Inhibition of IAA‐induced ethylene production in etiolated mung bean hypocotyl segments by 2,3,5‐triiodobenzoic acid and 2‐(p‐chlorophenoxy)‐2‐methyl propionic acid

Abstract

The effect of two auxin antagonists, 2,3,5‐triiodobenzoic acid (TIBA) and 2‐(p‐chlorophenoxy)‐2‐methyl propionic acid (CMPA) on IAA‐induced ethylene production in etiolated mung bean hypocotyl (Vigna radiata L. Rwilcz cv. Berken) segments was studied. Both TIBA and CMPA inhibited IAA‐induced ethylene production and CO2 production at concentrations from 0.001 mM to 0.1 mM and 0.01 mM to 1.0 mM, respectively. The optimum concentration for inhibition of ethylene production by TIBA was 0.05 mM and CMPA was 0.5 mM. At the optimum concentration of TIBA and CMPA, there was a significant decrease in IAA‐induced ethylene production without a decrease in respiration rates below control levels. After 18 h, mung bean hypocotyl segments treated with 0.05 mM TIBA for 6 h or 0.5 mM CMPA for 8 h showed a maximum inhibition of IAA‐induced ethylene production. Treatments longer than 8 h caused no further inhibition. The uptake of [14C]‐naphthaleneacetic acid by mung bean segments was greatly reduced by the addition of either TIBA (0.05mM) or CMPA (0.5 mM) to the incubation media. The results of treatment sequences showed that TIBA needed to be applied prior to IAA in order to inhibit IAA‐induced ethylene production, but CMPA caused the same inhibitory effect whether applied before or after IAA treatment. These findings provide evidence that TIBA inhibits auxin‐induced ethylene production in etiolated mung bean hypocotyl segments by blocking auxin movement into the tissue whereas CMPA may work on both auxin transport and action.