The rapid non-polar transport of auxin in the phloem of intact Coleus plants

Summary

Indole-3-acetic acid (IAA) is transported from a nearly mature leaf throughout an intact Coleus blumei Benth. plant in the phloem. A buffered solution of both ¹⁴C-methylene-labeled indoleacetic acid ([¹⁴C]IAA) and [6-³H]glucose was supplied in a glass capillary to the distal end of a severed main lateral vein of the leaf. Both labeled sugar and auxin move rapidly through the plant at velocities of ca. 16–20 cm h^-1 with closely similar, exponential profiles. This translocation is nonpolar; both auxin and sugar move upwards to the apex and young expanding leaves as well as downwards to the base of the shoot. Neither tracer appears in mature leaves; this eliminates the possibility that they enter the xylem. At the end of the transport period, 80–90% of the radioactivity recovered from various portions of the plants supplied with [¹⁴C]IAA is still identical chromatographically with IAA. In microautoradiographs prepared by techniques that minimize loss and redistribution of soluble compounds, radioactivity from [³H]IAA is concentrated in the phloem of the midrib and petiole of the fed leaf. A ring of triiodobenzoic acid (TIBA) strongly inhibits the polar auxin transport in sections isolated from the ringed region but does not significantly affect auxin translocation in the phloem of intact plants. TIBA does, however, reduce the entry of auxin into the collecting veins of the leaf. Thus steps in auxin transport sensitive to TIBA may occur during transfer through the leaf or into the phloem, but not during long distance translocation in the phloem.