Auxin Meaning and Definition
Auxins are a potent growth hormone released by plants in their natural state. They encourage cell division, stem and root growth and are present in shoot and root tips. They can also have a significant impact on plant orientation by encouraging cell division on one side of the plant in response to sunlight and gravity.
Plants require sunshine, water, oxygen, and minerals to grow and develop. There are outside influences. Aside from these, plants' growth and development are governed by certain intrinsic influences. Plant hormones, also known as phytohormones, are a form of a hormone produced by plants.
All growth and development processes, such as cell division, enlargement, flowering, seed formation, dormancy, and abscission, are regulated by plant hormones.
Plant hormones are classified into two groups based on their actions:
Plant Growth Promoters
Plant Growth Inhibitors
Auxin Definition
Auxin is a word that means "to rise." In agricultural and horticultural practices, they are widely used. They are found in the rising apices of roots and stems before migrating to other parts of the plant to perform their functions.
Natural: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
Synthetic: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid)
Auxin in Plants
Role of Auxins
Auxins stimulate shoot elongation by influencing gibberellins, which facilitate cell elongation. This lengthens the plant. Gibberellins, and thus auxins, increase the distance between nodes, allowing branch points to be spaced further apart.
Controlling Seedling Orientation
Charles Darwin and his son Francis were among the first to note the seedlings bend toward the sun. However, where auxins are located and how they affect cells inside the plant determine whether a new shoot develops into the soil or towards the light. Gravity can cause auxins to migrate downward and laterally away from light. Cells grow faster in areas of the plant where auxin levels are high.
When auxin is added to a cut stem, the stem will begin to form roots at the cut.
Auxins in the flower facilitate the maturation of the ovary wall as well as steps in the full growth of the fruit.
Auxins can be synthesized or formed naturally (by the plant) (in a lab). When synthesized, they can be used as a pesticide in high concentrations, causing rapid development. 2-4-D is an auxin-based herbicide that is specifically designed to cause dicots (plants like dandelions) to grow rapidly and uncontrollably, eventually destroying the plant.
What is Auxin Plant Hormone?
Auxin is a plant hormone derived from tryptophan, an amino acid. Auxins may be one of several molecules, but they are all involved in cellular regulation in some way. Plant hormones come in five different forms. Auxin molecules are one of them. Gibberellins, cytokinins, ethylene, and abscisic acid are the other main classes. Auxin was the first of these groups to be discovered, and it was isolated chemically in the 1930s. Indoleacetic acid, or IAA, is the most widely used auxin. IAA is an auxin that is essential for plant tissue growth and development. Scientists have been able to replicate similar structures, known as synthetic growth regulators, by studying auxin molecules.
Functions of Auxin in Plants
Stem and root cell elongation
IAA in the apical bud suppresses the development of lateral buds due to apical dominance.
Parthenocarpy, or the growth of fruit without fertilization, is induced, for example, in tomatoes.
Prevents leaves, flowers, and fruits from falling prematurely.
Useful in stem cuttings and grafting where it encourages flowering, such as in pineapples.
2,4-D is a commonly used herbicide for killing unwanted dicot weeds without harming monocot plants.
Cell division and xylem differentiation are aided by this substance.
FAQs on Auxin
1. What is auxin and where is it primarily found in a plant?
Auxin is a key class of plant hormones, also known as plant growth regulators, that plays a crucial role in coordinating many growth and behavioural processes in a plant's life cycle. The name 'auxin' is derived from the Greek word 'auxein', which means 'to grow'. These hormones are primarily synthesised in the apical meristems of shoots and roots, which are the growing tips, as well as in young leaves and developing seeds. From these production sites, auxin is transported to other parts of the plant to regulate their development. For a broader understanding, you can explore more about Plant Hormones.
2. What are the major functions of auxin in plant growth and development?
Auxins are vital for a plant's survival and growth, performing several important functions:
- Cell Elongation: Auxins promote the stretching and elongation of cells, particularly in the stem, which is a primary mechanism for plant growth.
- Apical Dominance: It promotes the growth of the main central stem (apical bud) while inhibiting the growth of lateral buds. This is why many plants grow taller before they grow wider.
