ETHYLENE - BIOSYNTHESIS AND PERCEPTION

Ethylene is a plant hormone influencing plant processes such as seed g ermination, diageotropism, flowering, abscission, senescence, fruit ri pening, and pathogenesis responses. Its biosynthesis involves multiste p enzymatic pathway converting methionine to ethylene. Conjugated form s of ACC, 1-(malonylamino)cyclopropane-1-carboxylic acid and 1-(gamma- L-glutamylamino)cyclopropane-1-carboxylic acid, and the enzymes cataly zing these reactions have also been demonstrated. The different ethyle ne biosynthesis enzymes are encoded each by a family of genes, which r aises questions about their functional significance. In addition to tr anscriptional control of gene expression, posttranscriptional regulati on seems to play an important role. Mutants of Arabidopsis and tomato have been used to dissect the ethylene signal transduction pathway. An alyses of ethylene-insensitive mutants in Arabidopsis and tomato have identified ETR1 gene or its homologues, which codes for a protein simi lar to microbial two-component regulators. Transgenic yeast expressing the ETR1 protein binds ethylene, suggesting that ETR1 is an ethylene receptor. Constitutive response mutants of Arabidopsis include eto typ es, which overproduce ethylene in the seedling stage, and ctr types, w hich resemble wild-type seedlings grown in ethylene. The CTR1 gene-pro duct was shown to encode a Raf-like kinase involved in the signal tran sduction pathway. Thus, molecular characterization of the components i nvolved in ethylene response and the delineation of a genetic structur e, from ethylene biosynthesis through ethylene detection and gene acti vation, have enabled the first and most advanced functional insight in to plant transduction of external events.