Introduction to Plant Coordination
In our previous chapters, we looked at how animals use nerves and hormones to stay in control. But what about plants? They don't have a brain or a nervous system, yet they can still "sense" the world around them. They grow toward light, roots grow toward gravity, and seeds know exactly when to sprout. This is all thanks to plant hormones (also called plant growth regulators) and special light-sensitive pigments called phytochromes. In this chapter, we will explore how plants use these chemicals to coordinate their growth and development.
Plant Hormones: IAA and Gibberellins
Plants use chemical messengers to communicate between cells. Unlike animal hormones which are made in specific glands, plant hormones are often produced in growing tissues like the tips of roots and shoots.
Auxin (IAA)
IAA (Indoleacetic Acid) is the most common type of auxin. It is primarily responsible for cell elongation. In the shoots, IAA moves to the shaded side of the plant, causing those cells to grow longer than the cells on the light side. This uneven growth causes the plant to bend toward the light.
How IAA works:
1. IAA is produced in the apex (tip) of the shoot.
2. It moves down the stem by diffusion and active transport.
3. It stimulates the pumping of hydrogen ions into the cell walls.
4. This makes the cell walls flexible and stretchy, allowing the cell to take in water and expand.
Gibberellins
Gibberellins are another group of hormones that play a massive role in stem elongation and seed germination. If a plant lacks gibberellins, it often stays "dwarf."
Analogy: If IAA is like a "stretching" coach for individual cells, Gibberellin is like the "growth spurt" signal for the whole plant stem.
Key Takeaway: IAA focuses on directional growth and stretching cells, while Gibberellins focus on overall height and "waking up" seeds to start growing.
Phytochromes: Sensing Light
Plants don't have eyes, but they have phytochromes. These are pigments that act like biological "light switches." They help the plant tell whether it is day or night, and how long the day is (which helps them decide when to flower).
The Two Forms of Phytochrome
Phytochromes exist in two states that can flip back and forth:
1. \(P_r\): Absorbs Red light (found in daylight).
2. \(P_{fr}\): Absorbs Far-red light (found in shade or at sunset).
The "Switching" Process:
- When \(P_r\) absorbs red light, it quickly converts into \(P_{fr}\).
- When \(P_{fr}\) absorbs far-red light (or is left in the dark for a long time), it slowly converts back into \(P_r\).
- Daytime: There is lots of red light, so the plant accumulates \(P_{fr}\).
- Nighttime: The \(P_{fr}\) slowly turns back into \(P_r\).
Don't worry if this seems tricky at first! Just remember: \(P_{fr}\) is the active form that usually triggers a biological response, like flowering or seed germination. The plant uses the ratio of \(P_r\) to \(P_{fr}\) to "calculate" the length of the night.
How Hormones Control Genes
You might wonder: "How does a tiny chemical like IAA actually make a plant grow?" The answer lies in transcription.
Both phytochromes and plant hormones like IAA and gibberellins can act as transcription factors or activate them. These are proteins that bind to DNA and "turn on" or "turn off" specific genes.
The Process:
1. The hormone enters the cell or binds to a receptor.
2. This triggers a signal that moves into the nucleus.
3. A transcription factor is activated.
4. This factor binds to a promoter region on the DNA.
5. mRNA is produced (transcription), which is then translated into proteins (like enzymes) that cause the growth response.
Comparison: Nervous vs. Hormonal Coordination
In Unit 5, you are expected to understand the differences between how animals and plants coordinate their bodies. While we have both systems, plants rely entirely on the hormonal style.
Nervous Coordination:
- Transmission: Electrical impulses along neurones.
- Speed: Very rapid (milliseconds).
- Duration: Short-lived response.
- Targeting: Very specific (to one muscle or gland).
Hormonal Coordination (Plants and Animals):
- Transmission: Chemicals moving through transport tissues (like phloem) or diffusion.
- Speed: Slower.
- Duration: Longer-lasting effects (like growth).
- Targeting: Can affect the whole organism or multiple tissues at once.
Core Practical 18: Amylase in Germinating Seeds
This practical investigates how gibberellins trigger the production of amylase in cereal grains (like barley or wheat).
The Biology:
Inside a seed, there is a food store called the endosperm (mostly starch). To grow, the embryo needs sugar. When the seed absorbs water, it releases gibberellins. These hormones travel to a layer called the aleurone layer and switch on the gene for the enzyme amylase. Amylase then breaks down starch into glucose for growth.
The Investigation:
1. Seeds are soaked in different concentrations of gibberellic acid.
2. The seeds are placed on starch agar plates.
3. After a period of time, the seeds are removed and the plates are flooded with iodine.
4. Observation: A clear zone (where the starch has been digested) will appear around the seed. The larger the clear zone, the more amylase was produced.
5. Variables: You must control the temperature and the soaking time of the seeds.
Common Mistake: Students often forget that it is the gibberellin that causes the transcription of the amylase gene. Without the hormone, the starch stays as starch, and the seed stays dormant!
Summary and Key Takeaways
- IAA: Responsible for cell elongation and phototropism.
- Gibberellins: Responsible for stem growth and initiating seed germination by activating amylase.
- Phytochromes: Light-sensitive pigments (\(P_r\) and \(P_{fr}\)) that control flowering and germination based on light quality and day length.
- Gene Expression: Plant hormones work by activating transcription factors to turn specific genes on or off.
- Coordination: Hormonal coordination is generally slower and longer-lasting than nervous coordination.