Plant Hormones (OCR A-Level Biology A): Revision Notes

Plant Hormones

Introduction to plant hormones

Plants produce chemical messengers that regulate various aspects of growth and development. These substances are called plant hormones or plant growth regulators.

Plant hormones coordinate responses including:

• Tropisms (directional growth responses)
• Leaf abscission
• Stomatal closure
• Seed germination
• Flowering and fruit formation
• Fruit ripening

Auxins

The phototropic mechanism

Phototropism is coordinated by the plant hormone indole-3-acetic acid (IAA), which belongs to a group of hormones called auxins. Auxins influence gene expression by switching certain genes on and others off.

IAA is synthesised in the meristem and moves down the stem to stimulate extension growth. It works by activating proteins called expansins in the cell wall. Expansins increase cell wall flexibility by loosening bonds between cellulose fibres, enabling shoots to elongate.

The precise mechanism of auxin transport across the tip remains under investigation. However, genetic studies on Arabidopsis thaliana have identified specialised channel proteins that appear to transport auxin across cell membranes.

Experimental evidence for auxin redistribution

Geotropism in shoots and roots

Negative geotropism in plant stems results from auxin accumulation on the lower side. H.E. Dolk demonstrated this by growing coleoptiles horizontally until they showed a geotropic response. He then decapitated them and placed the tips laterally on agar blocks divided by a razor blade.

Measurements revealed significantly more auxin in the lower block than the upper block. Gravity appears to modify auxin distribution, causing accumulation on the lower side and increasing growth rate there.

Apical dominance

Auxins produced at the apex (growing point) of a plant stem not only promote upward growth but also inhibit lateral bud development. This phenomenon is called apical dominance.

Experimental evidence demonstrates this effect: removing the apical bud allows lateral buds to grow, but immediately replacing the cut tip with an auxin-containing agar block restores lateral bud inhibition.

Gibberellins

Gibberellins are another group of plant hormones involved in growth regulation. They are produced in young leaves, seeds, and root tips.

Functions of gibberellins include:

• Stimulating seed germination
• Promoting stem elongation
• Triggering flowering

Dwarf plant varieties have very low gibberellin levels. Treating them with gibberellins causes them to grow to normal height, demonstrating the hormone's role in plant stature.

The graph shows that gibberellin-treated dwarf pea plants reached approximately $88\text{ mm}$ after $8$ days, whilst control plants only achieved around $64\text{ mm}$ – demonstrating the significant growth-promoting effect of gibberellins.

Hormones and leaf loss

Deciduous plants shed leaves during hot, dry conditions to reduce water loss. In temperate climates, leaf loss also occurs in winter when water absorption from frozen soils is difficult and photosynthesis is limited by low temperatures and reduced light.

The hormone ethene stimulates cell wall breakdown in the abscission layer, causing leaf detachment. Auxins also influence leaf loss – they normally inhibit abscission and are produced by young leaves, making leaf stalks insensitive to ethene. As leaves age, auxin concentration decreases, permitting ethene to trigger leaf loss.

Hormones and stomatal closure

Abscisic acid (ABA) regulates stomatal closure by guard cells. When soil dries out (decreased soil water potential), roots produce ABA which translocates to leaves, affecting guard cells. Guard cells lose water and turgor, causing stomata to close.

Hormones and seed germination

Seed germination is a complex process involving food reserve mobilisation and growth promotion. Environmental factors like temperature and water availability are important, but many seeds possess dormancy mechanisms preventing germination even under favourable conditions.

Dormancy is a state where seeds will not germinate (despite suitable conditions) until receiving a specific environmental stimulus. Several plant hormones regulate germination, with gibberellins, abscisic acid, and auxins being most important.

Role of gibberellins

Gibberellins stimulate germination and break dormancy. Evidence includes:

• Mutant Arabidopsis strains producing no gibberellins can germinate if treated with gibberellins
• Grand Rapids lettuce seeds normally requiring light will germinate in darkness when treated with gibberellins
• In cereals (wheat and barley), gibberellins from the embryo travel to the aleurone layer (cells surrounding the endosperm), causing it to produce amylase

Amylase breaks down stored starch into sugar, providing energy for embryo growth and breaking dormancy.

The hormonal balance

Commercial use of plant hormones

Since hormones control plant growth and development, they have numerous commercial applications:

Auxins as selective weed killers: Although auxins normally promote growth, high concentrations cause excessively rapid growth that distorts tissues (especially roots), creating damage and pathogen entry points. Synthetic auxins like $2,4\text{-D}$ are used at concentrations $100$ times higher than natural levels. This is particularly useful for cereal fields and lawns because grasses are far less sensitive than broadleaved weeds, so the treatment selectively kills weeds whilst leaving grass unharmed.