Control and Coordination in Plants (9700 A Level)

Hello future Biologists! Welcome to one of the most interesting topics: how plants control their lives without a nervous system. While animals use fast electrical signals (nerves) and hormones (endocrine system), plants rely almost entirely on chemical signals (hormones, often called plant growth regulators) to coordinate growth, movement, and development.

Don't worry, even though the processes seem abstract, we will break down the key mechanisms—from fast movements in carnivorous plants to the precise molecular control of growth and germination. Let's get started!

1. Rapid Movement: The Venus Fly Trap (Dionaea muscipula)

When we think of plant movement, we usually think of slow growth towards light (tropisms). However, the Venus fly trap demonstrates remarkably fast coordination, even without nerves!

Mechanism of Trap Closure (The Electric Snap)

The Venus fly trap has two modified leaf lobes that form a trap. Along the inner surface of these lobes are sensitive trigger hairs (or touch receptors).

The closure mechanism is a rapid response mediated by an electrical signal, followed by quick changes in water pressure (turgor).

  1. Stimulation: A touch (usually two contacts within about 20–40 seconds) stimulates the trigger hairs.
  2. Electrical Signal: This mechanical stimulus generates an electrical signal, similar to an animal's action potential (though slower and hydraulic-based), which travels across the leaf lobes.
  3. Turgor Change in Motor Cells: The electrical signal causes specialised cells along the middle rib (the hinge) of the trap, known as motor cells, to rapidly and almost simultaneously lose turgor.
    Did you know? This rapid turgor loss is primarily due to the active transport of ions (like \(K^+\)) out of the motor cells, followed quickly by water via osmosis.
  4. Trap Closure: When the motor cells lose water, they shrink. This sudden change in shape and rigidity causes the pre-stressed lobes (which are naturally curved outward) to snap shut instantly, trapping the prey.

Analogy: Think of the Venus fly trap like a plastic snap-bracelet (slap bracelet) that is held in a strained, curved shape. A quick stimulus (the ion/water movement) releases the strain, and the physical structure rapidly flips into a new stable shape (the closed trap).

Key Takeaway for Venus Fly Trap: Rapid response is achieved through an electrical signal triggering massive, sudden shifts in turgor pressure, not muscle contraction.

2. Auxin and Cell Elongation (The Acid Growth Hypothesis)

Auxin (specifically Indoleacetic acid, IAA) is a plant hormone critical for tropisms (growth responses to external stimuli like light or gravity). It promotes the irreversible stretching and elongation of plant cells.

Auxin's Role in Elongation Growth

Auxin facilitates elongation by making the cell wall more plastic (able to stretch). This is explained by the Acid Growth Hypothesis:

Step-by-step process of Auxin action:

  1. Binding and Activation: Auxin molecules bind to specific receptor proteins located on the cell surface membrane of target cells (e.g., in the shoot tip).
  2. Proton Pumping: This binding stimulates the activation of proton pumps (\(H^+\) pumps) embedded in the membrane.
    Important: These proton pumps use ATP to actively pump \(H^+\) ions (protons) out of the cytoplasm and into the cell wall (the region outside the plasma membrane, called the apoplast).
  3. Acidification: The accumulation of \(H^+\) ions lowers the pH of the cell wall material to approximately 4.5–5.0. This is the "acid growth" step.
  4. Wall Loosening: The low pH activates specialised enzymes (often called expansins) present in the cell wall. These enzymes weaken the structure by breaking or loosening the hydrogen bonds between the cellulose microfibrils and other polymers.
  5. Turgor-Driven Extension: Because the plant cell is turgid (high internal pressure) due to water uptake (osmosis), the now-loosened cell wall is unable to resist this internal pressure, leading to irreversible cell elongation.

Simple Trick: Remember the four 'A's of Auxin:
Auxin $\rightarrow$ Activates pump $\rightarrow$ Acidifies wall $\rightarrow$ Allows extension.

Accessibility Note: This is a great example of signal transduction. The hormone (Auxin) is the first messenger, which leads to a sequence of events (like activating the proton pump and enzymes) inside the cell that results in the final response (growth).

Key Takeaway for Auxin: Auxin promotes growth by stimulating proton pumping, which acidifies the cell wall, activating enzymes that loosen cell wall structure, allowing the cell to expand due to turgor pressure.

3. Gibberellin (GA) and Barley Germination

Gibberellins (GA) are another class of plant hormones, most famous for promoting stem elongation (alongside auxin) and, crucially for this syllabus, stimulating seed germination. We specifically look at the process in a barley grain.

The Role of Gibberellin in Barley Germination

The barley grain contains an embryo (the tiny plant), a large starch-rich food supply (the endosperm), and a surrounding tissue layer called the aleurone layer.

  1. Imbibition: The dormant barley seed absorbs water (imbibition).
  2. GA Production: The embryo is activated by the water and begins to synthesise and secrete gibberellin.
  3. Target Action: Gibberellin travels to the aleurone layer (the outer layer of the endosperm).
  4. Gene Activation: GA acts as a signal to the aleurone cells, stimulating the transcription and translation of genes responsible for synthesising digestive enzymes, primarily amylase.
  5. Starch Breakdown: Amylase is secreted from the aleurone layer into the endosperm, where it hydrolyses (breaks down) the stored starch into soluble sugars (maltose and glucose).
  6. Nutrient Supply: These soluble sugars are transported to the growing embryo, providing the energy and materials needed for cell division and growth (germination).
Connecting Gibberellin to Gene Control (Syllabus Link 16.3.4)

How does gibberellin actually switch on the amylase genes? This links directly to the concept of gene regulation in eukaryotes.

GA achieves its effect by removing the chemical "brakes" on germination:

  • The Brake: Genes that promote growth and amylase production are normally inhibited (switched off) by specific repressor proteins known as DELLA proteins.
  • GA Action (Releasing the Brake): When GA is present, it causes the breakdown of these DELLA protein repressors.
  • Activation: With the DELLA repressors removed, transcription factors that promote growth can now bind to the DNA, initiating the transcription of genes, such as the gene for amylase.
The Role of the Dominant Allele (Le vs. le)

The effect of gibberellin on stem elongation (plant height) is controlled by genes.

  • Dominant allele (Le): This allele codes for a functional enzyme necessary for the synthesis pathway of gibberellin. Since functional GA is produced, the plant grows tall (stem elongation).
  • Recessive allele (le): This allele codes for a non-functional enzyme in the GA synthesis pathway. Very little functional gibberellin is produced, resulting in reduced stem elongation and a dwarf phenotype.

This illustrates a fundamental principle: a gene codes for a protein (in this case, an enzyme), and the function (or lack thereof) of that protein directly determines the physical characteristic (phenotype).

Key Takeaway for Gibberellin: GA breaks seed dormancy by signalling the aleurone layer to produce amylase (via removing DELLA repressors). Genetically, the Le allele determines tall growth by producing a functional enzyme for GA synthesis.