Date of Award
Doctor of Philosophy
Brad M. Binder
Barry D. Bruce, Albrecht vonArnim, Andreas Nebenfuehr
The gaseous phytohormone ethylene regulates several physiological and developmental processes in higher plants. There are five ethylene receptor isoforms that mediate the responses to ethylene in the model plant Arabidopsis thaliana. Prior research has shown that these five ethylene receptor isoforms in Arabidopsis have both overlapping and non-overlapping roles in regulating diverse responses such as growth in air, growth recovery after removal of ethylene, and ethylene stimulated nutational bending. Functional divergence of ETR1 has been determined in controlling some of these traits and in some of these cases, ETR1 subfunctionalization requires the receiver domain. Using homology modeling and sequence alignment studies, we determined regions with the structural divergence between the receiver domains of ETR1 versus EIN4 and ETR2. I found that the ETR1 receiver domain has multiple functions where residues in the γ-loop are important for germination on salt and abscisic acid signaling, whereas residues on the C-terminal end of the receiver domain are essential for ethylene-stimulated nutations. Additionally, ETR1 and ETR2 have contrasting roles in the control of at least one trait, seed germination under salt stress. I expanded these results by showing that ETR1 and ETR2 have contrasting roles in the control of germination under a variety of inhibitory conditions. Using epistatic analysis, I also show that ETR1 and ETR2 do not require the canonical ethylene signaling pathway to regulate seed germination under stress conditions. The mechanism for this differential control is unclear but could involve differences in receptor-protein interactions. To explore the importance of receptor-protein interactions I conducted a yeast two-hybrid screen using the cytosolic domains of ETR1 and ETR2 against a root library. This uncovered unique interacting partners with ETR1 and ETR2. I focused on three of the stress-related proteins and confirmed their interactions with the receptors in yeast and in planta assays. Loss of these proteins resulted in faster germination in response to ABA showing that they are involved in ABA responses. Thus, ETR1 and ETR2 signal via a non-canonical pathway to control seed germination and affect ABA signal transduction. These results suggest that the receptors have both ethylene-dependent and –independent roles in plant cells.
Bakshi, Arkadipta, "Non-Canonical Signaling from the ETR1 and ETR2 Ethylene Receptors in Arabidopsis thaliana. " PhD diss., University of Tennessee, 2017.