Ethylene regulates plant growth, development, and responsiveness to a variety of stresses. Cloning of the Arabidopsis EIN2 gene identifies a central component of the ethylene signaling pathway. The amino-terminal integral membrane domain of EIN2 shows similarity to the disease-related Nramp family of metal-ion transporters. Expression of the EIN2 CEND is sufficient to constitutively activate ethylene responses and restores responsiveness to jasmonic acid and paraquat-induced oxygen radicals to mutant plants. EIN2 is thus recognized as a molecular link between previously distinct hormone response pathways. Plants may use a combinatorial mechanism for assessing various stresses by enlisting a common set of signaling molecules.
Ethylene is an important regulator of plant growth. We identified an Arabidopsis mutant, responsive-to-antagonist1 (ran1), that shows ethylene phenotypes in response to treatment with trans-cyclooctene, a potent receptor antagonist. Genetic epistasis studies revealed an early requirement for RAN1 in the ethylene pathway. RAN1 was cloned and found to encode a protein with similarity to copper-transporting P-type ATPases, including the human Menkes/Wilson proteins and yeast Ccc2p. Expression of RAN1 complemented the defects of a ccc2delta mutant, demonstrating its function as a copper transporter. Transgenic CaMV 35S::RAN1 plants showed constitutive expression of ethylene responses, due to cosuppression of RAN1. These results provide an in planta demonstration that ethylene signaling requires copper and reveal that RAN1 acts by delivering copper to create functional hormone receptors.
The Arabidopsis ethylene receptor gene ETR1 and two related genes, ERS1 and ETR2 , were identified previously. These three genes encode proteins homologous to the two-component regulators that are widely used for environment sensing in bacteria. Mutations in these genes confer ethylene insensitivity to wild-type plants. Here, we identified two Arabidopsis genes, EIN4 and ERS2 , by cross-hybridizing them with ETR2. Sequence analysis showed that they are more closely related to ETR2 than they are to ETR1 or ERS1. EIN4 previously was isolated as a dominant ethyleneinsensitive mutant. ERS2 also conferred dominant ethylene insensitivity when certain mutations were introduced into it. Double mutant analysis indicated that ERS2 , similar to ETR1 , ETR2 , ERS1 , and EIN4 , acts upstream of CTR1. Therefore, EIN4 and ERS2 , along with ETR1 , ETR2 , and ERS1 , are members of the ethylene receptor-related gene family of Arabidopsis. RNA expression patterns of members of this gene family suggest that they might have distinct as well as redundant functions in ethylene perception. INTRODUCTIONThe simple gas ethylene acts as an endogenous regulator of growth and development as well as a mediator of stress responses in higher plants. It is involved in many developmental processes, including seed germination, leaf and flower senescence, and fruit ripening (Abeles, 1992). It regulates basic cellular processes such as cell elongation (Abeles, 1992) and root hair formation (Tanimoto et al., 1995). It also acts as a signal in pathogen defense, wound responses, and nodule formation (Abeles, 1992;O'Donnell et al., 1996;Penninckx et al., 1996;Penmetsa and Cook, 1997). The diverse roles that ethylene plays and the specificity of its action suggest complexity in the regulation of its synthesis and in its signal transduction pathway. This complexity has been observed in the regulation of ethylene production. The biosynthetic pathway of ethylene has been elucidated (Yang and Hoffman, 1984). The precursor S -adenosyl-L -methionine is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Ethylene is produced from ACC by the action of ACC oxidase. These two key enzymes, ACC synthase and ACC oxidase, each encoded by a multigene family, are differentially expressed in response to developmental, environmental, and hormonal factors (Kende and Zeevaart, 1997).Recently, progress has been made in understanding the signal transduction pathway of ethylene. This has been achieved largely by molecular and genetic studies that use the model plant Arabidopsis. More than a dozen ethylene response mutants have been identified by screening for alterations in the triple response (Ecker, 1995). Triple response refers to the morphological changes of etiolated seedlings in response to ethylene, including short and thick hypocotyls, short roots, and exaggerated apical hooks (Bleecker et al., 1988;Guzmán and Ecker, 1990). The response mutants can be classified into two categories: ethylene-insensitive mutants and constitutive response mutants. T...
Ethylene is a gaseous plant growth regulator that controls a multitude of developmental and stress responses. Recently, the levels of Arabidopsis EIN3 protein, a key transcription factor mediating ethylene-regulated gene expression, have been demonstrated to increase in response to the presence of ethylene gas. Furthermore, in the absence of ethylene, EIN3 is quickly degraded through a ubiquitin͞proteasome pathway mediated by two F-box proteins, EBF1 and EBF2. Here we report the identification of ETHYLENE-INSENSITIVE5 as the 533 exoribonuclease XRN4. Specifically, we demonstrate that EIN5 is a component of the ethylene signal transduction cascade acting downstream of CTR1 that is required for ethylene-mediated gene expression changes. Furthermore, we find that the ethylene insensitivity of ein5 mutant plants is a consequence of the over-accumulation of EBF1 and EBF2 mRNAs resulting in the under-accumulation of EIN3 even in the presence of ethylene gas. Together, our results suggest that the role of EIN5 in ethylene perception is to antagonize the negative feedback regulation on EIN3 by promoting EBF1 and EBF2 mRNA decay, which consequently allows the accumulation of EIN3 protein to trigger the ethylene response.Arabidopsis ͉ growth regulation ͉ signal transduction
The Arabidopsis ethylene receptor gene ETR1 and two related genes, ERS1 and ETR2, were identified previously. These three genes encode proteins homologous to the two-component regulators that are widely used for environment sensing in bacteria. Mutations in these genes confer ethylene insensitivity to wild-type plants. Here, we identified two Arabidopsis genes, EIN4 and ERS2, by cross-hybridizing them with ETR2. Sequence analysis showed that they are more closely related to ETR2 than they are to ETR1 or ERS1. EIN4 previously was isolated as a dominant ethylene-insensitive mutant. ERS2 also conferred dominant ethylene insensitivity when certain mutations were introduced into it. Double mutant analysis indicated that ERS2, similar to ETR1, ETR2, ERS1, and EIN4, acts upstream of CTR1. Therefore, EIN4 and ERS2, along with ETR1, ETR2, and ERS1, are members of the ethylene receptor-related gene family of Arabidopsis. RNA expression patterns of members of this gene family suggest that they might have distinct as well as redundant functions in ethylene perception.
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