Auxin, Ethylene and Kinetin in a Carrier‐protein Model System for the Polar Transport of Auxins in Petiole Segments of Phaseolus Vulgaris

Summary The capacity of bean petiole segments to transport 14C‐indole‐3‐acetic acid decreases to approximately half within 5 hours of excision and to less than one‐tenth after 10 hours. Exposure of the segments to ethylene enhances this loss in capacity, but pretreatment with an auxin completely protects the transport system from both the natural and ethylene‐induced fall. A decrease in synthesis of protein accompanies the natural loss of transport capacity, and treatments with auxin or kinetin which retain synthesis also retain transport. Only auxin treatments protect against the effects of ethylene. To correlate these results, a theoretical model for polar transport is developed in which it is proposed that auxin molecules are bound to a specific carrier protein sited at the plasmalemma. Transport capacity could be regulated by the rate of synthesis of the carrier protein, with a secondary regulation of the activity of the carrier by the binding of either auxin or ethylene molecules to specific sites on the protein.