Ethylene is a naturally occurring gaseous plant hormone that influences a wide array of processes throughout a plant’s life cycle. It plays a role in plant growth, development, and responses to environmental changes. Understanding how plants perceive and react to ethylene is important for fundamental plant biology.
The Receptor’s Crucial Role
Plants possess specialized protein structures known as receptors, essential for perceiving external signals. These receptors function much like a “lock” activated by a specific “key,” the ethylene molecule. Without them, plants cannot recognize ethylene or initiate the necessary biological responses. The interaction between ethylene and its receptors is the initial step in a complex communication system.
Ethylene binding to its receptor triggers internal cellular events. This molecular recognition allows plants to “sense” ethylene, enabling them to adapt and respond to various cues. Receptors translate the external chemical signal into an internal cellular message. This process ensures appropriate physiological reactions.
The Receptor’s Intracellular Home
The primary ethylene receptors in plants, such as those of the ETR/ERS family, are predominantly found within the Endoplasmic Reticulum (ER) membrane. The ER is an extensive network of interconnected membranes and flattened sacs extending throughout the cytoplasm, continuous with the outer nuclear membrane.
The ER performs important functions, including protein synthesis, folding, and lipid production. Receptor localization to this internal membrane system positions them to interact with other signaling components. This placement means the receptors are embedded for processing and transmitting the ethylene signal.
From Receptor to Response
Ethylene perception by its receptors initiates a cascade of molecular events that alter plant behavior. In the absence of ethylene, ER-localized receptors actively maintain CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) in an active state. Active CTR1 then phosphorylates another protein, ETHYLENE-INSENSITIVE2 (EIN2), thereby repressing ethylene-responsive processes. This acts as a default “off” switch for ethylene signaling.
When ethylene is present, it binds to its receptors, leading to their inactivation. This binding causes a conformational change, deactivating CTR1, which allows EIN2 to become active. Once activated, a specific portion of EIN2, its C-terminal domain, is cleaved and travels from the ER to the nucleus. Inside the nucleus, this EIN2 fragment, along with transcription factors like EIN3 and EIL1, regulates gene expression, leading to the various physiological responses observed in the plant.
Ethylene’s Diverse Plant Actions
The signaling pathway initiated by ethylene’s binding to its ER-based receptors orchestrates a wide variety of plant actions. One of the recognized effects is the ripening of climacteric fruits, such as tomatoes, bananas, and apples. Ethylene triggers changes in their color, texture, and flavor.
Ethylene also plays a role in leaf senescence (aging and shedding) and influences the fading and wilting of flowers. Beyond these developmental processes, ethylene is involved in plant responses to various environmental stresses, including wounding, drought, and flooding. The hormone can also promote seed germination and root hair development, enhancing a plant’s ability to absorb water and nutrients.