The growth-promoting effects of a bacterial endophyte on lodgepole pine are partially inhibited by the presence of other rhizobacteria

To test the hypothesis that rhizobacteria naturally present in soils may in terfere with the extent of root colonization and plant growth promotion by plant growth-promoting rhizobacteria (PGPR), we studied two lodgepole pine PGPR (Bacillus polymyxa strains L6 and Pw-2) when inoculated singly and whe n coinoculated with a non-PGPR competitor (Curtobacterium flaccumfaciens PF 322). Bacillus polymyxa Pw-2 and Curtobacterium flaccumfaciens PF322 were c onsistently found as endophytes, while Bacillus polymyxa L6 was never found within the root interior. Strains Pw-2 and L6 differed in the rate and typ e of growth promotion. Strain Pw-2 increased root growth (branching and elo ngation) and shoot biomass accumulation 6 and 9 weeks, respectively after i noculation, while strain L6 increased primary root elongation and root biom ass accumulation after 12 weeks. Seedlings coinoculated with Pw-2 and PF322 had decreased shoot biomass and primary root lengths when compared with se edlings inoculated only with Pw-2. This effect was not linked to a decrease in the population size of Pw-2 in the rhizosphere or in the root interior of coinoculated treatments. Ln contrast, strain L6-mediated growth promotio n was not impaired by coinoculation with PF322. Strain L6 did interfere to some degree with the growth-promoting capability of strain Pw-2. These resu lts indicate that endophytic PGPR may be less adapted to microbial competit ion than external root-colonizing PGPR, and that the efficacy of endophytic PGPR may be reduced by the presence of other bacteria on external or inter nal root tissues.