Developmental regulation of the expression of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase genes in hypocotyls of etiolated mung bean seedlings

Differential expression of seven 1-aminocyclopropane-1-carboxylate (ACC) synthase and two ACC oxidase genes during the developmental process was investigated in etiolated mung bean seedlings. Among these genes, VR-ACS1, VR-ACS6, VR-ACS7, VR-ACO1, and VR-ACO2 were expressed in the subhook regions of hypocotyls. Except for VR-ACO1, the expression levels of these genes were correlated with the amount of ethylene produced. VR-ACO1 expression was largely constitutive. During hypocotyl growth, the expression of VR-ACS1, VR-ACS6, and VR-ACS7 was maintained at a high level. After the growth stopped, their expression levels became negligible, but the decrease in the VR-ACO2 transcript was much slower. These results suggest that the expression of each of the five genes in the subhook regions of hypocotyls is kinetically distinct, temporally and spatially regulated, and especially high in the tissues of rapidly elongating cells. However, endogenously produced ethylene had no stimulating effect on the growth of hypocotyls or roots. The expression of each of the five genes detected in intact hypocotyls was inducible with auxin. The expression of auxin-inducible VR-ACS1 and VR-ACS6 was negatively regulated in the presence of auxin action and transport inhibitors. However, the transcripts of VR-ACS7, VR-ACO1, and VR-ACO2 were rarely influenced by the treatment of these inhibitors. These results suggest that additional growth factor(s), besides auxin, was (were) involved in the regulation of endogenous ethylene synthesis in etiolated mung bean seedlings.