Auxin induces three genes encoding 1-aminocyclopropane-1-carboxylate synthase in mung bean hypocotyls

Abstract

By screening a cDNA library of auxin-treated mung bean ( Vigna radiata L.) hypocotyls, we have isolated two full-length cDNA clones, pVR-ACS6 and pVR-ACS7, for 1-aminocyclopropane-1-carboxylate (ACQ synthase, the rate-limiting enzyme in the ethylene biosynthetic pathway. While pVR-ACS6 corresponds to the previously identified PCR fragment pMBAl, pVR-ACS7 is a new cDNA clone. pVR-ACS6 is 1867bp long encoding 472 amino acids (Mr = 53.6 kDa), and pVR-ACS7 is a 1840 bp clone encoding 468 amino acids (Mr = 53.1 kDa). The coding regions of pVR-ACS6 and pVR-ACS7 share 81 and 88 % identity at nucleotide and amino acid levels, respectively. Genomic Southern blot analyses suggest the existence of only one copy of ACS6 and ACS7 genes in the mung bean genome. Previously, it was reported that mung bean ACC synthase cDNA clone (pAIM-1) representing ACS1 gene was expressed following auxin treatment. Northern blot analyses were carried out to compare the magnitudes and induction kinetics of the expression of the three genes, ACS1, ACS6 and ACS7 whose expressions are induced by auxin (100 µM IAA) in hypocotyls. Although all three genes are expressed in response to auxin treatment, the level of ACS1 transcript is lower than those of ACS6 and ACS7 transcripts during the entire period of auxin treatment. ACS6 and ACS7 are specifically induced by auxin, while ACS1 is induced by cycloheximide, and to lesser extent by excision and auxin treatment. A comparison of deduced amino acid sequences among auxin-induced ACC synthases reveals that these enzymes share a high degree of homology (65–75 %) to VR-ACS6 and VR-ACS7 polypeptides, but only about 50% to VR-ACS1 polypeptide. These results indicate that ACC synthase is encoded by a multigene family and expression of these genes is differentially regulated by auxin in mung bean hypocotyl tissue.