Effects of auxinic herbicides on growth/development, ethylene biosynthesis and gene expression were studied in Sinapis arvensis as well as wild type, auxin‐resistant and ethylene‐insensitive lines of Arabidopsis thaliana. Expansion of the radius of the root/hypocotyl and inhibition of root and hypocotyl elongation were observed in seedlings of S arvensis and wild‐type A thaliana treated with the auxinic herbicide 2,4‐D. All of these morphological changes, except for inhibition of root elongation, are caused by auxinic herbicide‐induced ethylene biosynthesis, which is caused by transcriptional activation of auxin‐inducible genes encoding 1‐aminocyclopropane‐1‐carboxylic acid synthase (ACS). We are the first to distinguish that two types of signal/mechanism exist; the first results from auxinic herbicide‐induced ethylene while the second mechanism is not mediated by ethylene. We identified and characterized seven members of ACS gene family in wild mustard and studied the auxin‐induced expression of these genes. The significance of these findings in understanding the molecular basis for the morphological changes associated with auxinic herbicide action is discussed. © 2000 Society of Chemical Industry

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