Competitiveness of Diverse Methylobacterium Strains in the Phyllosphere of Arabidopsis thaliana and Identification of Representative Models, Including M. extorquens PA1

Facultative methylotrophic bacteria of the genus Methylobacterium are consistently found in association with plants, particularly in the phyllosphere. To gain a better understanding of the mechanisms underlying the dispersal and occurrence of Methylobacterium on plants, diverse strains were isolated, identified, and studied with regard to their competitiveness on the model plant Arabidopsis thaliana. As a basis for this study a comprehensive collection of Methylobacterium isolates was established. Isolates were obtained from five different naturally grown A. thaliana populations and diverse other plant genera at these and further sites. They were classified using automated ribosomal internal spacer analysis (ARISA) and a representative subset was identified based on 16S rRNA gene sequence analysis. A comparison of their ARISA patterns with those generated based on a cultivation-independent approach from the same sampling material confirmed that the isolates were abundant colonizers of the studied plants. In competition experiments, colonization efficiency of the strains was found to be linked to phylogeny, rather than to the geographical origin or plant genus from which they were isolated. The most competitive colonizers were related to the species Methylobacterium tardum and Methylobacterium extorquens. Higher cell numbers were observed in the phyllosphere of A. thaliana when a mixture of different strains was applied relative to inoculation with only one strain, suggesting partial niche heterogeneity. Based on the results of the competition experiments, representative strains with different colonization efficiencies were selected, which will serve as models in future studies aiming at a better understanding of plant colonization by this bacterial genus. Among them is the meanwhile genome-sequenced strain M. extorquens PA1, which represents a competitive species of plant colonizers with a broad dispersal. This strain was characterized in more detail including physiological, morphological, and chemotaxonomical properties.