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Characterization of a Streptomyces sp. Isolated from Root Nodules of Ceanothus velutinus Dougl.¹

ABSTRACT

A procedure using a 20% solution of H₂O₂ as the sterilizing. agent was developed which allowed the isolation of a Streptomyces sp. from the root nodules of C. velutinus. This was repeated 17 times and a total of 136 similar streptomyces sp. were isolated from the nodules of this nonleguminous, nitrogen-fixing plant. Further characterization by standard bacteriological procedures suggested that the isolates belonged to a single species, which did not correspond to any previously recognized Streptomyces sp The inoculation of sterilized, germinated seeds with some of the isolates showed that they were capable of initiating the infection of root hairs on C. velutinus. Infection and nodulation of C. velutinus occurs in at least three distinct phases. At first there is a swelling at the tip of the root hair, which is followed by a marked swelling. As infection continues the root hair becomes swollen to its base. Shortly after this point has been reached the final phase of the process is recognized as the formation of visible nodules on the root surface. In the presence of some isolates and a sterile nodule extract, the infection process has been observed to the point where the entire root hair has become swollen. Without the sterile extract the infection was not as complete and progressed much slower. When the isolate is omitted and the seedling inoculated with the sterile nodule extract, a slight swelling at the root hair tip occurs. Despite the similarity of the isolates, infective ability of the isolates which appears to be a unique feature of the endophyte, no species name should be proposed until the demonstration of nodule formation with the isolate. Also it was suggested that some predisposing agent(s) may be required for the rapid initiation of the infection process in C. velutinus seedlings.

¹ Technical Paper no. 2075: Oregon Agr. Exp. Sta., Oregon State Univ., Corvallis. Presented before Division S-3, Soil Sci. Soc. America, Columbus, Ohio, Nov. 1, 1965. Portions of this material are from a thesis submitted by the senior author in partial fulfillment of the requirements of a Ph.D. degree. This study was supported in part by grant G24010 from the National Science Foundation.

² Assistant Professor of Forest Soils, Professor of Forest Soils, and Professor of Microbiology, respectively, Oregon State University.

Received for publication December 20, 1965. Accepted for publication April 12, 1966.