Ethylene is a gaseous hormone that influences a plant’s life, from seed germination to fruit ripening and the shedding of leaves. For a plant to respond to this gas, it must first detect its presence. This detection is performed by a receptor, a specialized protein molecule designed to recognize and bind to a specific signal. Understanding where this receptor is located inside the plant cell is key to understanding how this simple gas can have such profound effects on plant growth and development.
The Ethylene Receptor Protein Family
The cellular component that detects ethylene is not a single protein but a small family of related proteins. In the model plant Arabidopsis thaliana, researchers have identified five distinct but similar receptor proteins. These are known by names such as ETR1 (ETHYLENE RESPONSE 1) and ERS1 (ETHYLENE RESPONSE SENSOR 1). ETR1 was the first to be discovered and remains the most studied, but all five members work collectively to monitor ethylene levels.
These receptor proteins are structurally similar, each containing domains that allow them to bind ethylene and transmit a signal. They function as dimers, meaning two individual receptor proteins pair up to form a single functional unit. This pairing is stabilized by disulfide bonds. The existence of multiple, similar receptors provides a robust system for ethylene perception throughout the plant’s various tissues and developmental stages.
Location Within the Cell
Ethylene receptors are not found on the outer boundary of the plant cell, the plasma membrane. Instead, scientific studies have localized these proteins to the membrane of an internal organelle called the endoplasmic reticulum (ER). The ER is a vast network of interconnected membranes that extends throughout the cytoplasm of the cell. It functions as a cellular factory, involved in the synthesis of proteins and lipids and their transport.
The receptor proteins are integral components of this ER membrane, meaning they are physically embedded within the membrane structure. Parts of the protein are exposed to the interior of the ER (the lumen) and other parts face the cell’s cytoplasm. The N-terminal domain, which contains the site for binding ethylene, is situated within the ER lumen. This internal placement is possible because ethylene is a small gas that can readily diffuse through the cell to reach these internal receptors.
This localization has been confirmed through multiple lines of evidence, including membrane fractionation and immunoelectron microscopy. Studies using fluorescently tagged versions of the receptors have shown them distributed in a reticular network characteristic of the ER. While the ER is the primary site, it is an open question whether a small subset of receptors might exist in or move to other membrane systems under specific conditions.
Significance of the Receptor’s Location
The placement of ethylene receptors on the endoplasmic reticulum membrane is directly linked to how the signal is transmitted. When ethylene binds to its receptor on the ER, it initiates a signaling cascade. This process begins with the inactivation of the receptor, which, in the absence of ethylene, is in an active state. This active, ethylene-free receptor works with another protein called CTR1 to repress ethylene responses downstream.
Upon binding ethylene, the receptor complex changes conformation and ceases to activate CTR1. This inactivation of CTR1 relieves the repression on the next protein in the pathway, a transmembrane protein called EIN2, which is also located in the ER membrane. This allows for a part of the EIN2 protein to be cleaved off. This cleaved portion of EIN2 then travels from the ER membrane to the cell’s nucleus.
Once inside the nucleus, the EIN2 fragment activates transcription factors, such as EIN3. These transcription factors are proteins that can bind to specific regions of DNA and control which genes are turned on or off. By activating these factors, the signal that began on the ER results in widespread changes in gene expression. This alteration of genetic activity leads to the physical responses associated with ethylene, such as the softening of fruit or the fading of flowers.