Endophytic colonization and field responses of hybrid spruce seedlings after inoculation with plant growth-promoting rhizobacteria

Abstract

Bacterial colonization and growth responses of hybrid spruce ( Picea glauca × engelmannii) seedlings after inoculation with plant growth-promoting rhizobacteria (PGPR) were evaluated in controlled environment and field assays. Six antibiotic-resistant bacterial strains belonging to the genera Bacillus and Pseudomonas were used as inocula. For controlled environment assays, surface sterilized seeds were inoculated with log 6 Bacillus or log 8 Pseudomonas colony-forming units (cfu) under gnotobiotic conditions for assessment of rhizosphere and internal root and stem tissue colonization. In the field trial, one-year-old, container-grown hybrid spruce seedlings were inoculated with each of the Bacillus or Pseudomonas strains 1–2 days before planting at nine field sites in British Columbia and Alberta, Canada. All six strains colonized the rhizosphere with log 5.6−7.6 cfu g −1 root tissue under controlled environmental conditions. Two strains, Bacillus Pw-2R and Pseudomonas Sm3-RN, were also recovered from internal root and stem tissues with population sizes ranging from log 3.9 to log 5.0 cfu g −1 plant tissue. In the field trial, PGPR survived the winter in the rhizosphere with populations of log 2 to log 5 cfu g −1 root tissue 17 months after inoculation. In addition, spruce growth was significantly enhanced by bacterial inoculation at certain sites. The most effective growth-promoting strains were the endophytes, Sm3-RN and Pw2-R, and the external root colonizing strain, Ss2-RN. These three strains increased seedling dry weight up to 57% above noninoculated controls at five of the nine outplanting sites. Pseudomonas strain Sw1 and the other two Bacillus strains produced mean spruce dry weight increases at four of the sites. Seedling growth inhibition due to bacterial inoculation was detected at some sites. Our results confirm the short-term, site specific effectiveness of PGPR for reforestation of conifer seedlings, and indicate that benefits of a single inoculation at planting can extend through the second year in the field. In addition, bacteria capable of colonizing the seedling interior may be more effective PGPR for spruce than those restricted to the rhizosphere and the root surface. However, site specific seedling growth promotion may necessitate matching PGPR strains to outplanting sites for effective growth promotion.