Auxins
Who discovered auxins and How?
Auxins were discovered by Charles Darwin and his son Francis Darwin. They conducted experiments in the late 1800s and early 1900s that led to the discovery of auxins. They observed that when a plant is illuminated from one side, it tends to grow towards the light source. They hypothesized that there must be a growth-promoting substance produced in the tip of the plant that causes this bending, which they called auxin. Their work laid the foundation for our understanding of plant hormones and their role in plant growth and development.
Natural Auxins:
Auxins naturally occur in various parts of plants, including the apical meristems (tips of stems and roots), young leaves, developing seeds, and fruits. In plants, auxins are primarily produced in specialized cells called the apical meristems and are transported to other parts of the plant through vascular tissues, such as the phloem.
Auxins can also be found in certain fungi, bacteria, and algae. However, in the context of plant growth and development, the term "auxins" usually refers to the group of plant hormones that regulate various physiological processes, such as cell elongation, root development, apical dominance, and phototropism. These naturally occurring auxins include indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and naphthaleneacetic acid (NAA), among others.
It's important to note that while auxins are naturally occurring in plants, they can also be synthesized artificially and used in agricultural practices to manipulate plant growth and development.
Artificial Auxins:
Artificial auxins, also known as synthetic auxins or auxin-like compounds, are human-made substances that mimic the effects of naturally occurring auxins in plants. These synthetic auxins are often used in agricultural practices and horticulture for various purposes, such as promoting root formation, preventing premature fruit drop, and controlling the growth of weeds.
One example of an artificial auxin is 2,4-D (2,4-dichlorophenoxyacetic acid), which is widely used as a selective herbicide to control broadleaf weeds in crops. Another commonly used synthetic auxin is IBA (indole-3-butyric acid), which is used in rooting powders and gels to stimulate root growth in plant cuttings.
Artificial auxins can be formulated into different formulations, such as sprays, powders, and liquids, for ease of application. However, it's important to use these synthetic auxins according to the specified guidelines and regulations to ensure their safe and effective use without harming the environment or non-target plants.
The key functions of auxins in plants:
Auxins play a crucial role in various aspects of plant growth and development. Here are some of the key functions of auxins in plants:
1. Cell elongation: Auxins promote elongation of plant cells, especially in stems and coleoptiles (protective sheaths around emerging leaves). By loosening the cell wall, auxins facilitate cell expansion and contribute to overall plant growth.
2. Apical dominance: Auxins produced in the apical meristems (tips of stems and roots) inhibit the growth of lateral buds, promoting the dominance of the main shoot or terminal bud. This ensures upward growth and prevents the excessive branching of the plant.
3. Root development: Auxins are involved in root initiation and development. They promote the formation of lateral roots from the main root and enhance root growth and branching.
4. Phototropism: Auxins are responsible for phototropism, which is the directional growth of plants in response to light. In response to light, auxins accumulate on the shaded side of the stem, causing cell elongation and bending towards the light source.
5. Fruit development: Auxins contribute to the development and ripening of fruits. They promote fruit growth, seed development, and the production of enzymes that aid in fruit ripening.
6. Tropisms: Apart from phototropism, auxins are also involved in other tropisms, such as gravitropism (response to gravity) and thigmotropism (response to touch). They regulate the bending and growth of plant parts in response to these environmental stimuli.
7. Leaf abscission: Auxins inhibit the abscission or shedding of leaves. They help maintain leaf attachment to stems by regulating the production of enzymes that break down cell walls during leaf detachment.
It's important to note that the functions of auxins can vary depending on the concentration and distribution within the plant. Additionally, other plant hormones and environmental factors also influence the overall response and effects of auxins in plants.
Distribution of Auxins in plants:
Auxins are primarily synthesized in the apical meristems, which include the shoot apical meristem (SAM) located at the tip of the stem and the root apical meristem (RAM) located at the tip of the root. From these meristems, auxins are transported throughout the plant body, leading to their distribution in various plant tissues and organs.
The transport of auxins in plants occurs through two main pathways: polar transport and non-polar transport.
1. Polar transport: This type of auxin transport occurs primarily in the vascular tissues of plants, namely the xylem and phloem. In polar transport, auxins move in a directional manner from the apical meristems towards the other parts of the plant. This transport is facilitated by specific carrier proteins called PIN proteins. These proteins regulate the efflux of auxins from cells, controlling their movement and distribution. Polar transport of auxins allows for the coordinated growth and development of plant organs, such as roots, stems, and leaves.
2. Non-polar transport: Non-polar auxin transport refers to the diffusion and movement of auxins across cells and tissues, independent of the vascular system. This type of transport allows auxins to spread laterally within plant tissues and is important for local growth responses, such as cell elongation and tropic responses.
The distribution of auxins within plant organs and tissues is influenced by several factors, including:
- Gravitropism: Gravity affects the distribution of auxins in plant roots and shoots. In response to gravity, auxins accumulate on the lower side of roots (positive gravitropism) and on the upper side of shoots (negative gravitropism), influencing their growth and development.
- Phototropism: Light also plays a role in the distribution of auxins. In response to light, auxins typically accumulate on the shaded side of stems, causing cell elongation and bending towards the light source.
- Sink-source dynamics: The demand for auxins in various plant organs, such as growing tips, developing leaves, and developing fruits, affects their distribution. Auxins tend to move from source tissues (which produce and release auxins) to sink tissues (which require auxins for growth and development).
Overall, the distribution of auxins in plants is a dynamic process influenced by both internal and external factors, allowing for precise control of plant growth and development.
Different Forms of Auxins:
There are several types of auxins that have been identified in plants. However, the naturally occurring auxin in plants is indole-3-acetic acid (IAA). IAA is a small, water-soluble molecule and is the most studied and well-known plant hormone.
Apart from IAA, other synthetic forms of auxins have also been developed. These include:
1. Indole-3-butyric acid (IBA): IBA is a synthetic auxin that is commonly used in horticulture for rooting cuttings. It is more stable than IAA and is often used in commercial rooting hormone products.
2. 1-Naphthaleneacetic acid (NAA): NAA is a synthetic auxin that is widely used as a growth regulator in agriculture. It is more stable than IAA and can be used for various applications, such as fruit thinning, seed production, and plant growth regulation.
3. 2,4-Dichlorophenoxyacetic acid (2,4-D): 2,4-D is a synthetic auxin that is commonly used as a herbicide. It selectively kills broadleaf weeds while sparing grasses.
4. Picloram: Picloram is a synthetic auxin that is used as a herbicide and for control of invasive plants. It is particularly effective against woody plants and is often used for vegetation management in forests and rangelands.
It's important to note that different forms of auxins can have different effects on plant growth and development, and their usage should be carefully regulated to ensure optimal results.
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