The Effect of Morphactin (Methyl 2-Chloro-9-hydroxyfluorene-9-carboxylate) on Basipetal Transport of Indol-3-ylacetic Acid in Hypocotyl Sections of Phaseolus vulgaris L.

Abstract

The effect of morphactin (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate) on basipetal transport of auxin (Indol-3-ylacetic acid-2-¹⁴C) was studied in bean (Phaseolus vulgaris) hypocotyl with the donor-receiver block method. Morphactin (5 × 10⁻⁶m) reduced IAA (5 × 10⁻⁶m) transport intensity by an average of 83 per cent and auxin transport capacity by 90 per cent, but transport velocity was not affected. Morphactin did not inhibit uptake of IAA into hypocotyl tissue, but it did prevent transfer of IAA from the tissue into receiver blocks. Chromatographic analysis of the tissue after 4 h IAA-2-¹⁴C transport showed that 54 per cent of the total activity was in the form of IAA in the control and 42 per cent in the morphactin treated tissue. No difference was found in the rate of decarboxylation of IAA-1-¹⁴C between control and morphactin treated tissue sections. Nor could any difference between control and morphactin be shown in the radioactivity associated with a TCA ppt fraction. In a study of the transportable auxin pool, morphactin decreased the size of the pool and increased the half-life of decay of auxin transport from 1⋅22 h to 3⋅85 h. In a kinetic analysis of the reversal of morphactin (5 × 10⁻⁶m) inhibition by increasing concentration of IAA-2-14C (5 × 10⁻⁶m to 2 × 10⁻⁵m), it was shown that IAA transport resembles Michaelis-Menten enzyme reaction kinetics, and that inhibition by morphactin fitted a ‘mixed type’ model. IAA had a dissociation constant of 8⋅5 × 10⁻⁶m and morphactin that of 4⋅3 × 10⁻⁷m with a K_m for the transport process of 8⋅5 × 10⁻⁶m.