IAA and N6-benzyladenine inhibit ethylene-regulated expression of ACC oxidase and ACC synthase genes in mungbean hypocotyls

Regulation of expression of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (IT-AC01) and ACC synthase (VR-ACS1) genes by ethylene, indole-3-acetic acid (IAA), and N-6-benzyladenine (BA), was investigated in hypocotyl tissues. Exogenous ethylene markedly increased transcript level of IT-AC01 and reduced that of VR-ACSI, whereas aminooxyacetic acid (AOA), an inhibitor of ethylene biosynthesis, decreased the level of TIR-ACO1 mRNA and increased that of VR-ACS1, indicating that expression of VR-ACO1 and VR-ACS1 genes are under positive and negative feedback control by ethylene, respectively. However, IAA treatment reduced the level of VR-ACO1 transcripts and increased that of VR-ACS1, although the hormone greatly induced ethylene production. We have demonstrated that, in a system that separates the effect of IAA proper from the effect of IAA-induced ethylene, the amount of IAA-induced ethylene was enough to cause accumulation of VR-ACO1 mRNA and decrease of VR-ACS1 mRNA. We have also shown that the responsiveness of VR-ACO1 and VR-ACS1 to exogenous ethylene was greatly reduced in the presence of IAA. In addition, BA abolished ethylene responsiveness with respect to expression of VR-ACO1 and VR-ACS1. Based on these results, we suggest that IAA and BA inhibit ethylene action, resulting in suppression of TR-ACO1 expression and induction of PR-ACS1 expression.