EXTERNAL AND INTERNAL ROOT COLONIZATION OF LODGEPOLE PINE-SEEDLINGS BY 2 GROWTH-PROMOTING BACILLUS STRAINS ORIGINATED FROM DIFFERENT ROOT MICROSITES

Root colonization and in vitro carbon substrate utilization by two see dling growth-promoting Bacillus strains that originated from different root microsites were studied in greenhouse and growth chamber experim ents. Strain L6, identified as Bacillus polymyxa, was previously isola ted from rhizosphere soil containing roots of pasture plants, and Pw-2 , tentatively identified also as B. polymyxa, was isolated from within surface-sterilized lodgepole pine (Pinus contorta var. lattifolia (Do ugl.) Engelm.) roots. Rifamycin-resistant strains derived spontaneousl y from wild-type strains L6 and Pw-2, designated strain L6-16R and Pw- 2R, respectively, were used to monitor lodgepole pine root colonizatio n in a closed tube assay system. Three-week-old pine seedlings were in oculated with 10(5) colony-forming units (cfu) of strain Pw-2R or 10(6 ) cfu of strain L6-16R, and external and internal root colonization wa s assessed 2 and 4 weeks later. Strains L6-16R and Pw-2R were both rec overed from pine rhizosphere samples with >5 x 10(7) cfu/g fresh root tissue 2 weeks after inoculation, but neither strain was detected in t he root interior. When root colonization was assessed 4 weeks after in oculation, the rhizosphere populations of both strains had declined sl ightly to between 5 x 10(6) and 5 x 10(7) cfu/g fresh root tissue, but strain Pw-2R was also detected within root tissues with 10(5) cfu/g f resh root tissue. Lateral root formation was abundant 4 weeks after in oculation and may have facilitated colonization of internal root tissu es by strain Pw-2R. Both strains possessed pectolytic activity, althou gh differences between the strains were detected in in vitro substrate utilization capabilities using BIOLOG assays. These differences may b e related to their abilities to colonize internal root tissues. On the basis of our results, we hypothesize that internal root colonization by Bacillus strains is not a random event and that root-endophytic Bac illus strains possess specific physiological and (or) biochemical char acteristics that facilitate colonization of internal root tissues.