The eto1, eto2, and eto3 mutations and cytokinin treatment increase ethylene biosynthesis in Arabidopsis by increasing the stability of ACS protein

The Arabidopsis ethylene-overproducing mutants eto1, eto2, and eto3 have been suggested to affect the post-transcriptional regulation of 1-aminocyclopropane-1-carboxylic acid synthase (ACS). Here, we present the positional cloning of the gene corresponding to the dominant eto3 mutation and show that the eto3 phenotype is the result of a missense mutation within the C-terminal domain of ACS9, which encodes one isoform of the Arabidopsis ACS gene family. This mutation is analogous to the dominant eto2 mutation that affects the C-terminal domain of the highly similar ACS5. Analysis of purified recombinant ACS5 and epitope-tagged ACS5 in transgenic Arabidopsis revealed that eto2 does not increase the specific activity of the enzyme either in vitro or in vivo; rather, it increases the half-life of the protein. In a similar manner, cytokinin treatment increased the stability of ACS5 by a mechanism that is at least partially independent of the eto2 mutation. The eto1 mutation was found to act by increasing the function of ACS5 by stabilizing this protein. These results suggest that an important mechanism by which ethylene biosynthesis is controlled is the regulation of the stability of ACS, mediated at least in part through the C-terminal domain.