Abstract
Rhizosphere-associated Pseudomonas fluorescens are known plant growth promoting (PGP) and mycorrhizal helper bacteria (MHB) of many plants and ectomycorrhizal fungi. We investigated the spatial and temporal dynamics of colonization of mycorrhizal and non-mycorrhizal Aspen seedlings roots by the P. fluorescens strains SBW25, WH6, Pf0-1, and the P. protegens strain Pf-5. Seedlings were grown in laboratory vertical plates systems, inoculated with a fluorescently labeled Pseudomonas strain, and root colonization was monitored over a period of 5 weeks. We observed unexpected diversity of bacterial assemblies on seedling roots that changed over time and were strongly affected by root mycorrhization. P. fluorescens SBW25 and WH6 stains developed highly structured biofilms with internal void spaces forming channels. On mycorrhizal roots bacteria appeared encased in a mucilaginous substance in which they aligned side by side in parallel arrangements. The different phenotypic classes of bacterial assemblies observed for the four Pseudomonas strains were summarized in a single model describing transitions between phenotypic classes. Our findings also reveal that bacterial assembly phenotypes are driven by interactions with mucilaginous materials present at roots.
Highlights
Plant-growth-promoting (PGP) rhizobacteria and mycorrhizal fungi exert their beneficial effects on plants through direct and indirect interactions with roots, which lead to increased availability of soil nutrients to the plant, production of pathogen-antagonist metabolites, stimulation of plant systemic defenses, and increased plant resistance to biotic and abiotic stresses (Rodríguez and Fraga, 1999; Barea et al, 2005)
We found that Pseudomonas strains adopted a range of morphological phenotypes on the rhizoplane, ranging from micro-colonies to highly structured biofilms, with temporal patterns depending on the bacterial strain and community composition
L. bicolor is known to colonize Aspen by sheathing the roots within a dense mycelia mat (Felten et al, 2009). We examined this interaction between Laccaria and Aspen roots in our vertical agar plates (VAP) assay, which leads to functional ectomycorrhizae promoting plant growth (Figure 1C)
Summary
Plant-growth-promoting (PGP) rhizobacteria and mycorrhizal fungi exert their beneficial effects on plants through direct and indirect interactions with roots, which lead to increased availability of soil nutrients to the plant, production of pathogen-antagonist metabolites, stimulation of plant systemic defenses, and increased plant resistance to biotic and abiotic stresses (Rodríguez and Fraga, 1999; Barea et al, 2005). Rhizosphere microorganisms acquire photosyntheticallyderived carbon from the plant in form of sugars and organic acids. In nature, plants form symbiotic communities comprising mycorrhizal fungi and beneficial bacteria (Frey-Klett et al, 2007; Churchland and Grayston, 2014; Mitter et al, 2016). P. fluorescens species are highly abundant in the root microbiome of Populus (Aspen) trees (Gottel et al, 2011; Brown et al, 2012; Weston et al, 2012; Timm et al, 2015), which are widely distributed in the Northern Hemisphere and establish symbiotic interactions with mycorrhizal fungi (Burns and Honkala, 1990; Lammers et al, 2004; Tuskan et al, 2004). Numerous rhizobacteria, including P. fluorescens have been shown to improve mycorrhizal formation, and are considered mycorrhizal helper bacteria (MHB) (Garbaye, 1994; Founoune et al, 2002; Frey-Klett et al, 2007; Labbé et al, 2014)
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