Q. 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). Like many hormones, ethylene functions via an intracellular signal transduction pathway illustrated in Figure 1 above:
Figure 1. Signal transduction pathway of ethylene in plant cells. Key: 1. Ethylene Gas 2. Plasma Membrane 3. Receptor Protein 4. Endoplasmic Reticulum 5. Constitutive Triple Response Protein Kinase 1 (CTR1) 6. Ethylene Insensitive 2 protein (EIN2) 7. Ethylene Insensitive 3 protein (EIN3) 8. DNA 9. mRNA 10. Enzyme production.
In the absence of ethylene gas, a receptor protein activates the CTR1 protein kinase, which represses EIN2 function. Recent studies with plants genetically engineered to have reduced gene expression of an ethylene receptor (Tieman, et al. 1999) showed an increase in ethylene sensitivity, including accelerated fruit ripening and flower aging.
Which statement best explains how the ethylene response is regulated in plants?
