Recessive and dominant mutations in the ethylene biosynthetic gene ACS5 of Arabidopsis confer cytokinin insensitivity and ethylene overproduction, respectively (hormone biosynthesisyplant hormones)

We identified a set of cytokinin-insensitive mutants by using a screen based on the ethylene-mediated triple response observed after treatment with low levels of cytokinins. One group of these mutants disrupts ACS5 ,a member of the Arabidopsis gene family that encodes 1-amino- cyclopropane-1-carboxylate synthase, the first enzyme in eth- ylene biosynthesis. The ACS5 isoform is mainly responsible for the sustained rise in ethylene biosynthesis observed in re- sponse to low levels of cytokinin and appears to be regulated primarily by a posttranscriptional mechanism. Furthermore, the dominant ethylene-overproducing mutant eto2 was found to be the result of an alteration of the carboxy terminus of ACS5, suggesting that this domain acts as a negative regulator of ACS5 function.