Dynamics of Endogenous Indole-3-acetic Acid and Cytokinins During Adventitious Shoot Formation in Ipecac

For most plant species, auxins and cytokinins are added to the culture medium to induce adventitious shoot formation. In ipecac (Carapichea ipecacuanha (Brot.) L. Andersson), however, shoots form on internodal segments without phytohormone treatment. This creates an opportunity to analyze the dynamics of endogenous phytohormones during adventitious shoot formation. Ipecac is a medicinal plant whose root extract is used as an expectorant and emetic. We explored the relationships among adventitious shoot formation and levels of an auxin (indole-3-acetic acid) and four cytokinins in ipecac. Adventitious shoots that formed in the apical region of internodal segments were derived from epidermal cells. One of the shoots grew vigorously with a vascular bundle connected to that of the segment, and the others were suppressed in the outgrowth. When the biggest shoot was removed, another began to grow. During adventitious shoot formation, endogenous auxin accumulated in the basal region of segments, whereas cytokinins accumulated in the middle region. Thus, the distribution of auxin, not cytokinins, determined where adventitious shoots formed.