Location of Transported Auxin in Etiolated Maize Shoots Using 5-Azidoindole-3-Acetic Acid

A study was undertaken using the photoaffinity labeling agent, tritiated 5-azidoindole-3-acetic acid ([(3)H],5-N(3)IAA), to identify cells in the etiolated maize (Zea mays L.) shoot which transport auxin. Transport of [(3)H],5-N(3)IAA was shown to be polar, inhibited by 2,3,5-triiodobenzoic acid (TIBA) and essentially freely mobile. There was no detectable radiodecomposition of [(3)H],5-N(3)IAA within tissue kept in darkness for 4 hours. Shoot tissue which had taken up [(3)H],5-N(3)IAA was irradiated with ultraviolet light to covalently fix the photoaffinity labeling agent within cells that contained it at the time of photolysis. Subsequent microautoradiography showed that all cells contained radioactivity; however, the amount of radioactivity varied among different cell types. Epidermal cells contained the most radioactivity per area, approximately twofold more than other cells. Parenchyma cells in the mature stelar region contained the next largest amount and cortical cells, sieve tube cells, tracheary cells, and all cells in the leaf base contained the least amount of the radioactive label. Two observations suggest that the auxin within the epidermal cells is transported in a polar manner: (a) the amount of auxin in the epidermal cells is greatly reduced in the presence of TIBA, and (b) auxin accumulates on the apical side of a wound in the epidermis and is absent on the basal side. While these results indicate that auxin in the epidermis is polarly transported, this tissue cannot be the only pathway since the epidermis is only a small fraction of the shoot volume. The greater than twofold difference between the concentration of auxin in the epidermal and subtending cells demonstrates that physiological differences in the concentration of auxin can occur between adjacent cells.