Question

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?

• A. In the presence of ethylene gas (1), the ethylene receptors (3) activate CTR1 (5), resulting in the release of the EIN2 (6). EIN3 (7) can then continue the transduction pathway by acting as a transcription factor to mediate the expression of the particular genes involved.
• B. This pathway involves negative regulation by ethylene receptor proteins (3) because CTR1 (5) actively represses the release of EIN2 (6) in the absence of ethylene (1) but the response is activated when ethylene (1) is present or when the number of ethylene receptors (3) is reduced.
• C. In the absence of the hormone, ethylene receptors (3) activate CTR1 (5), resulting in the release of the EIN2 (6). EIN3 (7) can then continue the transduction pathway by acting as a transcription factor to mediate the expression of the particular genes involved.
• D. When ethylene (1) binds to its receptors (3) the EIN2 (6) is activated, which represses CTR1 (5) and allows for the transcriptional response to ethylene (1). It the absence of ethylene (1), EIN2 (6) is repressed and this causes CTR1 (5) to be activated, shutting down the transcriptional response to ethylene (1).

Answer 1

The correct option is B This pathway involves negative regulation by ethylene receptor proteins (3) because CTR1 (5) actively represses the release of EIN2 (6) in the absence of ethylene (1) but the response is activated when ethylene (1) is present or when the number of ethylene receptors (3) is reduced.

Ethylene response is a negative regulation by ethylene receptor proteins because, 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.

So the correct option is 'This pathway involves negative regulation by ethylene receptor proteins (3) because CTR1 (5) actively represses the release of EIN2 (6) in the absence of ethylene (1) but the response is activated when ethylene (1) is present or when the number of ethylene receptors (3) is reduced.'