Characterization of ETHYLENE INSENSITIVE SIX and the ENHANCER OF ETHYLENE INSENSITIVE in Arabidopsis thaliana

Ethylene is a gaseous plant hormone that affects diverse physiological processes throughout plant growth and development. These processes include seed germination, cell elongation, flower and leaf senescence, abscission, sex determination and fruit ripening. Its biosynthesis is mediated by the plants responses to both biotic and abiotic stresses such as pathogen attack, wounding, hypoxia, ozone, chilling and freezing. Molecular genetic studies in Arabidopsis thaliana have exploited the triple response to ethylene, and resulted in the identification of mutants defective in this response. After nearly two decades of work in Arabidopsis, this model for ethylene signaling has evolved into one of the best characterized signal transduction pathways of all plant hormones. Despite the breadth of knowledge in this field it is still unknown how many aspects of this pathway function. Additionally new components related to ethylene signaling continue to be discovered. In this work we describe two novel ethylene mutants that act together to cause an ethylene insensitive seedling phenotype. ETHYLENE INSENSITIVE6 (EIN6) when mutated causes an ethylene insensitive root phenotype in Arabidopsis etiolated seedlings. The protein encoded by this gene shares homology with Jumonji domain containing proteins that have recently been shown to act as histone demethylases in other eukaryotic organisms. ein6 mutants also display a pleiotropic range of microtubule related phenotypes not previously documented for other ethylene insensitive mutants. Mutants in the gene ENHANCER OF ETHYLENE INSENSITIVITY (EEN) do not demonstrate an obvious phenotype on their own, but enhance the ethylene insensitive root phenotype of ein6 plants to a full ethylene insensitive etiolated seedling phenotype. This is the first enhancer mutation found in the ethylene signal transduction pathway. This study sheds light on two uncharacterized components of the ethylene signal transduction pathway that may be associated with epigenetic control of transcriptional regulation by modification of core histone