Plant responses to environmental cuesEdexcel A-Level Biology A: Revision notes

Plants cannot move away from danger, so they respond by growth. They detect cues such as light, gravity and day length, and respond using growth regulators (such as IAA) and photoreceptors (such as phytochrome). These responses give a survival advantage, e.g. getting more light for photosynthesis.

Both phytochrome and IAA can alter transcription: they change which genes are switched on, so the cell makes different proteins.

Phytochrome is a light-sensitive pigment protein with two forms:

• Pr absorbs red light (660 nm) and is converted to Pfr. Pr is inactive.
• Pfr absorbs far-red light (730 nm) and is converted back to Pr. Pfr is the active form.

Pfr also slowly reverts to Pr in the dark. The ratio of Pfr to Pr tells the plant about its light environment. Pfr enters the nucleus and acts on transcription factors, which switches genes on or off.

• Germination: in light-requiring seeds (e.g. lettuce) Pfr stimulates genes for germination. A final flash of red gives germination; a final flash of far-red prevents it.
• Shade avoidance: leaves absorb red but transmit far-red, so shade has a low red to far-red ratio. This converts Pfr to Pr, so less Pfr inhibits stem elongation. The stem grows taller to reach light.
• Flowering: in short-day plants Pfr inhibits flowering, so flowering needs a long night, in which Pfr falls. A flash of red light in the night converts Pr to Pfr and stops flowering. Long-day plants respond the opposite way.

IAA (indoleacetic acid, an auxin) is made in the shoot tip. In phototropism:

1. Light is detected and IAA is moved by carrier proteins to the shaded side.
2. High IAA on the shaded side causes cell elongation: H⁺ ions are pumped into the cell wall, the wall loosens and cells take in water and lengthen (acid growth).
3. The shaded side grows faster, so the shoot bends towards the light.

Without the tip, there is no IAA and no bending. Covering the tip prevents light detection.

IAA binds to a receptor inside the cell. This causes a repressor protein to be broken down. The repressor normally prevents a transcription factor acting, so with it removed the transcription factor switches on genes for cell elongation and for proton pumps.

In gravitropism, IAA moves to the lower side. Shoot cells are stimulated by this high concentration, so the shoot grows upwards. Root cells are inhibited, so the root grows downwards. Roots and shoots respond differently to the same IAA concentration.