Effects of Gibberellic Acid on Utilization of Auxin Precursors by Apical Segments of the Avena Coleoptile

Abstract

The growth of apical segments of Avena coleoptiles was used as an index of the conversion of precursors to auxin. The growth induced by trypto-phan was small but considerably enhanced in the presence of gibberellic acid. The growth induced by indole-3-acetaldehyde approached that induced by indole-3-acetic acid. The minor effect of gibberellic acid on the growth induced by the aldehyde suggests that the action of the gibberellin in the conversion of the auxin precursor involves a transformation prior to the aldehyde stage. The growth induced by trypto-phan alone or in combination with gibberellic acid was inhibited by dimedone as it would be if conversion of tryptophan to auxin involves an aldehyde intermediate. The inhibition by dimedone indicates that gibberellic acid promotes conversion of the auxin precursor through an aldehyde intermediate. The growth effects of tryptamine and gibberellic acid were independent. The growth induced by tryptamine was only partially inhibited by dimedone, and some of the tryptamine must have undergone transformation to auxin through a pathway not involving an aldehyde.