Ethylene Biosynthesis and Signaling Networks

Abstract

Despite its simple two-carbon structure, the olefin ethylene is a potent modulator of plant growth and development (Ecker, 1995). The plant hormone ethylene is involved in many aspects of the plant life cycle, including seed germination, root hair development, root nodulation, flower senescence, abscission, and fruit ripening (reviewed in Johnson and Ecker, 1998). The production of ethylene is tightly regulated by internal signals during development and in response to environmental stimuli from biotic (e.g., pathogen attack) and abiotic stresses, such as wounding, hypoxia, ozone, chilling, or freezing. To understand the roles of ethylene in plant functions, it is important to know how this gaseous hormone is synthesized, how its production is regulated, and how the signal is transduced. Morphological changes in dark-grown (etiolated) seedlings treated with ethylene or its metabolic precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), have been termed the triple response. The exaggerated curvature of the apical hook, radial swelling of the hypocotyl, and shortening of the hypocotyl and root are the unmistakable hallmarks of this ethylene response. Over the past decade, the triple response phenotype has been used to screen for mutants that are defective in ethylene responses (Bleecker et al., 1988; Guzman and Ecker, 1990). Etiolated Arabidopsis seedlings with minor or no phenotypic response upon ethylene application are termed ethylene-insensitive (ein) or ethylene-resistant (etr) mutants. Mutants have also been identified that display a constitutive triple response in the absence of ethylene (Kieber et al., 1993; Roman and Ecker, 1995). This class can be divided into subgroups based on whether or not the constitutive triple response can be suppressed by inhibitors of ethylene perception and biosynthesis, such as silver thiosulfate and aminoethoxyvinyl glycine (AVG). Mutants that are unaffected by these inhibitors are termed constitutive triple-response (ctr) mutants, whereas mutants whose phenotype reverts to normal morphology are termed ethylene-overproducer (eto) mutants, which are defective in the regulation of hormone biosynthesis. The genetic hierarchy among ethylene biosynthesis and signaling pathway components in Arabidopsis has been established by epistasis analysis using these mutants (Solano and Ecker, 1998; Stepanova and Ecker, 2000). The intent of this review is not to cover all aspects of ethylene biology but to focus on recent findings. In particular, we examine interaction of ethylene and two other plant growth regulators, jasmonic acid (JA) and salicyclic acid (SA), and their roles in mediating responses to biotic and abiotic stresses. We begin by summarizing what is currently known about the mechanism and regulation of ethylene biosynthesis and by providing an update of our current understanding of the ethylene signaling pathway. Biosynthetic Pathway and Regulation of Ethylene. The formation of S-AdoMet is catalyzed by SAM synthetase from the methionine at the expense of one molecule of ATP per molecule of S-AdoMet synthesized. S-AdoMet is the methyl group donor for many cellular molecules (Methylated Acceptors), including nucleic acids, proteins, and lipids. In addition, S-AdoMet is the precursor of the polyamine synthesis pathway (Spermidine/Spermine biosynthesis pathway). ACC is the immediate precursor of ethylene. The rate-limiting step of ethylene synthesis is the conversion of S-AdoMet to ACC by ACC synthase under most conditions. MTA is the by-product generated along with ACC production by ACC synthase. Recycling of MTA back to methionine conserves the methylthio group and is able to maintain a constant concentration of cellular methionine even when ethylene is rapidly synthesized. Malonylation of ACC to malonyl-ACC (MACC) deprives the ACC pool and reduces the ethylene production. ACC oxidase catalyses the final step of ethylene synthesis using ACC as substrate and generates carbon dioxide and cyanide. Transcriptional regulation of both ACC synthase and ACC oxidase is indicated by dashed of ACC synthase is and be by and the by stresses. and of ACC synthase are the ACC synthase be or in is with ACC synthase at the and be from the it is by at the S-AdoMet is the precursor for ethylene biosynthesis (reviewed in and In to an of of cellular methionine is to S-AdoMet by S-AdoMet synthetase at the expense of ATP et al., 1998). S-AdoMet is the methyl donor in and is used as a substrate for many including and ethylene biosynthesis et al., 1998). In addition, S-AdoMet is involved in that proteins, and nucleic the of the cycle, the step of ethylene biosynthesis is the conversion of S-AdoMet to ACC by ACC synthase (reviewed in and In to ACC synthase also in this which is to methionine by using a methionine (reviewed in and 2000). This pathway the methyl group for of ethylene production. ethylene can be an pool of the the group of the methionine is also ACC is by ACC oxidase to and which is to