You must have done some reading and investigating on your own to know enough to ask this question! Good job.

Ethylene is actually a plant hormone -- most people do not realize that plants produce crucial "hormones" but they do. Typically, these hormones regulate plant growth and development, just as they do in animals, including humans. Ethylene is commonly known as the "senescence hormone" and has been well studied. It is actually a gaseous hormone and it hastens fruit ripening. You can look up the definition and description of plant "senescence" in a basic biology text.

The story of ethylene's discovery as a plant hormone is really interesting. Nearly 100 years ago, a student noticed that trees close to gas street lamps had leaves that "abscised" (look that up, too!) earlier than trees farther away from the gas lamps. By analyzing what components of the gas were responsible for hastening abscission, it was determined that ethylene (H₂C==CH₂) was the cause. Since that time, many researchers have worked on the mechanism of how ethylene gas exerts this abscission effect on plants as well as on fruit ripening. Using the small plant Arabidopsis Thaliana, scientists are using molecular genetics to identify the receptor that binds to ethylene and "signals" the plant cell to enter senescence. For example, a particular Arabidopsis mutant has been identified -- it is a strain carrying a mutation in the "ein2" gene. The phenotype (the characteristic) of this plant is insensitive to ethylene, meaning that it gets larger and larger (compared to the non-mutant strain). Suppose that you isolated a strain of plant carrying a mutation in a single gene. The phenotype of the plant is that it "acts like" it sees ethylene, even when no ethylene is present (it is very tiny). What are the possible roles of the mutant gene? (Hint: If the gene is not mutated, the plant is "normal" so what does the "normal form" of the gene do?).

You have asked a question that scientist do not completely understand. Cellular quantities of ethylene can reach a certain level and physiological changes called ripening will begin. The amount of ethylene can vary from fruit to fruit and is also affected by other gases like oxygen or carbon dioxide. The increase in ethylene follows changes in fruit texture (cell wall material is metabolized into smaller molecular weight units producing a softer texture), composition (generally increase in sugars and decrease in acids) and physiology (pigments - bananas go from green to yellow, and volatile flavor compounds increase in concentration.

The biochemical mechanisms by which ethylene initiates these changes is still being investigated by researchers. They are very complex, and how they are controlled within the cell is still not totally understood.

The quick and easy answer to your question is that it either acts as a transcription factor or affects the activation or synthesis of a transcription factor controlling the expression of ripening-specific genes.

What I can tell you about ethylene gas is that it is used by plants to "talk" to different parts of itself. Scientists call this "signal transduction". Because it is a plant messenger, we can use it to trick the plant into doing the things we want them do to. For example, vegetable growers use ethylene gas to cause the ripening of many different fruits and vegetables such as tomatoes. The growers pick the tomatoes when they're large and green, then gas them to make them ripen. There are A LOT of people who do research on ethylene gas, so if you go the library and look up ethylene gas in a scientific or agricultural journal I'm sure you'll lots for information.

I hope this gets you started. Good luck!