Plant Responses

AQA spec ref: 3.6.4 - Plant responses to the environment

Plants cannot move, but they can respond to their environment through growth and biochemical changes. Their responses are slower than animal nervous responses but are carefully regulated. The main mechanisms are: tropisms (growth responses to directional stimuli), nastic responses (non-directional), and the action of plant hormones (especially auxins, gibberellins, and abscisic acid). Understanding the role of IAA (auxin) in phototropism and gravitropism is the core of this section.

Plant Hormones Overview

Unlike animal hormones, plant hormones are often produced in multiple tissues and act locally as well as at a distance. They interact with each other, sometimes antagonistically. The key hormones for AQA are:

IAA (Indoleacetic Acid - Auxin)

IAA is the primary auxin in plants. It is produced mainly in the apical meristem (the growing tip of shoots). The key property of IAA: it promotes cell elongation in shoots at typical concentrations, but inhibits elongation in roots at the same concentrations.

How IAA promotes cell elongation: IAA activates a proton pump in the plasma membrane that pumps H⁺ ions into the cell wall. The lowered pH activates expansins - enzymes that loosen the hydrogen bonds between cellulose microfibrils, making the cell wall more plastic. The cell then takes in water by osmosis and expands. This is the acid growth hypothesis.

Phototropism (Response to Light)

When a shoot is illuminated from one side, it bends toward the light. This is positive phototropism.

Mechanism:

1. Light from one side is detected by photoreceptors in the shoot tip (the apical meristem region).
2. IAA, produced in the tip, moves laterally away from the light-exposed side to the shaded side. (The Cholodny-Went model)
3. IAA accumulates on the shaded side - higher concentration there than on the lit side.
4. Cells on the shaded side elongate more (because higher IAA = more elongation in shoot cells).
5. The shoot bends toward the light.

The role of the tip: if you remove the shoot tip, the shoot stops bending toward light, even when illuminated from one side. This confirms that IAA is produced in the tip and that the tip is needed for the directional response. If you replace the tip offset on one side (without light) the shoot bends away from the tip - the IAA from the tip causes one-sided elongation even without light.

This was shown by classic experiments:

• Boysen-Jensen: showed a diffusible substance from the tip causes bending (tip separated from shoot by mica gave no bending; separated by gelatine still allowed bending - gelatine allows diffusion, mica does not)
• Went: isolated the substance (auxin) by placing the tip on agar blocks, then applying agar blocks to decapitated shoots

Gravitropism (Response to Gravity)

Shoots show negative gravitropism (grow upward, away from gravity). Roots show positive gravitropism (grow downward, toward gravity).

The mechanism for root gravitropism:

1. When a root is placed horizontally, gravity causes starch-filled plastids (statoliths) in the columella cells (root cap) to settle to the lower side.
2. This somehow signals IAA to move laterally to the lower side of the root.
3. IAA concentration is now higher on the lower side.
4. In roots, IAA at higher concentrations inhibits elongation (unlike shoots, where it promotes it). This is because roots are much more sensitive to IAA - the optimal concentration for root elongation is much lower than for shoot elongation.
5. The upper side of the root (lower IAA) elongates more than the lower side (higher IAA, inhibited).
6. The root curves downward.

The different sensitivity of shoots and roots to IAA is key: the same concentration that maximally promotes elongation in shoots inhibits elongation in roots. This is why shoots curve toward light (more IAA on dark side promotes elongation) while roots curve away from the surface (more IAA on lower side inhibits elongation).

Gibberellins

Gibberellins are a large family of hormones that promote stem elongation, seed germination, and flowering. The AQA-relevant roles:

Stem elongation: gibberellins promote cell division in the subapical meristem and cell elongation. Genetic dwarfism in plants is often caused by mutations that prevent normal gibberellin synthesis or signalling. Application of gibberellin to dwarf plants restores normal height - this is how gibberellin was discovered (Kurosawa studying "foolish seedling disease" in rice, caused by the fungus Gibberella fujikuroi which produces gibberellin).

Seed germination: During germination, gibberellins secreted by the embryo diffuse to the aleurone layer of the seed. Gibberellin stimulates the aleurone cells to transcribe and translate genes for amylase. Amylase is secreted into the starchy endosperm, breaking down starch to maltose and glucose. This provides sugars for the growing embryo.

This is a commercial process: in brewing, barley is malted (allowed to germinate) specifically to produce amylases that will break down starch.

