Ethylene signalling and response pathway_ a unique signalling cascade with a multitude of inputs and

Ethylene signalling and response pathway_ a unique signalling cascade with a multitude of inputs and

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Review Ethylene signalling and response pathway: a unique signalling cascade with a multitude of inputs and outputs Anna N. Stepanova and Jose M. Alonso* North Carolina State University, Department of Genetics, PO Box 7614, Raleigh NC 27695 USA *Corresponding author, e-mail: jmalonso@unity.ncsu.edu Received 11 August 2004 Plants as immobile organisms need to constantly monitor the changes in the environment to modify and adjust developmental and metabolic pathways accordingly. The responses to these environmental cues require an integrative mechanism where external and internal signals are detected and processed to trigger an appropriate ‘reaction’ in the plant. Hormones play a key role in mediating some of these integrative processes and in generating the response reactions. The identification and char- acterization of the basic hormone signalling components and their interactions represent the first step towards comprehensive understanding of plant responses to intrinsic and extrinsic cues. A relatively well-characterized ethylene signalling and response pathway, together with numerous evidences of its interactions with other signalling/response pathways, provide an excellent example to illustrate our current knowledge and perspective on how signal integration occurs in plants. Introduction Ethylene is the simplest of the plant hormones. Despite this structural simplicity, it participates in the regulation of a large variety of developmental processes, from seed germination to cell elongation, fruit ripening, organ senescence and abscission (Abeles et al. 1992, Johnson and Ecker 1998). Most of these ethylene effects were first defined by classical physio- logical experiments in which the effects of exogenous hormone were studied. With the implementation of molecular genetic techniques and the adoption of a manageable plant genetic system, it became possible not only to test the role of endogenous ethylene in the regulation of all these processes, but also to start uncovering the molecular mechanisms involved in sensing and reacting to this hormone (Ecker 1995). More recently, the global analysis of ethylene-mediated changes in gene expression has uncovered hundreds of ethylene-regulated genes (Alonso et al. 2003a, Zhong and Burns 2003). These changes at the transcriptional level are the core components of the ethylene response and provide some of the basis to start dissecting the myriad of ethylene-mediated responses in plants. In spite of the large body of information about ethy- lene signalling and response, our current understanding of how this hormone mediates such a wide range of effects in diverse processes like differential cell growth in the apical hook or cell death in petal abscission zones (Johnson and Ecker 1998) is very limited. The advance in this area will require the extensive study of interactions between the ethylene signalling and response pathway with other hormonal, developmental, and environmental response mechanisms. In this review, we have compiled the latest information
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Ethylene signalling and response pathway_ a unique signalling cascade with a multitude of inputs and

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