The plant hormone ethylene regulates a variety of processes of growth and development. To identify components in the ethylene signal transduction pathway, we screened for ethylene-insensitive mutants in Arabidopsis thaliana and isolated a dominant etr2-1 mutant. The etr2-1 mutation confers ethylene insensitivity in several processes, including etiolated seedling elongation, leaf expansion, and leaf senescence. Double mutant analysis indicates that ETR2 acts upstream of CTR1, which codes for a Raf-related protein kinase. We cloned the ETR2 gene on the basis of its map position, and we found that it exhibits sequence homology to the ethylene receptor gene ETR1 and the ETR1-like ERS gene. ETR2 may thus encode a third ethylene receptor in Arabidopsis, transducing the hormonal signal through its ''twocomponent'' structure. Expression studies show that ETR2 is ubiquitously expressed and has a higher expression in some tissues, including inf lorescence and f loral meristems, petals, and ovules.

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...

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.

An Arabidopsis transgenic line was constructed expressing beta-glucuronidase (GUS) via the auxin-responsive domains (AuxRDs) A and B (BA-GUS) of the PS-IAA4/5 gene in an indoleacetic acid (IAA)-dependent fashion. GUS expression was preferentially enhanced in the root elongation zone after treatment of young seedlings with 10(-7) M IAA. Expression of the BA-GUS gene in the axr1, axr4, and aux1 mutants required 10- to 100-fold higher auxin concentration than that in the wild-type background. GUS expression was nil in the axr 2 and axr 3 mutants. The transgene was used to isolate mutants exhibiting altered auxin-responsive gene expression (age). Two mutants, age1 and age2, were isolated and characterized. age1 showed enhanced sensitivity to IAA, with strong GUS expression localized in the root elongation zone in the presence of 10(-8) M IAA. In contrast, age2 exhibited ectopic GUS expression associated with the root vascular tissue, even in the absence of exogenous IAA. Morphological and molecular analyses indicated that the age1 and age2 alleles are involved in the regulation of gene expression in response to IAA.