Relationship between Rapid, Firm Adhesion and Long-Term Colonization of Roots by Bacteria

For rhizobacteria to exert physiological effects on plant growth, the bacteria must first effectively colonize the root surface. To examine the relationship between long-term colonization of root systems and adherence to roots in the short term, a binding assay was developed. Adherence was determined by incubating roots of intact radish seedlings with bacteria, washing and homogenizing the roots, and dilution plating the resulting homogenate. Irreversible binding of bacteria was rapid, reaching half-maximum by 5 min. All of the rhizosphere bacteria tested showed similar, concentration-dependent binding (ranging from 10 to 10 CFU/ml), as well as long-term colonization of radish roots under sterile conditions. Escherichia coli, which is not a root colonizer, showed about 10-fold less binding, but still demonstrated concentration-dependent binding and rapid kinetics of adherence at high concentrations (10 to 10 CFU/ml). The bacteria tested were very different with respect to source or habitat and plant response, yet they showed similar concentration-dependent binding. There was no correlation between the relative hydrophobicities of the cell surfaces of strains and the adherence of the strains to roots. Binding of Pseudomonas fluorescens E6-22 was promoted by divalent cations (Ca and Mg) at concentrations of 5 to 10 mM, whereas monovalent cations (Na and K) had little effect; electrostatic phenomena may partially explain adherence in the short term, an important prelude to long-term colonization of root surfaces.