Auxins: The Plant Growth Hormone: Definition, Functions, & Uses

What Is Auxin?

Auxins are a class of naturally occurring and artificially synthesised hormones of plants. They play a significant role in the regulation of growth in plants. Auxins were initially extracted from human urine. Auxin means to "enlarge" or "increase". They induce cell division, differentiation and elongation. Charles Darwin discovered the phototropism movement of bending towards light in the coleoptile of canary grass. He demonstrated that there was some influencer at the tip of the coleoptile, responsible for the bending towards the light. Subsequently, Frits Went isolated and named the substance "Auxin", responsible for phototropic movement in oat coleoptile. Kenneth Thimann purified and elucidated the structure of primary auxins, e.g. IAA (Indoleacetic acid).

• Naturally Occurring Auxins: Indole-3-acetic acid, Indole butyric acid

• Artificially synthesized Auxins: 2,4-dichloro phenoxy acetic acid, Naphthalene acetic acid

Mechanism Of Action Of Auxin

• Auxin is mostly produced in the apical meristem of shoots, young leaves and seeds

• The movement of auxin is unidirectional or polar, it moves downwards from its site of production

• Polar transport results in an auxin concentration gradient, which stimulates specific responses

• Transport proteins specific for auxin in the plasma membrane regulate the transport of auxin out of the cell

• Plant hormones operate via signal transduction and many induce more than one cellular responses

• Auxin acts by binding to enzyme-linked receptors, which enhances the catalysis of a reaction

• When auxin binds to a receptor, it leads to the binding of a repressor protein for certain genes, namely auxin response genes, to ubiquitin, leading to degradation of the repressor protein and thus the transcription of auxin response genes progresses leading to cellular growth and development

Auxin Mechanism

• Cell Elongation: It promotes elongation in shoots and coleoptiles. The plasticity of the cell wall is increased by acidification

• Cell Division and Differentiation: Auxin promotes healing. It helps in cell differentiation and regeneration of vascular tissues phloem, xylem

• Callus Formation and Morphogenesis: Auxin along with cytokinin induces callus formation in explant and stimulates morphogenesis

• Secondary Growth: Auxin promotes secondary growth and induces cell division in the vascular cambium

• Root Initiation on Cuttings: For asexual propagation, NAA is used to initiate root formation in the stem cuttings

• Apical Dominance: It refers to the inhibition of the growth of axillary buds by the growth of apical meristem. Removal of the shoot tip induces the growth of lateral buds. Hence, this is employed to develop branching, e.g. hedge-making and tea plantations.

• Parthenocarpy: Application of auxin to some flowers leads to parthenocarpy, i.e. ovary enlarges and develops into seedless fruit unfertilized. Seedless tomatoes are widely produced by this method.

• Fruit development: Auxin is produced by seeds and it stimulates fruit development with gibberellins and delays senescence.

• Flowering: It delays the senescence of flowers. A high concentration of auxin promotes femaleness in some of the plants. It promotes flowering in litchi and pineapples.

• Herbicides: Synthetic auxins, e.g. 2,4-D and 2,4,5-T are used extensively to kill weeds. It does not affect grasses, i.e. monocotyledons.

• Promotes Tropism: The auxin induces phototropism, gravitropism and thigmotropism, i.e. movement in response to light, gravity and touch respectively.

Phototropic movement: It can be explained by the cell elongation due to auxins. Auxin concentration increases towards the dark side due to auxin migration. That means more cell elongation occurs at the shaded side compared to the side which receives light.

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