Auxin is a natural chemical compound within plants that plays a central role in coordinating their growth and development. As a plant hormone, it is essential for a plant’s overall body development. The term “auxin” comes from the Greek word “auxein,” meaning “to grow” or “to enlarge,” reflecting its primary function. Synthetic versions are also produced and utilized for various purposes.
The Discovery of Auxin
The journey to understanding auxin began with observations by Charles Darwin and his son, Francis, in the late 19th century. In their 1880 book, “The Power of Movement in Plants,” they described experiments with canary grass coleoptiles, noting that a signal from the tip of the coleoptile was responsible for its bending towards light, a phenomenon known as phototropism. If the coleoptile tip was covered or removed, the plant would not bend towards light, indicating the tip’s role in perceiving the stimulus and transmitting a growth-promoting influence downwards.
Further breakthroughs came in the 1920s with the work of Dutch biologist Frits Went. In 1926, Went isolated this growth-promoting substance. He placed oat coleoptile tips on agar blocks, allowing the substance to diffuse into the agar. These agar blocks then induced bending in decapitated coleoptiles, proving a transferable chemical signal influenced plant growth. Went named this chemical “auxin,” and its structural identification as indole-3-acetic acid (IAA) followed in the 1930s.
How Auxin Directs Plant Growth
Auxin influences plant growth primarily by stimulating cell elongation, particularly in stems and young leaves. It triggers processes that loosen cell walls, allowing cells to expand with water intake, which significantly contributes to the plant’s length. This effect supports upward growth and organ development.
Auxin also promotes root development, initiating new roots and growing existing ones. While higher concentrations of auxin promote shoot elongation, lower concentrations are necessary for root elongation. It is involved in the formation of lateral roots, which branch off from the primary root, and adventitious roots, which can form from non-root tissues like stems.
Auxin also plays a role in apical dominance. This is the phenomenon where the main, central stem of a plant grows preferentially, suppressing the growth of side branches or lateral buds. Auxin produced in the apical bud, located at the shoot tip, moves downwards and inhibits the development of lateral buds. If the apical bud is removed, the concentration of auxin decreases, allowing lateral buds to grow and leading to a bushier plant.
Auxin also influences the plant’s responses to environmental stimuli, known as tropisms. In phototropism, the plant’s growth response to light, auxin redistributes to the shaded side of the stem, causing cells on that side to elongate more rapidly. This differential growth results in the stem bending towards the light source. Similarly, in gravitropism, the plant’s response to gravity, auxin accumulation on the lower side of horizontal stems promotes upward growth, while in roots, it inhibits elongation on the lower side, directing root growth downwards.
Auxin also contributes to fruit development. After fertilization, auxin produced by developing seeds stimulates the growth and enlargement of the fruit, often in conjunction with other plant hormones like gibberellins. It influences cell division and expansion within the fruit tissues, helping to ensure proper fruit formation and delaying fruit senescence.
Practical Applications of Auxin
Auxin’s effects have led to numerous practical applications in agriculture, horticulture, and gardening. One widespread use promotes root growth in plant cuttings. Synthetic auxins like indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) are applied to stem cuttings to stimulate new root formation, significantly increasing propagation success. These rooting hormones are available in various forms, including powders, liquids, and gels.
Auxins are also used in fruit management. They can thin fruit on trees, ensuring larger, better quality fruits by reducing resource competition. Synthetic auxins can also prevent premature fruit drop, keeping fruits attached until full maturity. This is beneficial for crops like pears and lychees, where early fruit abscission causes significant yield losses.
At higher concentrations, some synthetic auxins are used as herbicides, controlling broadleaf weeds in lawns and cereal crops. Compounds like 2,4-D (2,4-dichlorophenoxyacetic acid) are effective because dicots (broadleaf plants) are more susceptible to high auxin levels than monocots (grasses). These herbicides cause uncontrolled, abnormal growth in susceptible plants, leading to their demise.