Fruit development: gibberellins promote fruit development. Applied to seedless grape varieties, they produce larger fruit.

Abscisic Acid (ABA)

ABA is generally a stress-response hormone - it inhibits growth and prepares the plant for unfavourable conditions.

Stomatal closure in drought: when a plant is water-stressed, ABA is produced in the leaves. It diffuses to guard cells and causes:

1. Inhibition of H⁺ ions being pumped out of the guard cell
2. K⁺ channels open, K⁺ leaves the guard cell
3. Reduced solute concentration inside guard cell→water leaves by osmosis
4. Guard cell loses turgor→stomata close
5. Water loss is reduced

This prevents excessive water loss through the stomata - a survival mechanism in drought.

Seed dormancy: ABA promotes dormancy - it prevents germination until conditions are favourable. Germination requires ABA levels to fall (and gibberellin levels to rise). The balance between ABA and gibberellin determines whether a seed germinates.

Ethene (Ethylene)

Ethene is a gaseous plant hormone. Key roles:

Fruit ripening: ethene triggers a cascade of events in ripening fruit - starch converts to sugars, cell walls soften (cellulase activity increases), colour changes as chlorophyll breaks down and carotenoids/anthocyanins accumulate. Ethene also stimulates more ethene production (positive feedback) - this explains why "one rotten apple spoils the barrel": ethene from a ripening or rotting apple accelerates ripening of neighbouring fruit.

Senescence and leaf abscission: ethene promotes the breakdown of chlorophyll and the formation of an abscission zone (the layer at the leaf stalk) that leads to leaf fall.

Response to mechanical stress: plants under wind or touch produce more ethene, which promotes wider but shorter stems - this is thigmomorphogenesis.

Apical Dominance

The apical meristem (shoot tip) suppresses the growth of lateral buds - this is apical dominance. It is maintained by IAA from the apical meristem inhibiting lateral bud outgrowth. When the tip is removed (pruning), IAA levels fall, lateral buds grow → the plant becomes bushier.

Cytokinin, produced in roots, promotes lateral bud growth. The ratio of IAA:cytokinin determines which buds grow. High IAA:cytokinin = apical dominance; low ratio = lateral growth.

Gardeners exploit this: pinching out the apical bud (removing it) promotes lateral branching and a bushier plant.

Summary

• IAA = auxin; promotes elongation in shoots, inhibits in roots (roots are more sensitive). Made in the apical meristem.
• Phototropism: IAA migrates to shaded side→more elongation on shaded side→shoot bends toward light
• Gravitropism in roots: statoliths settle to lower side→more IAA on lower side→lower side inhibited→root curves down
• Gibberellins: stem elongation; seed germination (trigger amylase in aleurone layer → starch breakdown); fruit development
• ABA: stomatal closure in drought (K⁺ loss from guard cells → loss of turgor); seed dormancy
• Ethene: fruit ripening; leaf senescence; positive feedback loop in ripening
• Apical dominance: IAA from tip inhibits lateral buds; removing tip → branching

AQA Exam Tips

• IAA concentration curve: in shoots, elongation is promoted up to an optimum IAA concentration, then inhibited at very high concentrations. In roots, the optimum is much lower - the same concentration that is optimal for shoots already inhibits roots. AQA gives graphs of this and asks you to compare.
• Phototropism mechanism - step-by-step: 1. IAA produced in tip. 2. IAA moves laterally to shaded side. 3. Higher IAA on shaded side. 4. Cells on shaded side elongate more. 5. Shoot bends toward light. Do not say "more light causes faster growth" - it's the unequal IAA distribution that matters.
• Gibberellin and germination: AQA loves asking the detailed sequence: embryo releases gibberellin → gibberellin diffuses to aleurone layer → aleurone cells produce amylase → amylase breaks down starch in endosperm → sugars for growth.
• Guard cell mechanism: ABA causes stomata to close by causing K⁺ to leave the guard cells → water potential rises in guard cell → water leaves by osmosis → turgor lost → pore closes.
• Apical dominance: explain in terms of IAA concentration. When the tip is intact, IAA levels in lateral buds are high enough to suppress them. When the tip is removed, IAA levels fall, lateral buds are released from inhibition and grow.
• Commercial applications: gibberellins in brewing (malting), fruit production; ethene in controlled fruit ripening during transport; ABA in agriculture.