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Inoculation of canola (Brassica campestris) seeds with a nitrogen-fixing strain of Pseudomonas putida (GR12-2) drastically increased the root length of seedlings grown in sterile growth pouches. Seed inoculation with inactive bacteria did not affect root lengths. Root elongation capacity was retained by nonnitrogen-fixing mutants of strain GR12-2. On the other hand, two other wild-type pseudomonads that do not fix nitrogen also increased root elongation. The addition of mineral nitrogen to the growth solution at concentrations of 1 mM or higher significantly inhibited root elongation of either inoculated or noninoculated seedlings. On the other hand, the addition of phosphate to the growth solution at similar concentrations stimulated root elongation of inoculated and noninoculated seedlings. The combined effects of bacterial inoculation and addition of phosphate on root and shoot elongation and on root and shoot weight were additive. Seed inoculation with P. putida GR12-2 increased the uptake of labelled phosphorus (32P) by seedlings grown in growth pouches and also enhanced the shoot elongation of seedlings grown in sterile soil. The capacity of P. putida GR12-2 to enhance phosphate uptake and to promote plant growth under gnotobiotic conditions may open the door to a new direction in the development of plant growth promoting inoculants.

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