DIFFERENTIAL ACCUMULATION OF TRANSCRIPTS FOR ACC SYNTHASE AND ACC OXIDASE HOMOLOGS IN ETIOLATED MUNG BEAN HYPOCOTYLS IN RESPONSE TO VARIOUSSTIMULI

Ethylene can be produced by a variety of developmental and environment al factors such as ripening, the plant hormone auxin, and mechanical w ounding via a biosynthetic pathway including AdoMet synthase, ACC synt hase, and ACC oxidase steps. ACC synthase and ACC oxidase are known to be encoded by multigene families, and are believed to be differential ly expressed in response to various stimuli. In mung bean, ACC synthas e is encoded by 7 genes, ACS1, ACS2, ACS3, ACS4, ACS5, ACS6, and ACS7, and ACC oxidase by 2 genes, ACO1 and ACO2. In this study, we have inv estigated differential accumulation of transcripts for ACC synthase an d ACC oxidase homologs in etiolated mung bean hypocotyls under various conditions by the Introduction semiquantitative RT-PCR method. Primer s which can specifically bind and amplify each cDNAs of ACS1, ACS2, AC S3, ACS4, ACS5, ACS6, ACS7, ACO1, and ACO2 were designed and used to m onitor the responses to various stimuli. Transcripts of ACO1 and ACO2 were accumulated constitutively in the hypocotyl segments even without any treatment. After cold treatment on intact plant, transcripts of A CS5, ACS6, and ACS7 were accumulated in the hypocotyl segments. We als o found that excision of hypocotyl segments and incubation in a buffer solution, a typical way of chemical treatments to hypocotyl segments, lowered the level of ACO2 transcripts with little change of the level of ACO1 transcripts. In response to incubation with IAA (0.1 mM) of e xcised hypocotyl segments, transcripts of ACS1, ACS6, and ACS7 were ac cumulated and the level of ACO2 transcripts was increased. Transcripts of ACS1, ACS2, ACS3, ACS5, ACS6, and ACS7 were induced by incubation with OGA (50 mu g/ml), while the transcripts of ACS4 were accumulated and the level of ACO2 transcripts was increased by incubation with 1 m M LiCl. Our results strongly suggest that all seven ACC synthase genes and two ACC oxidase genes must be active and each gene is differentia lly regulated by a different subset of the inducing factors.