Accumulating investigations reveal that ethylene signaling is involved in the salt response in Arabidopsis (Arabidopsis thaliana), and it has been reported that overexpression of a number of ethylene response factor (ERF) genes enhances salt tolerance; however, transcriptional regulation of the ethylene signal component ETHYLENE INSENSITIVE3 (EIN3) in the salt response has not been clearly defined. Consulting microarray data and transcriptional confirmations showed that three of the ERF genes were ethylene and salt inducible, named ESE1 to ESE3. Additionally, the expression of one of the ESE genes (ESE1) was suppressed in ein2, ein3-1, eil1-3, and ein3 eil1 but enhanced in EIN3-overexpressing (EIN3ox) lines. Inhibitors of ethylene biosynthesis, aminoethoxyvinylglycine, and ethylene action, AgNO 3 , reduced the expression of ESE1, while ethylene overproduction eto mutants enhanced the expression of ESE1, indicating that ESE1 is an ethylene-modulated gene downstream of EIN3/EIL1. Further analyses with biochemical and molecular approaches revealed that EIN3 physically binds to the ESE1 promoter, demonstrating that ESE1 was one target of EIN3. ESE1 in turn binds to promoters of salt-related genes, such as RD29A and COR15A. Moreover, either EIN3ox or ESE1ox was sufficient to enhance transcript levels of salt-related genes and salt tolerance. In addition, ESE1ox in ein3 enhanced the salt response during seed germination and seedling development, demonstrating that ESE1 is genetically downstream of EIN3. Thus, the evidence in this report reveals that the transcriptional complex of EIN3-ESE1 is a crucial event in the salt response, thereby connecting the transcriptional regulation of EIN3 and the downstream ERF protein ESE1 in the salt response.
SUMMARYThe phytohormones abscisic acid (ABA) and ethylene are known to play multiple roles in plant development and stress responses. Ethylene biosynthesis is affected by several factors including drought, cold and the phytohormone auxin, although the role of ABA is unclear. In this work ABA-responsive mutants were screened and a bZIP transcription factor HY5 was identified as a negative regulator of ethylene biosynthesis via modulation of the expression of the ethylene biosynthesis genes ACS2 and ACS5. Members of the ethylene response factor (ERF) family of transcriptional repressors in Arabidopsis have been shown to modulate ABA responses and three ERF members were found to carry putative HY5-binding cis-acting elements. Analyses with biochemical and molecular approaches revealed that HY5 specifically binds to the G-box region of the AtERF11 promoter to activate its transcription. We further demonstrate that AtERF11, which contains a repressor motif at its C-terminal, interacts with the dehydration-responsive element in the ACS2/5 promoters, to repress its expression, resulting in decreased ethylene biosynthesis. Moreover, an AtERF11 knockout mutant showed increased levels of ACS2/5 expression and ethylene emission, while treatment with ABA greatly suppressed ACS5 transcripts but not ACS2 expression and the ethylene content, indicating that AtERF11 is a key negative regulator for ABA-mediated control of ethylene synthesis. In addition, in ethylene over-producer mutants, ABA treatment was shown to suppress ACS5 transcripts and ethylene content, thereby affecting growth and development. Based on these data, in this research we present a model suggesting that the HY5-AtERF11 regulon is a key factor modulating ABA-regulated ethylene biosynthesis.
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