Question

Some etiolated seedlings develop hairs on their epidermis when exposed to dim light. Describe an experiment to test the hypothesis that a phytochrome is the photoreceptor for this effect.

Answer 1

Etiolation is a phenomenon in which seedlings grown in dark become elongated due to elongation of internodes, they have small cotyledons and also develop chlorosis. Phytochrome has two forms Pr and Pfr. Pr is converted to Pfr on exposure to red light (670nm) whereas, Pfr is converted to Pr on exposure to far-red light (730nm). Pfr is the active form which can lead to a response (in this case development of hairs on the epidermis). Since dim light consists of red wavelength (not far red), exposure of etiolated seedlings to it converts Pr to Pfr and the response (hairs on epidermis) is seen. Keeping this information in mind a very simple experiment can be conducted.

Procedure to conduct the experiment:

1. Grow seedlings in dark in two sets (A and B). A is the control and B is the test.

2. After the plants have grown and show etiolated features, expose both sets (A and B) to dim light. At this point Pr will be converted to Pfr in both sets.

3. In case of set B, re-expose it to far-red light, so Pfr in set B gets converted to Pr which is the inactive form of phytochrome.

4. Now keep both sets in dark for some more days and then observe. If phytochrome is the receptor for this effect, epidermal hair will be observed in set A and not in set B.

Explanation:

In set B exposure to far red light converts Pfr to inactive Pr. Thus, the response cannot be executed. However, in set B since no such inactivation occurs, the response (epidermal hairs) will be seen.