Phytohormones, Rhizobium Mutants, and Nodulation in Legumes : III. Auxin Metabolism in Effective and Ineffective Pea Root Nodules

High specific activity ³H]indole-3-acetic acid (IAA) was applied to the apical bud of intact pea (Pisum sativum L. cv Greenfeast) plants. Radioactivity was detected in all tissues after 24 hours. More radioactivity accumulated in the nodules than in the parent root on a fresh weight basis and more in effective (nitrogen-fixing) nodules than in ineffective nodules (which do not fix nitrogen).

For most samples, thin layer chromatography revealed major peaks of radioactivity at the R_F values of IAA and indole-3-acetylaspartic acid (IAAsp) and further evidence of the identity of these compounds was obtained by chromatography in other systems. Disintegrations per minute due to IAA per unit fresh weight were significantly greater for root than for nodule tissue, but were not significantly different for effective and ineffective nodules. Radioactivity due to IAAsp, expressed both on a percentage basis and per unit fresh weight, was significantly greater for nodule than for root tissue and significantly greater for the effective nodules than for the ineffective nodules. When ³H]IAA was applied to effective nodules, IAAsp was the dominant metabolite in the nodule. The data suggest that metabolism of auxins may be important for the persistence of a functional root nodule.