Diazotrophic endophytes of native black cottonwood and willow

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. Several of the isolates grew well in nitrogen-limited medium. The presence of nifH, a gene encoding one of the subunits of nitrogenase, was confirmed in several of the isolates including species of Burkholderia, Rahnella, Sphingomonas, and Acinetobacter. Nitrogenase activity (as measured by the acetylene reduction assay) was also confirmed in some of the isolates. The presence of these diazotrophic microorganisms may help explain the ability of these pioneering tree species to grow under nitrogen limitation.