The role of motility in the colonization of potato roots by Pseudomonas bacteria was studied. Four TnS-induced flagella-less mutants of the plant-growth-stimulating P. fluorescens WCS374 appeared to be impaired in their ability to colonize growing potato roots.Certain fluorescent Pseudomonas isolates are able to increase plant growth (3,10,16,28) or to protect plants against microbial pathogens (17,25). Our research is focused on strains that can be used to eliminate potato yield reductions as observed in Dutch fields in which potatoes are frequently grown (28). These yield reductions can be abolished by treatment of the seed potatoes with particular Pseudomonas strains (3, 16, 27, 28,). Essential for this beneficial effect of the bacteria on plants is the production of fluorescent siderophores, 22) which supposedly enable the Pseudomonas cells to scavenge most of the Fe3`ions from the Fe3+-poor soil, thereby depriving deleterious microorganisms of this essential element. A second factor which is supposed to be essential for efficient protection of the roots against deleterious microorganisms is the delivery of siderophores along the whole root system of the plant, which requires efficient colonization of the potato roots. Although root colonization is very important in nature, virtually nothing is known about it at the molecular level. We studied the role of motility of Pseudomonas bacteria in the colonization of potato roots. MATERIALS ANID METHODSStrains and growth conditions. The potato root isolate Pseudomonas fluorescens WCS374 (7, 9, 10) is resistant to nalidixic acid (25 ,ug/ml). Strain JM3741 is a TnS-marked derivative of WCS374 which was isolated by J. Marugg. It does not differ significantly from the parental strain in its root-colonizing ability, its siderophore production, its growth rate in the complex King B medium or in minimal medium, and its motility (P. A. H. M. Bakker, unpublished data). Escherichia coli CSH52 harboring the mobilizable plasmid pSUP202 (Apr Cmr Kmr Tcr) and strain S17-1 harboring pSUP2021 (=pSUP202 with Tn5 inserted into the gene for tetracycline resistance) were obtained from R. Simon (29). Pseudomonas strains were grown at 28°C and E. ccli strains were grown at 37°C in King B medium (15) under vigorous aeration. For the root colonization assay TnSlabeled strains were cultivated at 28°C for 48 h on King B medium solidified with 1.6% agar and supplemented with kanamycin (50 ,ug/ml), The TnS-labeled strains were resistant to kanamycin (200 ,ug/ml) and streptomycin (200 ,ug/ml) * Corresponding author.(TnS encodes both kanamycin and streptomycin resistance in Pseudomonas species).Isolation of Tn5-induced nonmotile mutants. TnS-induced mutants were obtained by the method of Simon et al. (29) with slight modifications as described by Marugg et al. (22). Briefly, E. coli S17-1 harboring pSUP2021 was mated with P. fluorescens WCS374 for 3 to 4 h at 28°C. Transconjugants were selected on King B agar plates containing 25 ,ug of kanamycin per ml and 20 ,g of nalidixic acid per ml. Colon...