The ethylene signaling pathway: new insights

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Abstract

During the past decade, molecular genetic studies on the reference plant Arabidopsis have established a largely linear signal transduction pathway for the response to ethylene gas. The biochemical modes of action of many of the signaling components are still unresolved. During the past year, however, progress in several areas has been made on several fronts. The different approaches used have included a functional study of the activity of the receptor His kinase, the determination of the ethylene receptor signaling complex at the endoplasmic reticulum and of the regulation of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) activity by these receptors, the identification of a unique MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) cascade, the cloning and characterization of numerous ETHYLENE INSENSITIVE3 (EIN3)/EIN3-like (EIL) transcription factors from many plant species, and the integration of the ethylene and jasmonate response pathways via the ETHYLENE RESPONSE FACTOR (ERF) family of transcription factors. The elucidation of the biochemical mechanisms of ethylene signal transduction and the identification of new components in the ethylene response pathway in Arabidopsis are providing a framework for understanding how all plants sense and respond to ethylene.

Introduction

Ethylene is a gaseous plant hormone that affects myriad developmental processes and fitness responses, including germination, flower and leaf senescence, fruit ripening, leaf abscission, root nodulation, programmed cell death, and responsiveness to stress and pathogen attack 1., 2.. A well-known effect of ethylene on plant growth is the so-called ‘triple response’ of etiolated dicotyledonous seedlings. This response is characterized by the inhibition of hypocotyl and root cell elongation, radial swelling of the hypocotyl, and exaggerated curvature of the apical hook. This highly specific ethylene response occurs at an early developmental stage (3 days post-germination), permitting large mutant populations of seedlings to be screened rapidly for ethylene response defects. Over the past decade, genetic screens that are based on the triple-response phenotype have been extensively conducted on Arabidopsis by many laboratories. More than a dozen unique mutants have been identified and these can be divided into three distinct categories: constitutive triple-response mutants (i.e. ethylene overproduction1 [eto1], eto2, eto3, constitutive triple response1 (ctr1) and responsive to antagonist1 [ran1]/ctr2); ethylene-insensitive mutants (i.e. ethylene receptor1 [etr1], etr2, ethylene insensitive2 [ein2], ein3, ein4, ein5, and ein6); and tissue-specific ethylene-insensitive mutants (i.e. hookless1 [hls1], ethylene insensitive root1 [eir1], and several auxin-resistant mutants) 1., 2., 3..

A combination of genetic and molecular analyses of these mutants has defined a largely linear ethylene response pathway leading from hormone perception at the membrane to transcriptional regulation in the nucleus. Briefly, ethylene is perceived by a family of membrane-associated receptors, including ETR1/ETR2, ETHYLENE RESPONSE SENSOR1 (ERS1)/ERS2 and EIN4 in Arabidopsis 4., 5., 6., 7.. Ethylene binds to its receptors via a copper co-factor, which is probably delivered by the copper transporter RAN1. Genetic studies predict that hormone binding results in the inactivation of receptor function [8]. In the absence of ethylene, therefore, the receptors are hypothesized to be in a functionally active form that constitutively activates a Raf-like serine/threonine (Ser/Thr) kinase, CTR1, which is also a negative regulator of the pathway [9]. EIN2, EIN3, EIN5, and EIN6 are positive regulators of ethylene responses, acting downstream of CTR1. EIN2 is an integral membrane protein whose function is not understood [10]. EIN5 and EIN6 have not yet been characterized at the molecular level. The nuclear protein EIN3 is a transcription factor that regulates the expression of its immediate target genes such as ETHYLENE RESPONSE FACTOR1 (ERF1) 11., 12.. ERF1 belongs to a large family of APETALA2-domain-containing transcription factors that bind to a GCC-box present in the promoters of many ethylene-inducible, defense-related genes [13]. Thus, a transcriptional cascade that is mediated by EIN3/EIN3-like (EIL) and ERF proteins leads to the regulation of ethylene-controlled gene expression. Several comprehensive reviews on the ethylene signaling pathway have been published recently 14., 15., 16.. Our intent in this review is not to redundantly describe aspects of ethylene biology that have been covered previously, but rather to discuss the new discoveries on ethylene signaling mechanisms that have emerged during the past two years.

Section snippets

Ethylene perception: the role of receptor kinase activity?

In Arabidopsis, ethylene is perceived by a family of five receptors (ETR1, ETR2, ERS1, ERS2 and EIN4) that share similarity with bacterial two-component regulators [17]. On the basis of structural similarities, the receptor family can be divided into two subfamilies. Members of the type-I subfamily, which include ETR1 and ERS1, contain an amino-terminal ethylene-binding domain (also called the sensor domain) and a well-conserved carboxy-terminal histidine (His) kinase domain. The type-II

Early signaling events: a receptor–CTR1 complex at the ER

The recent finding that ethylene perception and signaling occur at the endoplasmic reticulum (ER) is a major breakthrough in understanding the mechanism of ethylene signaling 29.••, 30.••. Previous studies suggested that the ethylene receptors may be localized at the plasma membrane. Recently, however, Arabidopsis ETR1 has been shown convincingly, on the bases of both sucrose density-gradient fractionation and immunogold electron microscopy, to associate with the ER [29••]. The amino-terminal

Signal transduction: a unique MAP kinase cascade uncovered?

Although CTR1 was shown to encode a protein with highest homology to Raf-like MAPKKKs a decade ago [9], its kinase activity has not been demonstrated until very recently [32••]. As predicted from sequence, the CTR1 protein expressed by Escherichia coli behaves as a Ser/Thr kinase and can phosphorylate myelin basic protein, an artificial substrate for numerous MAPKs. Ever since CTR1 was identified as a MAPKKK-like protein, it has been proposed that the ethylene signaling pathway includes a MAPK

Primary transcription factors: the action of EIN3/EIL proteins

Differential gene expression has been reported in many ethylene responses. The molecular characterization of EIN3 has provided direct evidence of ethylene-controlled transcriptional regulation [11]. In Arabidopsis, there are six members of the EIN3 family, in which EIN3 and EIL1 are the most closely related proteins [40••]. LOF mutations in the EIL1 gene have recently been isolated [40••]. Like ein3 mutants, eil1 plants show incomplete ethylene insensitivity, albeit eil1 plants are more

Secondary transcriptional regulation: integration of ethylene responses with other signals

The current information points towards a model in which the components of the primary ethylene signaling pathway, from the receptors to EIN3/EIL, are common to all ethylene responses (Figure 2). However, exposure to exogenous ethylene or increases in the synthesis of endogenous ethylene do not always induce the same ethylene responses in different tissue types or at different developmental stages. It could be that ethylene-independent signals are integrated with the primary ethylene signaling

Conclusions

Significant progress toward the delineation of ethylene action in plants has been made during the past decade using a combination of genetic and molecular-biology approaches. This work has made the ethylene pathway one of the most well-defined signaling pathways in plants. However, the detailed biochemical mechanisms of action of each of the known components of the ethylene signaling pathway are just beginning to be understood. Despite our current understanding of the mechanisms of ethylene

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • of special interest

  • ••

    of outstanding interest

Acknowledgements

The authors would like to thank Kelli Wise for help with the preparation of the manuscript. We apologize to those ethylene biologists whose research we were unable to discuss because of space limitations. Research in the Ecker laboratory is supported by grants from the National Science Foundation and the US Department of Energy. HG is supported by a postdoctoral fellowship from the National Institutes of Health.

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