Question

Explain how plants sense the ratio of Red:Far-red light and why this is important in the shade avoidance response.

Answer 1

Pr and Pfr are two interconvertible forms of Phytochromes. Pr absorbs red light and convert it to Pfr. Pfr absorbs far-red light and convert it back to Pr. Plant absorb red or far-red light causes a massive change in the shape of chromophore, resulting in the alteration of conformation and activity of the phytochrome protein to which they are attached, Pfr is physiologic form of protein, active during the red light exposure, exposure to far-red light results in the conversation of Pfr to Pr, hindering the phytochrome activity. Together they form the phytochrome system, this system act as a biological switch, it helps in the monitoring the level, intensity, duration and color of the light.

Plants use phytochrome system in its growth phase, Sunlight contains more of red light, plants in the unfiltered sunlight will get more of red light while the plants growing in the filtered light will get less amount of red light. Results in the Pfr active form which ads in the growth whereas the plants which grow in the filtered light will be exposed to a higher degree of Pr inactive resulting in the slow growth. Thus plants growing in the highly dense region will have a competition for the red light and those who get in abundance will grow well.

Darkness Far red light Cellular Response Pfr (active) Pr (inactive) Red light Phytochrome System