- Root Initiation: While high concentrations can inhibit root elongation, auxins are crucial for initiating the formation of adventitious roots, a principle widely used in horticulture for plant propagation from cuttings.
- Tropic Movements: Auxins mediate plant responses to environmental stimuli like light (phototropism) and gravity (gravitropism).
- Fruit Development: They are essential for the development of fruits and can be used to produce seedless fruits (parthenocarpy).
- Prevents Abscission: Auxin helps prevent the premature falling of leaves, flowers, and fruits.
3. What are the different types of auxins? Provide some examples.
Auxins can be classified into two main categories based on their origin:
- Natural Auxins: These are produced naturally by the plants themselves. The most common and important natural auxin is Indole-3-acetic acid (IAA). Another naturally occurring auxin is Indole-3-butyric acid (IBA), although its natural presence is debated in some species.
- Synthetic Auxins: These are artificially synthesised compounds that mimic the effects of natural auxins. They are widely used in agriculture for various purposes. Common examples include Naphthalene acetic acid (NAA), used to induce flowering and prevent fruit drop, and 2,4-Dichlorophenoxyacetic acid (2,4-D), which is used as a selective herbicide to kill broadleaf weeds.
4. Why is auxin called a 'plant growth regulator' instead of just a 'plant growth promoter'?
Auxin is termed a 'plant growth regulator' because its effect can be both promotive and inhibitory, depending on its concentration and the plant part it acts upon. While it famously promotes cell elongation in stems, the same high concentration that promotes stem growth can actually inhibit the growth of roots. Similarly, its role in apical dominance involves promoting the main shoot's growth while inhibiting the growth of lateral buds. This dual action—promoting growth in some contexts and inhibiting it in others—makes 'regulator' a more accurate term than the one-sided 'promoter'.
5. How does the function of auxin differ from that of gibberellin?
While both auxin and gibberellin are plant growth promoters, they have distinct roles and mechanisms. The primary difference lies in their main function: auxin's main role is cell elongation, whereas gibberellin's main role is to promote cell division and elongation, thus causing stem elongation. Other key differences include:
- Apical Dominance: Auxin is the primary hormone responsible for establishing and maintaining apical dominance, while gibberellins have little to no role in it.
- Rooting: Auxin is used to initiate rooting in stem cuttings, whereas gibberellins inhibit root formation.
- Breaking Dormancy: Gibberellins are famous for breaking seed and bud dormancy, a function not primarily associated with auxins.
You can explore a detailed comparison in this article on the Difference Between Auxin And Gibberellin.
6. How does auxin cause a plant to bend towards a light source?
The bending of a plant towards light, a phenomenon known as phototropism, is directly mediated by auxin. Here’s how it works:
- Auxin is produced at the tip of the plant shoot.
- When light shines on the plant from one side, the auxin migrates from the illuminated side to the shaded side of the shoot tip.
- This results in a higher concentration of auxin on the shaded side compared to the lit side.
- The increased concentration of auxin on the shaded side stimulates the cells there to elongate more rapidly than the cells on the lit side.
- This differential growth rate—faster on the shaded side and slower on the lit side—causes the shoot to bend and grow towards the light source. To learn more, read about Phototropism.
7. What are some key commercial applications of synthetic auxins in agriculture?
Synthetic auxins are extremely valuable in modern agriculture and horticulture due to their potent and specific effects. Key applications include:
- As Herbicides: Synthetic auxins like 2,4-D are widely used as selective herbicides. At high concentrations, they disrupt the normal growth patterns of broadleaf weeds (dicots), causing them to grow uncontrollably and die, while leaving narrow-leaf crops like corn and wheat (monocots) largely unaffected.
- Rooting Agent: Auxins like IBA and NAA are the active ingredients in most commercial rooting powders. Dipping stem cuttings in these powders stimulates rapid formation of adventitious roots, making plant propagation more successful.
- Preventing Fruit Drop: Spraying low concentrations of auxins like NAA can prevent the premature dropping of fruits such as apples and oranges from trees, thus increasing the yield.
- Inducing Parthenocarpy: Auxins can be applied to flowers to induce the development of fruit without fertilisation, leading to the production of seedless fruits like tomatoes and cucumbers.
