Auxin Hormone - Discovery, Types, Functions, and Mechanism

Auxin: Auxins are a class of plant hormones pivotal to the regulation of growth and developmental processes in plants. These organic compounds are fundamental to various developmental activities, including cell enlargement, initiation of root systems, differentiation of vascular tissues, and proliferation of buds and flowers. Auxins are necessary for plant morphological development and regulate gene expression through specific transcription factors and proteins that respond to environmental cues.

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Auxin Hormone

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Discovery of Auxin

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Auxin Types

1. Indole-3-acetic acid (IAA): IAA represents the predominant and most extensively researched natural auxin. It is produced in the apical regions of shoots and roots and is essential for regulating cellular elongation, division, and root development.
2. Indole-3-butyric acid (IBA): Similarly, IBA, another natural auxin, is produced in the shoots and roots. It promotes root growth and is commonly used in horticulture as a rooting hormone to aid in root formation in cuttings.

1. 2,4-Dichlorophenoxyacetic acid (2,4-D): As a prevalent herbicide, 2,4-D functions by emulating the activity of natural auxin. It causes erratic growth in broadleaf weeds, resulting in their demise.
2. Naphthalene acetic acid (NAA): NAA, a synthetic auxin, is used in a variety of horticultural practices, including promoting fruit set, inhibiting premature fruit drop, and enhancing root development in cuttings.

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Auxin Functions

• Distribution: Synthesised in the meristems, auxin is distributed towards the roots, facilitating coordination between overall plant size and tissue development through its concentration gradient.
• Molecular Influence: Auxin impacts cellular activities such as cytoplasmic streaming and enzyme regulation, essential for plant vitality.
• Regulation of Growth: It governs key developmental processes including flowering, fruit development, and the formation of tubers and bulbs, by directly controlling growth and cell proliferation.
• Phototropism and Apical Dominance: Auxin is important for phototropism, enabling plants to optimise photosynthesis by orienting towards the light. It also establishes apical dominance, which suppresses the growth of new shoots under a dominant meristem.

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Mechanism of Action of Auxin

1. Auxin Binding: Auxin molecules begin their action by attaching to specific receptors located on the cell membrane.
2. Auxin-Induced Protein Degradation: Upon binding, the auxin-receptor complex leads to the breakdown of certain proteins called Aux/IAA (Auxin/Indole-3-Acetic Acid). These proteins normally suppress gene activity, meaning they prevent genes from being activated.
3. Activation of Auxin Response Factors: With the suppressive Aux/IAA proteins removed, another group of proteins called Auxin Response Factors (ARFs) are able to start the process of gene transcription. ARFs are special proteins that bind to DNA and initiate the production of mRNA by starting the activity of an enzyme called RNA polymerase.
4. Gene Expression and Cellular Response: The mRNA produced is then translated into proteins that cause the visible effects of auxin, like stretching of cells, growth of roots, or formation of vascular tissues.

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Auxin Applications

• Root Induction: Synthetic auxins, such as indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA), are widely utilised to enhance root formation in stem cuttings. Auxins promote root growth, increasing cuttings survival and plant establishment.
• Fruit Set and Development: Auxins play an indispensable role in the setting and development of fruits. They stimulate cell division and expansion within the ovary, contributing to the growth of the fruit. Applying auxin sprays helps prevent the premature dropping of fruit in certain crops.
• Weed Management: Some synthetic auxins function as herbicides by interfering with plant growth hormones. These herbicides mimic the action of natural auxins at significantly elevated levels, disrupting normal growth processes and ultimately leading to the demise of the plant.
• Apical Dominance: Auxin regulates apical dominance, a phenomenon in which the primary stem of a plant grows more rapidly while suppressing the growth of lateral shoots. This mechanism enables the plant to grow taller and compete more effectively for light. By pruning the main stem, gardeners can mitigate the effects of apical dominance, promoting a bushier growth habit.