Ethylene gas is a plant hormone that regulates plant growth, development and response to environmental stress. It is produced from leaves, roots, stems, flowers and fruits, and it plays a major role in fruit ripening, flowering and abscission (the natural shedding of leaves, flowers and fruits). The biosynthesis of ethylene in plant cells occurs in a two-step biochemical pathway illustrated in figure below:
Figure : Ethylene biosynthesis starts when S-adenosyl-L-methionine (SAM) is converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS). ACC is then converted to ethylene by the enzyme ACC oxidase (ACO). The ACS and ACO genes are expressed in response to internal developmental cues and environmental stresses, such as wounding, flooding, drought, and pathogen attack.
Atta-Aly, Brecht and Huber (2000) researched ethylene feedback mechanisms in tomato and strawberry fruit tissues. Their studies revealed that immature fruit had a reduced concentration of ACC when exposed to ethylene gas, but ethylene application on mature fruit increased ACC oxidase (ACO) activity.
Which statement best explains how different plant tissues respond to ethylene gas?