植物學(xué)及園藝學(xué)英文版_botany_and_horticulture_(28)-全文預(yù)覽
【正文】 properties to the gas ethylene. ETR1 was found to act as a dimer that localizes to a cellular membrane system. ● A series of elegant studies performed in the niies revealed that ethylene receptors are encoded by a small gene family that in Arabidopsis consists of five members: ETR1, ethylene response2 (etr2), ethylene insensitive4 (ein4), ethylene resistant1 (ers1), and ethylene resistant2 (ers2). ● Their sequence similarity and structural anization, the five receptors are categorized into two subfamilies. Subfamily I members (ETR1 and ERS1) harbor three hydrophobic transmembrane domains in the aminoterminus followed by a conserved histidine kinase domain. Subfamily II members (ETR2, ERS2, and EIN4) possess four predicted aminoterminal hydrophobic transmembrane regions followed by a less conserved kinase domain that lacks several of the canonical features required for histidinekinase activity. Furthermore, three of the five receptors, ETR1, ETR2, and EIN4, also possess a carboxylterminal receiver domain. ● A major breakthrough came with the isolation of lossoffunction (LOF) alleles of the receptors. Each of the single LOF alleles was still able to respond to ethylene, indicating a high degree of functional redundancy among the receptors. Furthermore, triple and quadruple LOF mutants displayed a constitutive ethylene response in the absence of the hormone. ● Both receptor subfamilies appear to be able to sense ethylene, as double etr1。ers1 LOF mutants displayed severe phenotypes, including miniature rosettes, fertility defects, and altered flower morphology. All of these effects were dependent on a functional ethylene signaling pathway, implying that the observed growth defects arose from a misregulation of ethylene responses. Overexpression of the subfamily II members was unable to rescue the observed phenotypes, whereas ectopic expression of either wildtype ETR1 or ERS1 restored normal development, further supporting the notion of a unique role for the subfamily I receptors. ● Ethylene was found to bind to the receptors through a transition metal cofactor, copper. ● Moreover, the copper cofactor was shown to be essential for ethylene binding, and thus, proper receptor function. The current structural model for the ethylene binding domain suggests that the copper(I) cofactor is located in the electronrich hydrophobic pocket formed by the Nterminal transmembrane domains of the receptors. In particular, residues Cys65 and His69 are thought to play a fundamental role in this proteinmetalhormone interaction. ● In planta, the relevance of this interaction was further confirmed with the identification of RESPONSIVE TO ANTAGONIST1 (RA\N1). RAN1 was isolated using a screening for mutants with altered specificity in hormone binding by employing the ethylene antagonist transcyclooctene (TCO). ran1 plants are defective in a copper transporter similar to Ptype ATPases. ● The lack of the metal cofactor impairs receptor function in ran1 mutants by causing altered ligand specificity and thus rendering the plants responsive to the antagonist TCO. Furthermore, a strong LOF ran1 allele results in a constitutive ethylene response phenotype in the absence of the gaseous hormone. ● Autophosphorylation activity has been demonstrated for ETR1 and all other members of the receptor family. However, while ETR1 autophosphorylates in the predicted conserved histidine residue, ERS1 and all of the subfamily II members display a serinekinase activity in vitro. ERS1 also possesses a histidinekinase activity, whereas its serine autophosphorylation is thought not to be significant in vivo. ● Another interesting study addressed the roles of the kinase activity and of the carboxylterminal receiver domain of ETR1, ETR2, and EIN4 in ethylene signaling by looking at the effect of expressing truncated versions of ETR1 in a triple LOF etr1。eil1 double mutants display almost plete insensitivity to the ethylene gas. In addition, regulation of EIN3 by the SCF/26S proteasome pathway serves as an efficient mechanism for the prompt and finetuned control of this transcriptional cascade. ● In order to gain a better understanding of the ethylene effects in transcription, several research groups have performed genomescale studies using microarrays and other highthroughput techniques. ● Early studies relied on cDNA microarrays covering only a fraction of the genome, and thus Schenk and coworkers analyzed the expression of
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