Search

The CONSTITUTIVE PHOTOMORPHOGENIC9 (COP9) complex is a nuclear localized, multisubunit protein complex essential for repression of light-mediated development in Arabidopsis. Mutations that abolish the complex result in constitutive photomorphogenic development in darkness and pleiotropic developmental defects in both light and darkness. Here, we report the identification of two apparently redundant genes, AJH1 and AJH2, that encode a subunit of the COP9 complex. Both AJH1 and AJH2 share high amino acid sequence identity (62 and 63%, respectively) with JAB1, a specific mammalian coactivator of AP-1 transcription. The proteins encoded by these two genes are present in both complex and monomeric forms, whereas complex formation is in part mediated by the direct interaction with FUSCA6. In addition, the stability of the monomeric AJH proteins requires functional COP1 and DEETIOLATED1 loci. Together with the fact that the previously known subunit FUSCA6 is an Arabidopsis homolog of human GPS1, a negative regulator of AP-1 transcription, our data suggest that the COP9 complex may contain both negative and positive regulators of transcription. Therefore, the COP9 complex may achieve its pleiotropic effects on Arabidopsis development by modulating activities of transcription factors in response to environmental stimuli.

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.

Alterations in the response of dark-grown seedlings to ethylene (the "triple response") were used to isolate a collection of ethylene-related mutants in Arabidopsis thaliana. Mutants displaying a constitutive response (eto1) were found to produce at least 40 times more ethylene than the wild type. The morphological defects in etiolated eto1-1 seedlings reverted to wild type under conditions in which ethylene biosynthesis or ethylene action were inhibited. Mutants that failed to display the apical hook in the absence of ethylene (his1) exhibited reduced ethylene production. In the presence of exogenous ethylene, hypocotyl and root of etiolated his1-1 seedlings were inhibited in elongation but no apical hook was observed. Mutants that were insensitive to ethylene (ein1 and ein2) produced increased amounts of ethylene, displayed hormone insensitivity in both hypocotyl and root responses, and showed an apical hook. Each of the "triple response" mutants has an effect on the shape of the seedling and on the production of the hormone. These mutants should prove to be useful tools for dissecting the mode of ethylene action in plants.