Ethylene — Biosynthesis and perception

Abstract Ethylene is a plant hormone influencing plant processes such as seed germination, diageotropism, flowering, abscission, senescence, fruit ripening, and pathogenesis responses. Its biosynthesis involves multistep enzymatic pathway converting methionine to ethylene. Conjugated forms of ACC, l‐(malonylamino)cyclopropane‐1‐carboxylic acid and l‐(γ‐1‐glutamylamino)cyclopropane‐1‐carboxylic acid, and the enzymes catalyzing these reactions have also been demonstrated. The different ethylene biosynthesis enzymes are encoded each by a family of genes, which raises questions about their functional significance. In addition to transcriptional control of gene expression, posttranscriptional regulation seems to play an important role. Mutants of Arabidopsis and tomato have been used to dissect the ethylene signal transduction pathway. Analyses of ethylene‐insensitive mutants in Arabidopsis and tomato have identified ETR1 gene or its homologues, which codes for a protein similar 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 types, which overproduce ethylene in the seedling stage, and ctr types, which resemble wild‐type seedlings grown in ethylene. The CTR1 gene‐product was shown to encode a Raf‐like kinase involved in the signal transduction pathway. Thus, molecular characterization of the components involved in ethylene response and the delineation of a genetic structure, from ethylene biosynthesis through ethylene detection and gene activation, have enabled the first and most advanced functional insight into plant transduction of external events.