Abstract
Maize coleoptiles synthesize IAA within the apical 2-mm tip. However, specific IAA synthetic cell(s) and the IAA synthetic pathway in the tip have not been elucidated. We performed immunohistochemistry of IAA. Coleoptile tips treated with NPA accumulated IAA in the apical 0-2 mm region, and the IAA level per tissue was highest in the 0.5-1.0 mm tip. Immunolocalization showed that the IAA signal was the most intense in the upper-epidermis. 5-Methyl-tryptophan treatment substantially reduced the IAA level; the IAA immuno-signal in the upper-epidermis almost disappeared. These data indicate that IAA synthetic activity is highest in the upper-epidermis of the 0.5-1.0 mm region of coleoptiles. Immunogold labeling of IAA showed that IAA was localized in cytosol, plastids and mitochondria, but not in vacuoles or cell walls. These findings suggest that IAA is synthesized via a tryptophan-dependent pathway in the cytoplasm (including plastids and mitochondria) of maize coleoptile tip cells.
