Control and Coordination · Lesson 9 of 11
Chemical Communication and Plant Hormones
“Plant hormones quietly manage growth without holding a single meeting.”
- Compare electrical and chemical communication. - Explain why multicellular organisms require chemical messengers. - Describe how plant hormones reach their sites of action. - Explain auxin redistribution and phototropic bending. - State the functions of auxins, gibberellins, cytokinins and abscisic acid. - Distinguish growth-promoting and growth-inhibiting plant hormones.
Electrical impulses are excellent for urgent messages, but a growing organism also needs instructions that are widespread, gradual and persistent. A plant may need to lengthen a stem, divide cells in a developing seed or slow growth during unfavourable conditions. Chemical messengers are well suited to this kind of coordination.
Limitations Of Electrical Impulses
Electrical impulses travel rapidly, but in animals they normally reach only cells connected through nervous pathways. In addition, after generating an impulse, a nerve cell requires time to restore the conditions needed for another impulse. Electrical signalling is therefore not ideal for continuously instructing every cell in a growing tissue.
Chemical Communication
A stimulated cell can release a chemical compound that spreads through nearby tissue or is carried to distant regions. Only cells with suitable receptor molecules respond strongly. Chemical communication is generally slower than a nervous impulse, but it can influence many cells, continue over a longer period and coordinate processes such as growth and development.
A chemical messenger produced in one part of an organism that acts on responsive cells elsewhere to coordinate growth, development or physiological activity.
Plant hormones are produced in particular growing or active regions and move to their sites of action, often by diffusion or through transport tissues. Their effects depend on the hormone, its concentration, the receiving tissue and the developmental state of the plant.
| Feature | Electrical communication | Chemical communication |
|---|---|---|
| Speed | Very rapid | Usually slower |
| Path | Along connected excitable cells | Can spread or be transported to many responsive cells |
| Duration | Usually brief | May be sustained |
| Best suited for | Immediate responses | Growth, development and widespread adjustment |
Auxin
Auxin is produced mainly near shoot tips and promotes cell elongation in suitable shoot tissues. When light reaches a shoot from one side, auxin becomes more concentrated on the shaded side. Cells there elongate more than cells on the illuminated side. Because one side becomes longer, the shoot curves towards the light.
The same principle of unequal growth helps explain tendril coiling: contact leads to different growth rates on opposite sides. It is important not to say that auxin pulls the plant towards light. Auxin changes growth rate; the geometry of unequal elongation produces bending.
Gibberellins
Gibberellins promote growth of stems. By encouraging elongation in responsive tissues, they contribute to an increase in stem length. Their effect is part of coordinated development rather than an isolated command to every cell.
Cytokinins
Cytokinins promote cell division. They are therefore found in greater concentration in regions where cells divide actively, including developing fruits and seeds. Cell division increases cell number, while hormones such as auxin and gibberellins influence patterns of enlargement and elongation.
Abscisic Acid
Plants also require signals that restrict growth. Abscisic acid inhibits growth and is associated with responses such as wilting of leaves. Growth inhibition is not a failure of coordination; it can help a plant adjust its activities when continued rapid growth would be unsuitable.
| Plant hormone | Main effect emphasised here | Growth role |
|---|---|---|
| Auxin | Cell elongation and directional bending | Promotes growth |
| Gibberellins | Stem growth | Promotes growth |
| Cytokinins | Cell division in active regions | Promotes growth |
| Abscisic acid | Growth inhibition and wilting-related responses | Inhibits growth |
Problem
A developing tissue shows rapid cell division. Which plant hormone is most directly associated with this process?
- 1.The key observation is rapid cell division.
- 2.Cytokinins promote cell division.
- 3.Therefore cytokinin activity is the most direct association among the hormones considered.
Problem
Light reaches a young shoot from the left. Explain the resulting bend.
- 1.The right side is shaded.
- 2.Auxin becomes more concentrated on the shaded right side.
- 3.Right-side cells elongate more than left-side cells.
- 4.The longer right side makes the shoot curve left, towards the light.
Problem
A substance blocks auxin movement from the shoot tip while the shoot receives one-sided light. Predict the result.
- 1.The tip may still detect the directional light.
- 2.Blocking auxin movement prevents the normal shaded-side concentration pattern from reaching elongating cells.
- 3.The difference in elongation across the shoot is reduced.
- 4.Phototropic bending is therefore weakened or absent even though light is present.
Hormones do not act as food and auxin does not physically drag a shoot. Hormones are information-carrying chemicals; responsive cells alter growth after detecting them.
Quiz
Which plant hormone promotes cell division?
Why is chemical communication useful for growth?
Where does auxin accumulate when light reaches a shoot from one side?
Which hormone inhibits growth?
What directly causes a shoot to curve towards light?
Practice Problems
- Give two limitations of electrical impulses as a general coordinating system. Model answer: They mainly reach cells connected through nervous tissue, and an activated cell needs recovery time before producing another impulse.
- Explain why chemical communication is slower yet useful. Model answer: Chemicals move by diffusion or transport rather than rapid conduction, but they can reach many responsive cells and act steadily for longer periods.
- Explain phototropic bending through auxin. Model answer: One-sided light produces greater auxin concentration on the shaded side; those cells elongate more, so unequal growth curves the shoot towards light.
- Compare gibberellins and cytokinins. Model answer: Gibberellins promote stem elongation, while cytokinins promote cell division, especially in rapidly growing tissues.
- Why is a growth inhibitor useful to a plant? Model answer: Coordination requires both promotion and restriction. Inhibiting growth can conserve resources or produce suitable responses under unfavourable conditions.
Key Takeaways
• Electrical signals are rapid but limited to connected excitable pathways and brief transmission. • Chemical messengers are slower but can coordinate many cells persistently. • Plant hormones are produced in one region and act on responsive cells elsewhere. • Auxin promotes elongation and its unequal distribution produces phototropic bending. • Gibberellins promote stem growth and cytokinins promote cell division. • Abscisic acid inhibits growth and contributes to wilting-related responses. • Growth promotion and growth inhibition are both necessary forms of coordination.