Indoleacetic Acid Biosynthesis in Avena Coleoptile Tips and Excised Bean Shoots

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Abstract
Avena coleoptiles did not elongate when incubated with tryptophan under sterile conditions. Indole, anthranilic acid, and tryptamine promoted elongation. Under the same conditions, the tissue converted tryptophan-14C to IAA-14C. More IAA-14C was produced from indole-14C than from tryptophan-14C; however, the free tryptophan content of the tissue was also greatly increased by the indole treatment. Tryptophan-14C was readily taken up by the tissue but was mainly incorporated into protein and did not increase the free tryptophan level. When bean shoots were labeled with tryptophan-14C or indole-14C, the label incorporation into IAA-14C was very nearly the same. In this tissue the free tryptophan level in the tryptophan-14C and indole-14C treatments was also about equal. These results suggest that failure of exogenously supplied tryptophan to promote the elongation of Avena coleoptiles is a result of its predominant incorporation into protein and consequent unavailability for conversion to IAA.