ISOLATION AND CHARACTERIZATION OF MUTANTS OF THE PLANT GROWTH-PROMOTING RHIZOBACTERIUM PSEUDOMONAS-PUTIDA CR12-2 THAT OVERPRODUCE INDOLEACETIC-ACID

Following transposon Tn5 mutagenesis of the plant growth-promoting rhi zobacterium Pseudomonas putida GR12-2, mutants that were able to grow in the presence of the tryptophan analog 5-fluorotryptophan were selec ted. Seven of the 50 5-fluorotryptophan-resistant mutants overproduced the phytohormone indoleacetic acid (IAA). Of these seven mutants, the highest level of IAA was observed with strain P. putida GR12-2/aux1, which produced four times the amount of indoleacetic acid synthesized by the wild-type strain. Strain P. putida GR12-2/aux1, in contrast to the wild type, lost the ability to stimulate the elongation of the roo ts of canola seedlings under gnotobiotic conditions. The growth rate, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminas e activity of mutant strain P. putida GR12-2/aux1 were identical to th ose of the wild-type strain. The role of IAA in the mechanism of plant growth stimulation by P. putida GR12-2 and other plant growth-promoti ng rhizobacteria is discussed.