Definition and Function of Auxin as a Plant Hormone


In the world of plant biology, hormones play a crucial role in regulating various physiological processes. One such hormone is auxin, which is involved in numerous aspects of plant growth and development. Auxin is a key player in controlling cell elongation, root and shoot growth, phototropism, gravitropism, and many other important plant responses. In this article, we will explore the definition and function of auxin as a plant hormone, shedding light on its role in shaping the growth and behavior of plants.

Definition of Auxin

Auxin is a class of plant hormones that are primarily responsible for regulating cell elongation and growth. The term “auxin” is derived from the Greek word “auxein,” meaning “to grow.” The most common and well-studied auxin in plants is indole-3-acetic acid (IAA). However, there are other naturally occurring auxins, such as indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA), which also exhibit auxin-like activity.

Auxin is synthesized in the apical meristems, young leaves, and developing seeds of plants. It is then transported from the site of synthesis to other parts of the plant, where it exerts its effects. The movement of auxin within the plant is predominantly polar, meaning it moves in a specific direction, either from the shoot apex downward (basipetal transport) or from the root apex upward (acropetal transport).

Functions of Auxin

  • 1. Cell Elongation and Growth: One of the primary functions of auxin is to promote cell elongation, leading to the growth of plant organs such as stems and roots. Auxin stimulates the loosening of cell walls by activating enzymes called expansins, which break down the cross-linking of cellulose microfibrils. This allows the cell to expand and elongate, resulting in increased growth.
  • 2. Apical Dominance: Auxin plays a crucial role in maintaining apical dominance, which refers to the inhibition of lateral bud growth by the apical bud. The apical bud produces a higher concentration of auxin, which suppresses the growth of lateral buds. This ensures that the plant’s energy is directed towards vertical growth rather than lateral branching.
  • 3. Root Development: Auxin is essential for root development and gravitropism, which is the plant’s response to gravity. In roots, auxin accumulates on the lower side of the root, inhibiting cell elongation and promoting root curvature downwards. This helps the root to grow in the right direction, allowing it to penetrate the soil and anchor the plant.
  • 4. Phototropism: Auxin is also involved in phototropism, which is the plant’s response to light. When light is detected by the plant, auxin accumulates on the shaded side of the stem, causing cells to elongate more rapidly on that side. This results in the bending of the stem towards the light source, optimizing photosynthesis.
  • 5. Fruit Development and Ripening: Auxin plays a crucial role in fruit development and ripening. It promotes the growth of fruit and helps in the formation of seeds. Additionally, auxin regulates the ripening process by controlling the production of enzymes that break down cell walls, leading to softening and color changes in the fruit.
  • 6. Tissue Differentiation: Auxin is involved in the differentiation of plant tissues, including the formation of vascular tissues, cambium, and the initiation of lateral roots. It promotes the differentiation of undifferentiated cells into specialized cell types, allowing for the development of complex plant structures.


Auxin is a vital plant hormone that regulates various aspects of plant growth and development. It controls cell elongation, root and shoot growth, phototropism, gravitropism, and many other plant responses. Auxin promotes cell elongation by loosening cell walls, maintains apical dominance, directs root growth, mediates phototropism, regulates fruit development and ripening, and influences tissue differentiation. Understanding the functions of auxin is crucial for unraveling the complexities of plant growth and behavior, and it provides valuable insights for agricultural practices, horticulture, and plant biotechnology.

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