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13.4.2 - The Discovery of Plant Growth Regulators

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Introduction to the Discovery of PGRs

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Teacher
Teacher

Today, we're going to explore how the world of plant growth regulators, or PGRs, was discovered. Can anyone name a phenomenon they know about where plants respond to light?

Student 1
Student 1

Isn't that phototropism? Plants grow towards the light?

Teacher
Teacher

Exactly! Charles Darwin and his son, Francis, observed this in canary grass. They inferred that a substance at the tips of the coleoptiles influenced this growth. What do you think they concluded?

Student 2
Student 2

That the tip of the coleoptile was sending signals to the rest of the plant?

Teacher
Teacher

Right! They were discovering what we now know as auxin. It's intriguing how many scientific discoveries emerge from simple observations.

Student 4
Student 4

So, who isolated auxin?

Teacher
Teacher

Great question! Auxin was isolated by F.W. Went from oat coleoptiles, confirming Darwin's idea. This shows how crucial these early experiments were in identifying PGRs.

Teacher
Teacher

Let’s summarize: Darwin's observations led to the start of understanding auxins, which play a critical role in plants growing towards light.

Gibberellins and Fungal Influence

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Teacher
Teacher

Now, let’s talk about gibberellins. Has anyone heard of the 'bakanae' disease?

Student 3
Student 3

I think it's related to rice plants, right?

Teacher
Teacher

Yes! It's caused by the fungus *Gibberella fujikuroi*. E. Kurosawa first linked this disease to gibberellic acid in 1926, when he showed that extracts from the fungus caused symptoms in rice seedlings. What might this mean for farming?

Student 1
Student 1

It could help us find treatments or prevent the spread of the disease?

Teacher
Teacher

Exactly! Understanding the role of gibberellins allows farmers to manage crops more effectively. This highlights the importance of research in agriculture.

Skoog's Contributions to PGRs

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Teacher
Teacher

Let’s shift gears to F. Skoog. Can anyone tell me what he discovered about plant growth?

Student 2
Student 2

He worked with tobacco plants and found out something about cell division?

Teacher
Teacher

That's right! Skoog noticed that callus tissue from tobacco only grew when paired with nutrients like yeast extract or coconut milk. This led to the identification of cytokinins. Why do you think cytokinins are important?

Student 4
Student 4

They probably help in promoting growth, right?

Teacher
Teacher

Yes! They are crucial for cell division and can help break apical dominance, allowing lateral buds to grow. Remember this connection!

Teacher
Teacher

In summary, Skoog's research was pivotal in broadening our understanding of plant hormones.

The Identification of Ethylene

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Teacher
Teacher

Next, let's discuss ethylene. Who can tell me what role it plays in plants?

Student 1
Student 1

I know it helps with fruit ripening!

Teacher
Teacher

Exactly! H.H. Cousins determined that ripened oranges released a gas that accelerated the ripening of bananas. What does this tell us about plant interactions?

Student 3
Student 3

It shows plants communicate chemically!

Teacher
Teacher

Absolutely! Ethylene is a powerful signaling molecule that influences many processes, including senescence and abscission. Let's remember how connected plant physiology is.

Introduction & Overview

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Quick Overview

The discovery of plant growth regulators unfolded through a series of accidental findings related to plant behavior and physiology.

Standard

Early investigations into plant reactions to light and disease led to the identification of five major groups of plant growth regulators, each with unique roles that influence plant growth and development. This section explores significant discoveries and the key researchers behind them, including Darwin, Kurosawa, and Skoog.

Detailed

The Discovery of Plant Growth Regulators

The discovery of plant growth regulators (PGRs) was primarily accidental, stemming from various observations and experiments by notable scientists. Charles Darwin and his son, Francis, noted that canary grass coleoptiles grew towards light, a phenomenon known as phototropism. Their findings indicated that the apex of the coleoptile was crucial for this movement, eventually leading to the isolation of auxin by F.W. Went from oat seed coleoptiles.

Another pivotal discovery involved the ‘bakanae’ disease affecting rice, which was associated with the fungus Gibberella fujikuroi. E. Kurosawa (1926) found that this fungal pathogen produced gibberellic acid, leading to the symptoms of the disease.

F. Skoog and his co-workers discovered that callus from tobacco stem segments only proliferated when auxins were supplemented with other nutrients, which paved the way for the identification of cytokinins, a group of PGRs known for promoting cell division. Eventually, researchers purified and characterized several growth inhibitors, which were identified as abscisic acid (ABA).

The final major PGR, ethylene, was recognized through experiments where H.H. Cousins confirmed that ripe oranges released this gas, which hastened the ripening of bananas. These discoveries laid a foundation for understanding the intricate control PGRs exert on plant growth and development.

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Audio Book

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The Initial Observation

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Interestingly, the discovery of each of the five major groups of PGRs have been accidental. All this started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused that tip of the coleoptile is the source of auxin.

Detailed Explanation

The discovery of plant growth regulators (PGRs) began with an unexpected observation made by Charles Darwin and his son. They noticed that the coleoptiles, which are the first leaves of grass seedlings, bent toward a light source. This bending, known as phototropism, indicated that something from the tip of the coleoptile was influencing growth. Their research led them to conclude that this tip secreted a substance responsible for the bending, which we now understand to be auxin, a key plant hormone.

