Control and coordination in plants

Plants, unlike animals, are largely anchored organisms that lack a nervous system and muscle tissue. Despite these differences, they possess sophisticated communication systems that allow them to detect changes and coordinate responses to their internal and external environments, which is essential for their survival. Plant responses are generally less dramatic and often involve growth movements rather than rapid whole-body movements.

Plant coordination relies primarily on two main types of communication:

Chemical Communication: Plant Growth Regulators (Hormones)

Plant growth regulators are chemical messengers that influence plant growth and development. Unlike animal hormones, they are not produced in specialized glands but in various tissues, particularly growing regions like shoot and root tips, and can have profound effects even at very low concentrations. They move by diffusion and active transport over short distances, and via the phloem or xylem for longer distances. Plant growth regulators can interact synergistically (reinforcing effects) or antagonistically (opposing effects).

Key plant growth regulators include:

• Auxins (e.g., Indoleacetic acid - IAA):

  • Production: Mainly synthesized in the growing tips (meristems) of shoots and roots, and young leaves.

  • Mechanism of Action: Auxins stimulate cell elongation by promoting proton pumps in the cell surface membrane to move hydrogen ions (H+) into the cell wall. This acidification loosens the bonds between cellulose microfibrils and the cell wall matrix, allowing the cell walls to stretch as cells absorb water by osmosis and increase turgor pressure.

  • Effects:

    • Phototropism: Auxin moves to the shaded side of shoots and roots, causing cells on the shaded side of shoots to elongate more, bending the shoot towards light. In roots, high auxin concentration on the shaded side inhibits growth, causing the root to bend away from light.

    • Gravitropism (Geotropism): Auxin moves to the underside of shoots and roots. In shoots, this stimulates elongation, causing upward growth. In roots, high auxin concentration on the underside inhibits growth, causing downward growth.

    • Apical Dominance: Auxins produced in the apical bud inhibit the growth of lateral buds, promoting upward growth.

    • Other roles include promoting fruit growth and inhibiting leaf fall.

• Gibberellins (GAs):

  • Production: Synthesized in the embryos of seeds and in young leaves.

  • Effects:

    • Stem Elongation: Gibberellins stimulate extension growth of stems. This involves activating enzymes (like XET) in cell walls that loosen bonds between cellulose microfibrils. Dwarf plants, for example, may have a recessive allele (le) that codes for a non-functional enzyme in the gibberellin-synthesis pathway, making them unable to produce active gibberellin (GA1). Applying active gibberellin can make these dwarf plants grow tall.

    • Seed Germination: Gibberellins play a crucial role in breaking seed dormancy and initiating germination. In germinating seeds (e.g., barley), gibberellins activate the synthesis of amylase enzymes. These enzymes break down stored starch into soluble maltose, providing energy and raw materials for the growing embryo. The mechanism involves gibberellin causing the breakdown of DELLA proteins, which normally inhibit transcription factors responsible for amylase gene expression.

• Abscisic Acid (ABA):

  • Production: Known as a "stress hormone," ABA is produced in mature leaves, ripe fruits, seeds, and any cells containing chloroplasts or amyloplasts, particularly under water stress (e.g., drought conditions).

  • Effects: Its primary role is to trigger stomatal closure. This conserves water by reducing transpiration. The mechanism involves ABA binding to receptors on guard cells, which inactivates proton pumps, causing calcium ions (Ca2+) to act as a second messenger. This leads to the efflux of potassium ions and subsequent water loss from guard cells, making them flaccid and closing the stomatal pore. ABA is also involved in inducing bud and seed dormancy.

Electrical Communication in Plants

While plants lack a nervous system, their cells have electrochemical gradients and resting potentials similar to animal cells. Some plant responses are coordinated by action potentials. These plant action potentials are triggered by membrane depolarization, but unlike animal neurones, depolarization can result from the outflow of negatively charged chloride ions (Cl-) (rather than Na+ influx), with repolarization achieved by potassium ion (K+) outflow. These electrical signals travel along cell membranes and through plasmodesmata (cytoplasmic channels between cells), though they are generally slower and last longer than animal nerve impulses.

• Examples: Action potentials can be triggered by various stimuli such as touch, chemicals (e.g., acid, insect feeding), and are thought to coordinate responses to damage and stress.

• Venus Fly Trap: A notable example is the rapid closure of the Venus fly trap (Dionaea muscipula), a carnivorous plant. The trap's leaves have stiff, sensitive hairs. If two hairs are stimulated within 20-35 seconds, action potentials are generated, causing the leaf lobes to quickly change shape and close. This mechanism prevents false closures from rain or small debris, as small insects can escape. After trapping, calcium ions are involved in the exocytosis of vesicles containing digestive enzymes.

Tropic Responses

Tropisms are plant growth responses to directional stimuli. Growth towards the stimulus is a positive tropism, and growth away is a negative tropism.

• Phototropism: The growth of a plant in response to light. Shoots are positively phototropic, growing towards light to maximize light absorption for photosynthesis. Roots are typically negatively phototropic, growing away from light.

• Gravitropism (Geotropism): The growth of a plant in response to gravity. Shoots are negatively gravitropic, growing upwards. Roots are positively gravitropic, growing downwards into the soil for anchorage, water, and mineral ions.

• Mechanism: Tropic responses are often explained by the uneven distribution of auxins within the plant, leading to differential growth rates.

These coordinated responses allow plants to optimally position their organs for essential processes like photosynthesis and nutrient absorption, and to adapt to changing environmental conditions.