Elucidating the mechanisms involved in ripening of climacteric fruit and the role that ethylene plays in the process are key to understanding fruit production and quality. In this review, which is based largely on research in tomato, particular attention is paid to the role of specific isoforms of ACC synthase and ACC oxidase in controlling ethylene synthesis during the initiation and subsequent autocatalytic phase of ethylene production during ripening. Recent information on the structure and role of six different putative ethylene receptors in tomato is discussed, including evidence supporting the receptor inhibition model for ripening, possible differences in histidine kinase activity between receptors, and the importance of receptor LeETR4 in ripening. A number of ethylene-regulated ripening-related genes are discussed, including those involved in ethylene synthesis, fruit texture, and aroma volatile production, as well as experiments designed to elucidate the ethylene signalling pathway from receptor through intermediate components similar to those found in Arabidopsis, leading to transcription factors predicted to control the expression of ethylene-regulated genes.

Ethylene regulates many aspects of the plant life cycle, including seed germination, root initiation, flower development, fruit ripening, senescence, and responses to biotic and abiotic stresses. It thus plays a key role in responses to the environment that have a direct bearing on a plant's fitness for adaptation and reproduction. In recent years, there have been major advances in our understanding of the molecular mechanisms regulating ethylene synthesis and action. Screening for mutants of the triple response phenotype of etiolated Arabidopsis seedlings, together with map-based cloning and candidate gene characterization of natural mutants from other plant species, has led to the identification of many new genes for ethylene biosynthesis, signal transduction, and response pathways. The simple chemical nature of ethylene contrasts with its regulatory complexity. This is illustrated by the multiplicity of genes encoding the key ethylene biosynthesis enzymes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase, multiple ethylene receptors and signal transduction components, and the complexity of regulatory steps involving signalling relays and control of mRNA and protein synthesis and turnover. In addition, there are extensive interactions with other hormones. This review integrates knowledge from the model plant Arabidopsis and other plant species and focuses on key aspects of recent research on regulatory networks controlling ethylene synthesis and its role in flower development and fruit ripening.

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The hydra mutants of Arabidopsis are characterized by a pleiotropic phenotype that shows defective embryonic and seedling cell patterning, morphogenesis, and root growth. We demonstrate that the HYDRA1 gene encodes a ⌬ 8-⌬ 7 sterol isomerase, whereas HYDRA2 encodes a sterol C14 reductase, previously identified as the FACKEL gene product. Seedlings mutant for each gene are similarly defective in the concentrations of the three major Arabidopsis sterols. Promoter::reporter gene analysis showed misexpression of the auxin-regulated DR5 and ACS1 promoters and of the epidermal cell file-specific GL2 promoter in the mutants. The mutants exhibit enhanced responses to auxin. The phenotypes can be rescued partially by inhibition of auxin and ethylene signaling but not by exogenous sterols or brassinosteroids. We propose a model in which correct sterol profiles are required for regulated auxin and ethylene signaling through effects on membrane function. INTRODUCTIONSterols are essential components of fungal, plant, and animal membranes. They regulate fluidity and interact with lipids and proteins within the membrane, and they are the precursors for the brassinosteroid (BR) hormones in plants (Hartmann, 1998). The sterol biosynthetic pathway in plants, therefore, can be viewed as comprising two parts: one branch produces the bulk membrane sterols (the principal sterols in Arabidopsis being stigmasterol, campesterol, and sitosterol), and the second part represents the BR synthesis branch. Sterol biosynthesis has been well characterized in yeast, supported by a powerful system of genetic analysis. In animals, and more recently in plants, sterol biosynthetic enzyme function has been confirmed via the functional complementation of yeast mutants (Gachotte et al., 1996). Functional analysis of sterol function in plants has involved a range of approaches, but recently, genetic studies have provided useful information on the requirement for particular enzymes in sterol and BR biosynthesis and, for BRs, perception and signal transduction (Clouse, 2000;Diener et al., 2000;Schaeffer et al., 2001).In animals, sterols appear to be important to maintain correct cell-signaling activities. For example, drugs such as the ligand SR31747A, which inhibits the activity of the receptor (emopamil binding protein [EBP], which has ⌬ 8-⌬ 7 sterol isomerase activity), cause defects in a diversity of cellular processes, including the inhibition of mammalian lymphocyte proliferation in response to mitogens (Derocq et al., 1995) and the inhibition of graft rejection in mouse via the modulation of gene expression (Carayon et al., 1995), and they may influence lipoprotein functions leading to immunosuppressive effects (Dussossoy et al., 1999). In plants, a lack of detailed pharmacological studies has precluded analogous investigations of the role of sterols in plant cell biology.However, mutational and transgenic studies have given new insight into the roles of sterols in plant development. sterol methyltransferase1 ( smt1 ) mutants accumulate cholesterol...

Controlling the rate of softening to extend shelf life was a key target for researchers engineering genetically modified (GM) tomatoes in the 1990s, but only modest improvements were achieved. Hybrids grown nowadays contain 'non-ripening mutations' that slow ripening and improve shelf life, but adversely affect flavor and color. We report substantial, targeted control of tomato softening, without affecting other aspects of ripening, by silencing a gene encoding a pectate lyase.