Ethylene signaling in Arabidopsis: Events from the membrane to the nucleus

Ethylene is a plant hormone that regulates a diverse set of developmental and physiological processes including seed germination, abscission, and senescence. Molecular genetic analysis of ethylene signaling in Arabidopsis thaliana, has led to the identification of a number of genetic loci that are required for normal ethylene responses. Cloning of several of the mutant loci has provided significant insight into the nature of the ethylene signaling pathway. The Raf kinase homolog CTR1 is predicted to negatively regulate the ethylene response pathway through a MAP kinase cascade. ETR1 has been demonstrated to bind ethylene, and a family of homologous genes encode products that may function in a receptor complex. EIN3 and three homologous gene products contain several protein motifs that are reminiscent of transcription factors, and transduce the ethylene signal downstream. Additional loci are defined by the ein2, ein5/ain1, ein6, and ein7 mutations, and a number of mutations defining tissue-specific mediators of ethylene responses have also been identified. In this review, the different ethylene-related mutations and their gene products are considered in detail, and possible models of the manner in which the pathway functions are proposed.