Colonization of Soybean Roots byPseudomonasandSerratiaSpecies: Relationship to Bacterial Motility, Chemotaxis, and Generation Time

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Abstract
Thirty-two bacterial strains representing Pseudomonas putida, P. fluorescences, and Serratia spp. were isolated from soil or water. All strains colonized soybean [Glycine max] roots in laboratory, greenhouse, and field assays when applied as seed inoculants. Colony-forming units (cfu) ranged from log 1.9 to 6.1 cfu/g of root. All strains colonized to soybean seeds at values ranging from 1.4 to 7.0 cfu per seed. Mean generation times in cultured media were not significantly different among the three bacterial types and did not correlate to root or seed colonization levels. P. putida and P. fluorescens exhibited significantly (P = 0.05) greater motility and chemotaxis toward soybean exudates in soft agar (0.2%) and capillary assays than did Serratia spp. There was no different positive correlation between motility or chemotaxis and root or seed colonization by the bacteria, with one exception: chemotaxis of P. fluorescens on exudate agar significantly correlated to root colonization, but only in the laboratory assay, where overhead watering was eliminated. In the same assay, Serratia spp. showed a significant negative correlation between motility, and root colonization. Motility was not required for root colonization. A Tn5 nonmotile mutant of P. putida RW3 (RW3-) colonized roots and was distributed along them as well as the motile parent. Thus, use of motility, chemotaxis, or laboratory generation time assays will not likely improve the isolation and identification of superior root-colonizing bacteria.