting. Four new species of Glomus were isolated from under actinorrhizal shrubs in central Oregon and England: Glomus gerdemannii, G. halonatus, G. lacteus, and G. scintillans are described herein. An actinomycete was isolated from the rhizoplane of nitrogenfixing nodules of Ceanothus velutinus and was identified as an isolate of Streptomyces griseoloalbus. This isolate is a strong antagonist to three destructive root-rot pathogens: Phellinus weirii, Fomes annosus, and Phytophthora cinnamomi. This organism may confer protection to the nodule by presenting an antimicrobial barrier at the nodule-soil interface. The stability and longevity of the antimicrobial substance, its consistent effect on the pathogens on all substrates examined, and its ability to colonize wood suggest biological control possibilities for this organism in the Pacific Northwest
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