Bacterial endophytes in cotton: location and interaction with other plant-associated bacteria

Investigations were conducted to determine if biological control agent Pseudomonas fluorescens 89B-61 could colonize cotton tissues systemically and if internal colonization by a known endophytic bacterium, Enterobacter asburiae JM22, was influenced by the presence of other plant-associated bacteria. Following seed treatment, Pseudomonas fluorescens 89B-61 colonized cotton roots both externally and internally at mean population densities of 8.7 × 105 CFU/g and 1.1 × 103 CFU/g, respectively. However, bacteria were not detected in cotyledons, leaves, or stems. After inoculation onto leaves, Pseudomonas fluorescens 89B-61 established a mean internal population density of 1.6 × 104 CFU/g leaf tissue. Following stem injection, Pseudomonas fluorescens 89B-61 did not colonize roots or leaves. Pseudomonas fluorescens 89B-61 was localized on the root surface concentrated in grooves between epidermal cells, below collapsed epidermal cells, and in intercellular spaces close to the root epidermis, as identified by immunogold labeling of the bacterial membrane. Combined application of E. asburiae JM22 with another endophyte, Paenibacillus macerans Tri2-10, resulted in significantly lower internal populations of E. asburiae JM22 compared with treatment with E. asburiae JM22 alone. However, when coinoculated with a rhizosphere colonist, Micrococcus agilis strain 2RD-11, the colonization density of E. asburiae JM22 was not negatively affected. The results suggest that the internal colonization of cotton by bacteria with biological control activity may be an important aspect in their capacity to protect host plants against plant pathogens. The extent of internal colonization was shown to be influenced by other bacterial colonists.Key words: endophytic bacteria, location, interaction, cotton.