Characterization of EER4 and SAR1 in Relation to Their Role in Ethylene Signaling and Dampening Responses
Ethylene is a plant hormone important for many agriculturally significant developmental processes such as fruit ripening. In Arabidopsis, two mutants with an enhanced ethylene response, eer4 and sar1-7, result in decreased gene expression of AtEBP, an ethylene inducible gene. EER4 encodes a transcription factor related to TAF12. In Drosophila, TAF12's associate with the TFIID and SAGA complexes. The TFIID complex regulates expression of housekeeping genes, while the SAGA complex regulates expression of stress inducible genes. Genetic analysis determined that EER4 acts downstream of CTR1 and EIN2. The eer4;ein3-1 has a greater response than ein3-1 alone, indicating the reset mechanism is EIN3 independent. EER4 interacts with the EIN3 and ERF1 transcription factors possibly regulating expression of ethylene inducible genes. EER4 is also responsible for the regulation of the ethylene dampening mechanism independent of EIN3. SAR1 encodes a nucleoporin originally identified for its ability to restore auxin responsiveness to the auxin resistant mutant, axr1-12. SAR1 has pleiotropic effects due to the fact loss of function alleles of SAR1 have also been pulled out using screens for enhanced ethylene response and altered cold tolerance. This is not surprising because loss of function sar1 alleles accumulate poly A mRNA in the nucleus. Genetic analysis indicates SAR1 acts downstream of CTR1. Again, the sar1-7;ein3-1 double has a greater response than ein3-1 alone further supporting the ethylene reset mechanism is EIN3 independent. Due to sar1-1's involvement with auxin response, it was tested to whether this is a synergistic relationship between auxin signaling and level of ethylene response. Auxin response is a negative regulator of ethylene dampening response.