Examples & Analogies

Think of plants as being like people living in a crowded city. Just as people might navigate through a crowd to reach a place of interest (like food or entertainment), plants 'navigate' toward light for photosynthesis. The tip of the coleoptile acts like a person at the front of the crowd, sensing where to go. This phenomenon demonstrates how plants adapt to their environment, much like we do.

Discovery of Gibberellins

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The ‘bakanae’ (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.

Detailed Explanation

The discovery of another major group of PGRs, known as gibberellins, arose from the study of 'bakanae' disease in rice. This disease leads to abnormal growth where seedlings become excessively tall and spindly. E. Kurosawa found that treating these seedlings with filtrates from the fungus causing the disease resulted in similar symptoms, indicating that the fungus produced a substance responsible for this growth. This substance was eventually identified as gibberellic acid, which plays a significant role in plant growth regulation.

Examples & Analogies

Imagine if a friend who always eats too much candy starts to grow taller than their peers, not because they are naturally taller but because something they consumed is influencing their growth. Similarly, gibberellins are like that 'something' in the fungus that makes rice seedlings grow abnormally tall.

Cytokinins Discovery

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F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins, the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Miller et al. (1955) later identified and crystallised the cytokinesis promoting active substance that they termed kinetin.

Detailed Explanation

Cytokinins were discovered during experiments that involved growing tobacco stem segments. Researchers noticed that if they supplied auxins with certain nutrient-rich extracts, the cells would proliferate into callus tissue—a growth of undifferentiated plant cells. Further work by Miller et al. resulted in the identification of kinetin, a substance that initiates cell division, which was crucial in understanding how plant growth can be stimulated.

Examples & Analogies

Think of growing a plant like baking a cake. Just as you need the right ingredients (like flour, sugar, and eggs) to make a cake rise properly, plants need specific substances like auxins and cytokinins to thrive and grow. Kinetin acts like a special ingredient that boosts the growth process.

Discovery of Abscisic Acid

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During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).

Detailed Explanation

In the mid-1960s, researchers discovered three different growth inhibitors while studying plant processes and found that they all shared the same chemical nature. This led to the identification of these substances as abscisic acid (ABA), which is critical for regulating processes such as seed dormancy and leaf abscission. The ability to purify and characterize these substances allowed scientists to better understand their roles in plant growth and stress responses.

Examples & Analogies

Consider how alarm systems can prevent unauthorized entries into a building. Abscisic acid acts like that alarm system for plants, signaling when it's time to pause growth during unfavorable conditions, like drought, thereby helping to protect the plant's resources.

Gaseous PGR - Ethylene

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H.H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.

Detailed Explanation

The discovery of ethylene, a gaseous PGR, emerged from observations of ripened oranges. H.H. Cousins noted that when ripe oranges emitted a gas, it sped up the ripening process of unripe bananas stored nearby. Scientists later identified this gas as ethylene, which plays a significant role in processes like fruit ripening and flower wilting.

Examples & Analogies

Imagine going into a kitchen where fresh cookies are baking. The smell (or 'volatile substance') invites everyone to the table—it creates excitement to eat! Just like that aroma, ethylene sends signals to nearby fruits to ripen, making them ready for harvest.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Discovery of PGRs: The initial discoveries were made through observations of plant behavior, leading to significant findings about plant hormones.

  • Auxins: First isolated by F.W. Went, auxins are crucial for plant growth and respond to light.

  • Gibberellins: Identified as a result of research into the 'bakanae' disease, gibberellins facilitate significant growth responses.

  • Cytokinins: Found during studies of tobacco cell growth, cytokinins promote cell division and influence shoot development.

  • Ethylene: Recognized for its role in fruit ripening, ethylene is a crucial gaseous hormone influencing plant growth.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Darwin's observation of the bending of coleoptiles towards light led to the isolation of auxin.

  • The discovery of gibberellins resulted from studying the effects of a fungus on rice seedlings, leading to treatments for crop diseases.

  • Skoog's findings highlighted the importance of cytokinins when he observed their role in promoting callus growth in tobacco.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • In the dark the auxins play, growing plants the light's own way.

📖 Fascinating Stories

  • Once upon a time, a curious scientist named Darwin observed a plant leaning towards the sunlight. He discovered that a special substance at the tip of the plant helped it find its way—a substance named auxin. This led to a deeper adventure into plant growth.

🧠 Other Memory Gems

  • Remember 'A Giant Can Eat' to memorize PGRs: Auxin, Gibberellin, Cytokinin, Ethylene.

🎯 Super Acronyms

PGR - Plant Growth Regulators

  • Perfect Growth Releases (the essence of what they do).

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Auxin

    Definition:

    A class of plant hormones that promote cell elongation and are involved in growth responses.

  • Term: Gibberellin

    Definition:

    A group of hormones that promote stem elongation, seed germination, and flowering.

  • Term: Cytokinin

    Definition:

    Plant hormones that promote cell division and growth in plants.

  • Term: Ethylene

    Definition:

    A gaseous plant hormone associated with fruit ripening and senescence.

  • Term: Abscisic Acid (ABA)

    Definition:

    A plant hormone that regulates various processes including seed dormancy and stress responses.