Ethylene is a simple two-carbon hydrocarbon gas that acts as a powerful signaling molecule, or phytohormone, across the entire life cycle of a plant. This gaseous hormone orchestrates a diverse array of developmental and physiological processes. Ethylene is widely known for its ability to stimulate fruit ripening, promote the wilting of flowers, and cause the shedding of leaves (abscission). The molecule also plays a role in helping plants respond to various stresses, such as flooding and pathogen attack. Detecting this gaseous molecule requires a specialized cellular mechanism to translate the signal into a physical response.
The Receptor’s Precise Location
The primary receptors that bind to ethylene are anchored within a specific internal structure of the plant cell. These receptor proteins are predominantly found embedded in the membrane of the Endoplasmic Reticulum (ER), which is a vast network of sacs and tubules within the cell. The ER acts as a cellular factory, responsible for the synthesis, folding, and transport of proteins and lipids. This location is relevant because the ethylene molecule is nonpolar and highly hydrophobic, meaning it can easily diffuse through the outer cell membrane and the watery cytoplasm.
Once inside the cell, the gas reaches the ER membrane, where the receptor’s binding domain is positioned to capture it. The receptor’s structure is designed to hold a copper ion, which is a necessary cofactor for the actual binding of the ethylene molecule. By localizing the receptor to the ER, the plant isolates the initial perception event, allowing the binding to directly influence the downstream signaling components tethered to the membrane.
Molecular Identity of the Receptor
The molecules responsible for ethylene perception belong to a family of proteins known as ethylene receptors, often referred to by names like ETR1 (Ethylene Response 1) and ERS1 (Ethylene Response Sensor 1). These receptors are transmembrane proteins that span the lipid bilayer of the ER membrane. The N-terminal portion of the protein, which contains the ethylene-binding domain, is embedded in the membrane and faces the lumen of the ER.
The structure of the receptor is organized as a dimer, where two identical protein units are linked together, and this dimeric structure is required for the receptor to function. The C-terminal region extends into the cytoplasm and includes domains that classify the protein as a histidine kinase or pseudo-kinase. This cytoplasmic tail acts as the signal transmitter, translating the binding event into a biochemical change inside the cell. Not all members possess the full histidine kinase activity, with some being pseudo-kinases that still regulate the overall signal.
The Mechanism of Action
The ethylene signaling pathway operates in an unusual manner, resembling an “off” switch that is constantly held in the “on” position by the receptor complex. In the absence of ethylene, the receptor complex is active and functions as a negative regulator of the downstream response.
This repression is primarily mediated by CTR1 (Constitutive Triple Response 1), a Raf-like kinase that associates with the receptor’s cytoplasmic domain at the ER membrane. When the receptor is active without ethylene, it stimulates the kinase activity of CTR1, which then initiates a cascade that prevents the ethylene-responsive genes in the nucleus from being activated.
When ethylene binds to the copper cofactor within the receptor, it causes a conformational change that inactivates the receptor. This binding event effectively flips the “off” switch, leading to the deactivation of the associated CTR1 protein. The loss of CTR1 activity removes the negative regulation on the downstream signaling components, particularly EIN2 (Ethylene Insensitive 2).
Once freed from the inhibitory influence of CTR1, the C-terminal portion of EIN2 is cleaved and travels to the nucleus, carrying the signal that ethylene is present. In the nucleus, this signal stabilizes transcription factors like EIN3, which can then bind to the regulatory regions of DNA and activate the expression of hundreds of ethylene-responsive genes. The perception of ethylene acts not by activating a signal, but by releasing the plant from a state of constant repression.