Plant Responses: Introduction to Plant Hormones | A-level Biology | OCR, AQA, Edexcel

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The key points covered of this video include:

1. Apical Dominance
2. The Role of Plant Hormones in Leaves
3. Gibberellins and Stem Elongation
4. Gibberellins and Seed Germination

Apical Dominance

We previously saw that auxins are a type of plant growth factor which control phototropism and geotropism in plant shoots and roots. Auxin also plays a role in inhibiting the growth of side branches from lateral buds present further down the shoot. This inhibition of lateral buds is called apical dominance. It is thought apical dominance occurs because high auxin levels cause there to be high levels of the hormone abscisic acid in the shoot. Abscisic acid inhibits the growth of buds and therefore contributes to apical dominance. It is also thought that apical dominance occurs because high levels of auxin keep the levels of cytokinins in the shoot low. Cytokinins are chemicals which promote bud growth therefore if there are no cytokinins it is more difficult for lateral buds to grow.

The Role of Plant Hormones in the Leaves

In cold climates, deciduous plants shed their leaves in autumn to reduce transpiration and the loss of water in the winter. Cytokinins normally delay leaf senescence (aging) because cytokinins help maintain the nutrient supply in the leaves. Auxins present in leaves also inhibit the leaf abcission (the falling of leaves) In the winter, cytokinin and auxin levels in the leaves decrease which causes leaf senescence and leaf abscission. When there is low water availability, abscisic acid also causes the stomata in the leaves to close. This further reduces the loss of water through transpiration.

Gibberellins in Stem Elongation

Gibberellins are plant hormones which control stem elongation and seed germination in plants. A fungus in Japan causes a disease which causes rice plants to grow very tall. This is because the fungus secretes a type of gibberellin called gibberellic acid. Gibberellic acid can be used to make dwarf varieties of plants grow taller - suggesting it is responsible for plant stem growth. GA1, a type of gibberellin, is directly responsible for causing stem elongation. It has been discovered that GA1 is formed from the conversion of another type of gibberellin called GA20. This process was discovered to be catalysed by an enzyme encoded by the Le gene.

Gibberellins in Seed Germination

Gibberellin is also released by the plant embryo during seed germination. When the seed absorbs water it causes the embryo to release gibberellin. The gibberellin enables the production of the enzyme amylase which can break down starch to glucose. This allows glucose to be used for respiration so the seed embryo can grow. The glucose can also be used by the seed for protein synthesis - also allowing it to grow.

Summary

Plant hormones coordinate plant responses to external stimuli
The plant hormone auxin is present in the tip of plant shoots and helps control plant growth
Auxin promotes the growth of plant shoots but inhibits the growth of lateral buds below the shoot - this inhibition is called apical dominance
Abscisic acid also inhibits the growth of lateral buds whereas cytokinins promote the growth of lateral buds
Cytokinins prevent leaf senescence and auxins prevent leaf abscission
In deciduous plants the levels of cytokinins and auxins in the leaves decrease in the winter, causing leaves to be shed
Gibberellins promote stem elongation in plants
Gibberellins also promote seed germination because they enable the production of amylase
Amylase breaks starch down to glucose which can then be used in respiration by the seed embryo