We have isolated four cDNA clones of ACC synthase from etiolated mungbean seedlings treated with auxin. pVR-ACS2, pVR-ACS3 and pVR-ACS6 contained the same sequences as the previously reported DNA fragments, pMAC2, pMAC3 (Botella et al. 1992b) and pMBA1 (Kim et al. 1992), respectively. pVR-ACS1 was identical with pAIM-1 (Botella et al. 1992a). VR-ACS6 was specifically induced in response to the auxin signal. The IAA-induction of VR-ACS6 was very rapid (within 30 min) and insensitive to cycloheximide treatment at concentrations up to 100 microM. Significant accumulation of VR-ACS6 mRNA was detected at 1 microM IAA. The IAA-induced expression of VR-ACS6 was suppressed by ABA and ethylene, but enhanced by BA. These characteristics of VR-ACS6 expression were well correlated with the physiological data of auxin-induced ethylene production in mungbean hypocotyls. VR-ACS1 was strongly induced by cycloheximide, but was found to be not auxin-specific. Inhibitors of either ethylene biosynthesis (AOA) or action (NBD) increased the basal level of VR-ACS1 mRNA.