RECESSIVE AND DOMINANT MUTATIONS IN THE ETHYLENE BIOSYNTHETIC GENE ACS5 OF ARABIDOPSIS CONFER CYTOKININ INSENSITIVITY AND ETHYLENE OVERPRODUCTION, RESPECTIVELY

We identified a set of cytokinin-insensitive mutants by using a screen based on the ethylene-mediated triple response observed after treatme nt with low levels of cytokinins. One group of these mutants disrupts ACS5, a member of the Arabidopsis gene family that encodes 1-amino-cyc lopropane-1-carboxylate synthase, the first enzyme in ethylene biosynt hesis. The ACS5 isoform is mainly responsible for the sustained rise i n ethylene biosynthesis observed in response to low levels of cytokini n and appears to be regulated primarily by a posttranscriptional mecha nism, Furthermore, the dominant ethylene-overproducing mutant eto2 was found to be the result of an alteration of the carboxy terminus of AC S5, suggesting that this domain acts as a negative regulator of ACS5 f unction.