Effects of Indole-3-acetic Acid and Brassinosteroid on Ethylene Biosynthesis in Etiolated Mung Bean Hypocotyl Segments

Brassinosteroid (BR) and indole-3-acetic acid (IAA) alone and in combination were evaluated for their effects on ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), and ACC-synthase production in etiolated mung bean hypocotyl segments. Eighteen hours following treatment with BR (0.25 to 2.0 μM) or IAA (0.5 to 10.0 μM) alone promoted an increase in ethylene and ACC production. When BR and IAA were used in combination there was a synergistic increase in ethylene and ACC production at all concentrations tested 18 h following treatment. Aminooxyacetic acid (AOA), cycloheximide (CHI), and cobalt chloride (COCl₂) treatments after 18 h inhibited BR, IAA, and IAA + BR-induced ethylene and ACC production at 2 mM, 100 μM and 10 mM concentrations respectively. Segments treated with 10 μM IAA showed an increase in ACC and ACC-synthase production 3 h after treatment initiation with little change between 3 to 6 h. This plateau was followed by a sharp decline in both ACC and ACC-synthase from 6 to 12 h following treatment. BR treated segments showed a sharp increase in both ACC and ACC-synthase between 6 to 24 h following treatment. When IAA was used in combination with BR there was a sharp increase in ACC and ACC-synthase for the first 12 h following treatment, followed by a plateau. Cobalt chloride dramatically inhibited BR-induced ACC and ACCsynthase over a 24 h period whereas it had a stimulatory effect on both IAA and BR + IAA-induced ACC and ACC-synthase production shortly following treatment. AOA stimulated IAA-induced ACCsynthase activity but not BR-induced. Both IAA and BR induced ethylene and ACC production were both inhibited by AOA.