text: Ethylene signaling pathway is a signal transduction in plant cells to regulate important growth and developmental processes. Acting as a plant hormone, the gas ethylene is responsible for promoting the germination of seeds, ripening of fruits, the opening of flowers, the abscission (or shedding) of leaves and stress responses. It is the simplest alkene gas and the first gaseous molecule discovered to function as a hormone.
  Most of the understanding on ethylene signal transduction come from studies on Arabidopsis thaliana. Ethylene can bind to at least five different membrane receptors. Although structurally diverse, the ethylene receptors all exhibit similarity (homology) to two-component regulatory system in bacteria, indicating their common ancestry from bacterial ancestor. Ethylene binds to the receptors on the cell membrane of the endoplasmic reticulum. Although homodimers of the receptors are required for functional state, only one ethylene molecule binds to each dimer.
  Unlike in other signal transductions, ethylene is the suppressor of its receptor activity. Ethylene receptors are active without ethylene due to binding with other enzymatically active co-receptors such as constitutive triple response 1 (CTR1) and ethylene insensitive 2 (EIN2). Ethylene binding causes EIN2 to split in two, of which the C-terminal portion of the protein can activate different transcription factors to bring about the effects of ethylene. There is also non-canonical pathway in which ethylene activates cytokinin receptor, and thereby regulate seed development (stomatal aperture) and growth of root (the apical meristem).