Production of B-group vitamins by plant growth-promoting Pseudomonas fluorescens strain 267 and the importance of vitamins in the colonization and nodulation of red clover

We have estimated the production of water-soluble B vitamins by plant growt h-promoting rhizobacterium Pseudomonas fluorescens strain 267 in a minimal medium with different C sources and at different pH values. In the minimal medium, strain 267 produced large amounts of niacin (0.92 mug ml(-1)) and p antothenic acid (0.75 mug ml(-1)), but also other vitamins such as biotin, thiamine, cobalamine and pyridoxine. The production of B vitamins was depen dent on the C source and pH of the growth medium. By random Tn5 mutagenesis , thiamine and niacin auxotrophs were isolated from P. fluorescens strain 2 67 and mutants were used to evaluate the vitamin production on colonization of clover roots under controlled conditions. Red clover root colonization decreased by about 1 order of magnitude in the case of the niacin auxotroph . The vitamin auxotrophs of P. fluorescens in a mixed inoculation of clover with R. leguminosarum by. trifolii strain 24.1 showed no plant growth-prom otion activity.