Poplar and willow are economically-important, fast-growing tree species with the ability to colonize nutrient-poor environments. To initiate a study on the possible contribution of endophytes to this ability, we isolated bacteria from within surface-sterilized stems of native poplar (Populus trichocarpa) and willow (Salix sitchensis) in a riparian system in western Washington state. Severalof the isolatesgrew well in nitrogen-limited medium. The presenceof nifH, a gene encodingone of the subunits of nitrogenase, was confirmed in several of the isolates including species of Burkholderia, Rahnella, Sphingomonas, and Acinetobacter. Nitrogenase activity (as measuredby the acetylene reduction assay) was also confirmed in someof the isolates. The presenceof these diazotrophic microorganisms may help explain the ability of these pioneering tree species to grow under nitrogen limitation.
Endophytic bacteria associated with sweet potato plants (Ipomoea batatas (L.) Lam.) were isolated, identified and tested for their ability to fix nitrogen, produce indole acetic acid (IAA), and exhibit stress tolerance. Eleven different strains belonging to the genera, Enterobacter, Rahnella, Rhodanobacter, Pseudomonas, Stenotrophomonas, Xanthomonas and Phyllobacterium, were identified. Four strains were shown to produce IAA (a plant growth hormone) and one strain showed the ability to grow in nitrogen free medium and had the nitrogenase subunit gene, nifH. To determine if IAA production by the endophytes had any role in protecting the cells against adverse conditions, different stress tests were conducted. The IAA producer grew well in the presence of some antibiotics, UV and cold treatments but the response to pH, osmotic shock, thermal and oxidative treatments was the same for both the IAA producer and the no IAA producer. To determine if IAA produced by the strains was biologically relevant to plants, cuttings of poplar were inoculated with the highest IAA producing strain. The inoculated cuttings produced roots sooner and grew more rapidly than uninoculated cuttings. These studies indicate that endophytes of sweet potato plants are beneficial to plant growth.
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