Dynamics of Aspen Roots Colonization by Pseudomonads Reveals Strain-Specific and Mycorrhizal-Specific Patterns of Biofilm Formation

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.

Increase in phenolic compounds of Coriandrum sativum L. after the application of a Bacillus halotolerans biofertilizer

“…It is commonly accepted that efficient PGPR strains colonize roots under a wide range of conditions, predominantly by forming biofilms . Bacterial plant root attachment is essential for successful colonization.…”

“…29 It is commonly accepted that efficient PGPR strains colonize roots under a wide range of conditions, predominantly by forming biofilms. 35 Bacterial plant root attachment is essential for successful colonization. This process relies on a variety of cell, such as pili, flagella and extracellular polysaccharides, which play a major role in the initial surface adhesion.…”

Interrogation of genes controlling biofilm formation using CRISPR interference in Pseudomonas fluorescens

“…We assessed the functionality of our CRISPRi/dCas9 system in three different P. fluorescens strains: SBW25, WH6 and Pf0-1 that expressed a chromosomal copy of the mNG gene, encoding the mNeonGreen fluorescent protein under control of a constitutive Pc promoter 11 . This construct was inserted at similar chromosomal locations in all strains 11 .…”

“…Often associated with chronic infections and resistance to antibiotic treatments, biofilms are generally considered harmful to human health 5,6 . In contrast, biofilm-forming non-pathogenic bacteria can effectively protect from infection by pathogens 7 , and can promote the growth of plants and stimulate symbiotic interactions between mycorrhizal fungi and plant roots [8][9][10][11][12] . Thus, there is considerable interest in deciphering at a molecular level the regulatory mechanisms of biofilm formation and dispersion to combat biofilms but also to better control them.…”

“…Here, we focus on studying c-di-GMP-associated regulators that control biofilm formation in the rhizobacterium P. fluorescens. Soil and plant-associated bacteria such as P. fluorescens isolates can provide beneficial ecological services to various plants [42][43][44] and form dynamic and highly structured biofilm at aspen roots 11 . Genomic comparisons between multiple P. fluorescens strains revealed a high genomic heterogeneity with a conserved core genome and a widely diverse pan-genome encoding specialized activities, many being relevant for colonization of the rhizosphere and plant-bacteria interactions 45,46 .…”