The Mechanism of Ethylene Perception

Almost one century ago, Anton Nicolyovitch Neljubov determined that the gaseous olefin ethylene was capable of altering the development of higher plants in a variety of ways. In the ensuing decades, the involvement of endogenously produced ethylene in plant growth and development was firmly established (reviewed by Abeles et al., 1992). Ethylene has been implicated as a factor that controls the timing of seed germination, the rate and dimensions of etiolated seedling growth and leaf expansion, the initiation and progression of abscission and fruit ripening, and the expression of a number of stress-related responses in plants (Abeles et al., 1992). More recently, the biochemical and molecular characterization of the biosynthetic pathway for ethylene has provided insights into the mechanisms by which plants control the interna1 concentrations of this hormone (reviewed by Kende, 1993). Understanding the mechanisms by which plant cells perceive and transduce the ethylene signal has been a more daunting problem. Physiochemical considerations have prompted a number of researchers to postulate that ethylene might interact with a receptor through a protein-bound transition metal such as Cu(1) (Burg and Burg, 1967; Kovacic et al., 1991; Sisler, 1991). High-affinity, saturable binding sites for ethylene have been detected in plant tissues and extracts (reviewed by Sisler, 1991). However, the physiological relevance of these sites with respect to ethylene responses has never been demonstrated, nor have the biochemical agents responsible for binding been isolated and characterized from plants. Recent advances in understanding ethylene signal transduction have come from pursuing a genetic approach with Arabidopsis thaliana. A number of mutants affecting ethylene responses in Arabidopsis have been identified (reviewed by Ecker, 1995), and these define a common initial pathway for all recognized ethylene-induced changes in the plant. In this Update, we will describe the initial pathway for ethylene signal transduction as it is currently understood and emphasize, in particular, evidence that the ETRZ gene codes for an ethylene receptor. We will also