Chapter 38, Problem 1TYU

Summary Introduction

Introduction: Plant hormones are chemical molecules produced by plants in extremely low concentration for regulating the growth and development. There are five major hormones involved in the growth process. They are auxin, gibberellin, ethylene, abscisic acid, and cytokinin. Each of these hormones has its own functions at specific locations. The presence of these hormones in a definite amount is very essential for the normal growth and development of the plant.

Answer to Problem 1TYU

Correct answer: In the signal transduction process for the hormone auxin, the molecule ubiquitin tags certain proteins for destruction.

Hence, the correct answer is option (c).

Explanation of Solution

Reason for the correct answer:

General mechanism of action of auxin hormone:

Auxins are a group of related hormones responsible for a wide range of consequences on the growth and development of plants. Within a certain concentration, Auxin stimulates cell elongation in stems and coleoptiles. Auxin inhibits elongation growth by increasing the cell wall extensibility, according to the acid growth hypothesis. The main effects of auxin are to promote cell elongation according to the concentration.

Many plant hormones bind to the receptors which trigger the enzymatic reaction and results in the changes for cell growth and development of the plants. Both external and internal signal triggers the auxin hormone synthesis. The cytosol or the nucleus of the cell contains one receptor with three-dimensional shape (TIR1 receptor) that binds to auxin molecule. As the auxin binds to its receptor, ubiquitin molecule attaches to the repressor molecule and inhibits the auxin response genes. Thereafter, the ubiquitinylated protein is targeted and degraded into peptide fragments in a proteasome. This causes transcription of auxin response gene which acidifies the cell wall of target cells. The acidified target cell walls become more plastic which enables it to expand due to increased force of the cell’s turgor pressure. Thus, the action of auxin hormone causes cell expansion without cell division.

Option (c) is given as “tags certain proteins for destruction”.

The small regulatory protein called ubiquitin attaches to other proteins and are used to tag certain proteins for destruction. This process of tagging is referred to as ubiquitination and it is a post-translational modification. Ubiquitin-mediated proteolysis normally takes place during auxin signaling in plants.

Hence, the correct answer is option (c).

Reasons for the incorrect answers:

Option (a) is given as “absorbs blue light”.

Phototropins are the proteins that absorb blue light and they help to control the photosynthetic efficacy of plants, not auxins.

Hence, option (a) is incorrect.

Option (b) is given as “becomes phosphorylated”.

Phototropins are the blue-light receptors that control the photosynthetic efficacy of plants located at the shoot tips. These proteins become phosphorylated in response to blue light.

Hence, option (b) is incorrect.

Option (d) is given as “interacts antagonistically with gibberellins”.

Gibberellin is a growth-regulating hormone that enhances the elongation of stems and leaves and in the development of fruits. During auxin signaling in plants, the ubiquitin molecule does not antagonistically interact with gibberellins. Instead, ubiquitin-mediated proteolysis takes place in the signal transduction process in auxin.

Hence, option (d) is incorrect.

Option (e) is given as “binds to a receptor in the plant cell’s plasma membrane”.

During auxin signaling in plants, the ubiquitin molecule does not bind to a receptor in the plasma membrane of the plant cell. Instead, ubiquitin-mediated proteolysis takes place in the signal transduction process in auxin.

Hence, option (e) is incorrect.

Hence, options (a), (b), (d), and (e) are incorrect.

Conclusion

As the auxin binds to its receptor, ubiquitin molecule attaches to the repressor molecule in order to target it for destruction.