Mechanism Of Action Of The Arabidopsis Ethylene Receptor Signaling

Ethylene is an important phytohormone regulating plant growth and development in many aspects. There are five ethylene receptors in Arabidopsis and genetics studies indicate that ethylene receptor genes are functionally redundant. According to homology in amino-acid sequences and protein structures, these five receptors can be categorized into two subfamilies and share structural similarity with prokaryotic histidine-kinase proteins. However, biological significance of the classification remains to be studied. Function of ETR1 N terminus has been studied by several laboratories and distinct results and interpretations have been made. We also analyzed possible role of ETR1 N terminus in receptor signal output.To investigate biological and functional significance of the receptor classification, we studied receptor signal output of the dominant etr1-1, etr2-1 and ers2-1 recptor proteins, representing dominant receptor signal output of the two subfamilies. Our results show that the dominant receptor activity of subfamilyâ…¡receptors is dependent on subfamilyâ… ; while dominant receptors of both subfamilies can be functionally independent on subfamilyâ…¡. These data suggest subfamilyâ…¡receptors may act upstream to subfamilyâ… ,and that wild-type receptors are essential for mediation of ethylene insensitivity. Furthermore, results of protein-protein interaction indicate that the GAF domain may be involved in signal transfer between receptors of the two subfamilies. Domain swapping shows that function of the GAF domain and kinase domain of ETR1 and ETR2 can not be substituted by the other.To study whether ETR1 N terminus can mediate receptor signal output, we transform etr1(1-349) and etr1-1(1-349) to different mutants backgrounds. The etr1(1-349) and etr1-1(1-349) are able to restore etr1-7 ers1-2 rosette growth and flower fertility to a similar extent. In contrast, neither etr1(1-349) nor etr1-1(1-349) was capable of signal output in etr1-7 ers1-3.The ERS1 transcript was detectable in ers1-2 but not in ers1-3, implying that ETR1 N-terminal signaling is subfamilyâ… dependent. Loss of subfamilyâ…¡receptor gene did not perturb etr1-1(1-349)-mediated ethylene insensitivity. This result suggest that the loss of subfamily II has little eaffect on ETR1 N-terminal signal output. Possible roles of the covalent linkages in ETR1 N terminal signaling was next studied. When the disulfide-forming residues were mutated, mutant etr1 variants were capable to receptor signal output, indicating that ETR1 may not relay on covalent homodimerization to be functional.