Stimulated Nodulation of Soybeans by Rhizobium Japonicum Mutant (b.14075) That Catabolizes the Conversion of Tryptophan to Indol·3yl· Acetic Acid*

Nodulations of Glycine max L. seedlings by parental Rhizobium japonicum (USDA strain 26) and a tryptophan catabolic mutant (tan 4b, NRRL strain B-14075) were evaluated for effectiveness. Seedlings grown in plastic pouches were compared based on increases in root weight (wt.), nodule volume (vol.) and acetylene-reducing (AR) activity. The tan 4b mutant, which can produce extracellular indol-3yl-acetic (IAA) and indol-3yl-pyruvic (IPA) acids asymbiotically, enhanced nodulation significantly. Nodulation by strain 26 was most effective when a basal nutrient was supplemented with sucrose, CaCO" EDTA, nicotinic acid and glutamate. In contrast, effective nodulation by tan 4b did not require an exogenous glutamate (0.34 mM)-nicotinate (81 ,11M) combination. Like strain 26 inoculum supplemented with exogenous IAA (0.1 mM), taproot formation associated with unsupplemented tan 4b was inhibited by exposure to direct light.