Structure-Function of the Ethylene Binding Domain
One topic of interest is to further define the structure and function of the ethylene binding domain. We have used both mutational and chemical analyses to uncover more information about how ethylene binds to the receptor and transduces the signal through the protein. Interestingly, ethylene receptors are metalloproteins containing copper ions. While copper is the natural co-factor, we have shown that the other group 11 transition metals (silver and gold) support ethylene binding to the receptors while other metals do not. This observation is of interest because silver, but not gold, blocks ethylene action in the plant. These observations support the idea that silver may block conformational changes in the receptor because it is larger than copper. Future work in the lab will address this model. Another chemical approach we have used is to analyze ethylene binding and receptor function in the presence of various strained alkenes. Finally, we have used alanine-scanning mutagenesis of conserved residues in the binding domain to define regions necessary for ethylene binding, turning the receptor off when ethylene binds, and maintaining a functional receptor. We are currently expanding this to include other techniques of analysis to determine conformational changes that occur in the binding domain during ethylene binding and transduction. For more information refer to: Rodriguez et al., 1999; Wang et al., 2006; Pirrung et al., 2008; Binder et al., 2010.

Kinetics of Growth Inhibition and Recovery
A second area is to uncover new details about the ethylene signal transduction pathway downstream of the receptors. A major effort recently has been to use a computer-driven, time-lapse image acquisition system to study the kinetics of growth changes in etiolated seedlings of Arabidopsis thaliana. This system has revealed transient and subtle changes due to ethylene that would have otherwise remained unknown. Combining this approach with genetics and molecular biology has refined our understanding about ethylene receptor function and down-stream signal transduction components and has provided links between events at the molecular level with those at the organ level.  In particular, there appear to be two phases to the ethylene response which can be genetically and pharmacologically distinguished. The second, slower phase response to ethylene is dependent on the EIN3 and EIL1 transcription factors. In contrast, the events leading to the first phase response remain unknown. Efforts continue to define the central roles for EIN3 and EIL1 and to uncover more details about the control of the first phase response. We also have found that recovery kinetics after removal of ethylene depends on certain receptor isoforms. We continue to investigate the basis for this dependency. For more information refer to: Binder et al., 2004; Binder et al., 2004; Kim et al., 2011.

Growth responses in Arabidopsis
(click on image to link to time-lapse movies)
 

Nutation response in Arabidopsis stimulated by ethylene
(click on image to link to time-lapse movie)
 

Specific Function for the ETR1 Ethylene Receptor
A third area of current research is based on our recent observation that ethylene stimulates nutational bending of hypocotyls that are dependent on the ETR1 receptor. Nutations (also called circumnutations) are nodding or coiling movements and are thought to be important in allowing the roots and shoots to penetrate the soil. Thus, they are likely to be important in seedling survival. Loss-of-function mutants of the ETR1 receptor results in loss of the nutation phenotype while loss-of-function mutant combinations of the other receptor isoforms results in constitutive nutations in air. The basis for this unique role for ETR1 is now being investigated. The observation that ETR1 has a unique role in ethylene-stimulated nutations suggests that the other receptor isoforms might have unique roles in  other developmental and physiological processes such as senescence, abscission and responses to abiotic stresses. This idea is currently being tested. We have also found that auxin transport is involved in ethylene-stimulated nutations. This provides a good system to explore the molecular links between ethylene signaling and auxin transport. For more information refer to: Binder et al., 2006; Kim et al., 2011.

Ethylene & Other Species
A relatively new area for the lab is to explore ethylene response kinetics in other plant species. Previous studies using endpoint analysis showed that there is a wide variety of responses to ethylene. We are expanding upon this by documenting kinetic differences between species with the goal of better understanding the diversity of ethylene related traits as well as adding to our overall understanding of ethylene signaling. It is believed that ethylene receptors were acquired during the endosymbiotic event that led to chloroplasts. We are now studying putative ethylene receptors in various cyanobacteria.

Growth Responses in non-Arabidopsis species
(click on image to link to time-lapse movies)