Differential Accumulation of Transcripts for ACC Synthase and ACC Oxidase Homologs in Etiolated Mung Bean Hypocotyls in Response to Various Stimuli

Abstract

Ethylene can be produced by a variety of developmental and environmental factors such as ripening, the plant hormone auxin, and mechanical wounding via a biosynthetic pathway including AdoMet synthase, ACC synthase, and ACC oxidase steps. ACC synthase and ACC oxidase are known to be encoded by multigene families, and are believed to be differentially expressed in response to various stimuli. In mung bean, ACC synthase 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 investigated differential accumulation of transcripts for ACC synthase and ACC oxidase homologs in etiolated mung bean hypocotyls under various conditions by the semiquantitative RT-PCR method. Primers which can specifically bind and amplify each cDNAs of ACS1, ACS2, ACS3, ACS4, ACS5, ACS6, ACS7, ACO1, and ACO2 were designed and used to monitor 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 ACS5, ACS6, and ACS7 were accumulated in the hypocotyl segments. We also 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 excised hypocotyl segments, transcripts of ACS1, ACS6, and ACS7 were accumulated and the level of ACO2 transcripts was increased. Transcripts of ACS1, ACS2, ACS3, ACS5, ACS6, and ACS7 were induced by incubation with OGA (50 μg/ml), while the transcripts of ACS4 were accumulated and the level of ACO2 transcripts was increased by incubation with 1 mM LiCl. Our results strongly suggest that all seven ACC synthase genes and two ACC oxidase genes must be active and each gene is differentially regulated by a different subset of the inducing factors.