13.4.3.1. Auxins

Auxins (from Greek ‘auxein’: to grow) was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.

Auxins are a type of plant growth regulator responsible for promoting growth. The name 'auxin' comes from the Greek word 'auxein,' meaning 'to grow.' The most common auxin is indole-3-acetic acid (IAA), which can be naturally occurring or synthetic. Auxins are produced at the tips of the plant's stems and roots. From these growth apices, they move to other parts of the plant to exert their effects.

Think of auxins as the 'growth hormones' for plants, much like how human growth hormone causes children to grow. Just as children have specific areas in their bodies where growth is initialized, such as the growth plates in bones, auxins are generated at the tips of plants to trigger growth in other parts of the plant.

Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices. They help to initiate rooting in stem cuttings, an application widely used for plant propagation.

Both natural and synthetic auxins are used in agriculture and gardening to enhance plant growth. For example, they can be applied to stem cuttings to encourage them to grow roots. This is a common technique in horticulture to propagate plants more efficiently.

When you cut a branch from a plant and want it to grow roots in water or soil, using a rooting hormone that contains auxins can increase your chances of success. It's similar to how some people take vitamins to support their growth and health.

Auxins promote flowering, e.g., in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.

Auxins play a crucial role in flower development as well. For instance, they can induce flowering in certain plants like pineapples. Additionally, auxins can delay the dropping of fruits and leaves, ensuring that the plant retains them until they are mature. However, they also signal the plant to shed older parts that are no longer useful.

When you pick apples from a tree, some fruits may still be clinging to the branches. Using auxins can keep those fruits from dropping prematurely, much like how a coach ensures that the best players stay on the team until the end of the season.

In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds.

Apical dominance is a growth pattern where the main shoot of the plant grows more than the side shoots. This is due to auxins produced at the top of the plant, restricting the growth of lateral buds. When the top of the plant is removed, the restriction is lifted, allowing the lateral buds to start growing.

Think of a team leader in a group project. If the leader is strongly guiding the group (like an apical bud with auxins), the other members may not contribute as much. If the leader steps back, team members can take on more significant roles, similar to how lateral buds grow when the apical bud is removed.

Auxins are widely used as herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants. It is used to prepare weed-free lawns by gardeners.

Auxins, particularly synthetic ones like 2, 4-D, are used in agriculture as herbicides. They specifically target broad-leaf weeds without harming grass or cereal crops. This targeted application makes lawns and fields more manageable and productive.

Imagine having a garden where you want to eliminate dandelions without harming your grass. Auxins act like a selective herbicide, similar to how a precision tool can remove just the weeds while keeping the desired plants intact.

Auxin also controls xylem differentiation and helps in cell division.

Auxins are important not just for overall growth but also for specific processes, such as the differentiation of xylem tissue. They encourage the development of xylem cells, which are vital for water transport within the plant. Additionally, they stimulate cell division, promoting overall plant growth.

You can think of auxins as coaches who specialize in different parts of the team. Just as a coach might focus on training new players to improve specific skills, auxins support the development of crucial parts of the plant, like xylem, ensuring everything works together for stronger growth.

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