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White lupin (Lupinus albus L.) develops proteoid or cluster root morphology when subjected to phosphate deficiency. These densely clustered, short lateral roots are the site of organic acid and acid phosphatase secretion and increased phosphoenolpyruvate carboxylase (PEPC) activity. While auxin has been implicated in the initiation and development of proteoid roots, little evidence to support this hypothesis is available. To investigate th relationship between auxin and the induction of proteoid roots, attempts were made to modify auxin transport during the early stages of lateral root development. In separate experiments, auxin transport inhibitors NPA (N-(1-napthyl) phthalamic acid) and TIBA (2, 3, 5-triiodobenzoic acid) were applied to lupin plants at 4 and 8 days after emergence. NPA and TIBA were added to both+ P and-P nutrient solutions and used to water plants growing in sand culture. The inhibitors were found to dramatically reduce proteoid root formation in the-P treatments, while having no apparent affect on root formation in+ P treatments or shoot morphology in either treatment. In the-P treatments, the addition of NPA and TIBA reduced PEPC and malate dehydrogenase activities in the proteoid root segments compared to the proteoid segments of the-P control plants. The addition of IAA to the nutrient solution did not affect proteoid root formation in-P plants. In the+ P treatments, the addition of IAA to the nutrient solution resulted in approximately 30% more proteoid root segments compared to the+ P control plants. This research was supported in part by the NRICGP grant USDA/93-371000-8941.