Endophytic colonization of spruce by plant growth-promoting rhizobacteria

Abstract

We evaluated internal tissue colonization of hybrid spruce (Picea glauca×P. engelmannii) seedlings by two plant growth-promoting rhizobacteria (PGPR) strains, Bacillus polymyxa strain Pw-2R and Pseudomonas fluorescens strain Sm3-RN, using surface sterilization-dilution plating and immunofluorescent antibody staining assays. Both strains were consistently detected inside spruce root and stem tissues 5 months after seed inoculation according to a surface sterilization-dilution plating assay. Internal tissue population sizes ranged from log 3.9 to log 5.0 cfu g−1 plant tissue. Visualization of bacteria using immunofluorescent antibody staining suggested that these microorganisms colonized root hair and cortical cells as well as stem vascular tissues. Our results confirm the capability of these two plant growth-promoting rhizobacterial strains to enter spruce root tissues after soil inoculation and ultimately colonize stem vascular tissue without causing visible symptoms of disease.