Plant responses: phytochrome, auxin and gibberellin

Phytochrome

Phytochrome is a light-absorbing pigment that exists in two interconvertible forms:

• Pr absorbs red light (about 660 nm) and is converted to Pfr.
• Pfr absorbs far-red light (about 730 nm) and is converted back to Pr. In darkness, Pfr slowly converts back to Pr.

Daylight contains more red than far-red light, so Pfr builds up during the day and falls at night. Pfr is the active form: it moves into the nucleus and interacts with transcription factors, switching genes on or off.

Phytochrome controls germination of some seeds (red light promotes it), flowering in response to day length (photoperiodism), and stem elongation and greening (plants grown in the dark are tall, thin and yellow: etiolated).

Auxin and phototropism

Auxin (IAA) is made in shoot tips and moves down the shoot. It causes cells to elongate: auxin stimulates proton pumps, H+ is pumped into the cell wall, the lower pH activates proteins that loosen bonds between cellulose fibres, and the cell takes up water and stretches. Auxin also changes gene transcription by causing breakdown of repressor proteins.

Phototropism: when light comes from one side, auxin moves to the shaded side. Cells there elongate more, so the shoot bends towards the light.

Gibberellins

Gibberellins (GA) cause stem elongation and are involved in seed germination:

1. The seed absorbs water and the embryo releases gibberellin.

2. GA reaches the aleurone layer and causes breakdown of DELLA proteins, which normally repress growth genes.

3. A transcription factor can then switch on the gene for amylase.

4. Amylase is secreted into the endosperm and hydrolyses starch to maltose, giving sugars for the embryo to respire as it grows.

Worked example

A seed that needs light to germinate is given a flash of red light followed by a flash of far-red light. Predict whether it germinates and explain.

1. Red light converts Pr to Pfr.
2. Far-red light then converts Pfr back to Pr.
3. The last flash decides: little Pfr remains.

Answer: It does not germinate, because Pfr, the active form needed, has been converted back to Pr.

Check yourself

1. What happens when Pr absorbs red light?
   • It is destroyed
   • It is converted to Pfr
   • It becomes auxin
   • It is converted to chlorophyll
   Show answer

   It is converted to Pfr. Red light converts Pr to the active Pfr.

2. Which form of phytochrome is biologically active?
   • Pr
   • Neither form
   • Only the form present in darkness
   • Pfr
   Show answer

   Pfr. Pfr switches genes on or off.

3. What happens to Pfr levels during a long night?
   • They rise as more red light is absorbed
   • They fall as Pfr slowly converts back to Pr
   • They stay the same
   • Pfr is converted to auxin
   Show answer

   They fall as Pfr slowly converts back to Pr. In darkness Pfr reverts to Pr.

4. Why does a shoot lit from one side bend towards the light?
   • Auxin moves to the lit side and cells grow faster there
   • Light kills cells on the lit side
   • Auxin moves to the shaded side, where cells elongate more
   • Gibberellin is destroyed on the shaded side
   Show answer

   Auxin moves to the shaded side, where cells elongate more. Unequal elongation bends the shoot.

5. How does auxin make cells elongate?
   • It causes H+ to be pumped into the cell wall, loosening the cellulose
   • It makes cell walls thicker
   • It stops water entering cells
   • It makes cells divide by meiosis
   Show answer

   It causes H+ to be pumped into the cell wall, loosening the cellulose. Acid conditions loosen bonds between cellulose fibres.

6. How does gibberellin switch on the amylase gene in cereal seeds?
   • It binds directly to starch
   • It is converted into amylase
   • It causes DELLA repressor proteins to be broken down
   • It blocks phytochrome
   Show answer

   It causes DELLA repressor proteins to be broken down. Removing the repressor lets a transcription factor activate the gene.

7. A seedling grown in darkness is tall, thin and yellow. What is this called?
   • Etiolation
   • Phototropism
   • Habituation
   • Photoperiodism
   Show answer

   Etiolation. Without Pfr, stems elongate and chlorophyll is not made.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• College of Saint Benedict and Saint John's University: Phytochrome lecture notes
• AGRIS (FAO): Gibberellin signaling in barley aleurone cells. Control of SLN1 and GAMYB expression

Last updated 7 November 2026. Spotted a mistake? Tell us.