Auxin, the distribution and role of auxin in plants

One of the most important functions of auxin is to promote rooting. For shoots that can be cuttings, soaking them in auxin solution before cuttings can promote the rooting of cuttings and grow into independent plants.

In plants, auxin is mainly produced in leaf primordium, young leaves, and developing seeds. Mature leaves and root tips also produce auxin, but in small quantities. Auxin is widely distributed in higher plants, but most of them are concentrated in vigorous growth sites (such as germ sheaths, meristems of buds and root tips, cambium, ovary after fertilization and young seeds, etc.), but less in tissues and organs that tend to senescence.

The transport of auxin in the plant body is mainly transported from the upper end of the plant morphology to the lower end, and cannot be reversed for transportation.

Physiological effects of auxin After long-term and in-depth research, scientists have found that the reason why plants can show phototropism is that under the irradiation of unilateral light, auxin is more distributed on the backlit side than on the phototropic side. In this way, the cells on the backlit side elongate and grow rapidly, resulting in a bend of the stem towards the slow-growing side, i.e., towards the side of the light source. The effect of auxin on plant growth is often twofold.

Auxin can both promote and inhibit plant growth; It can both promote and inhibit germination; It can not only prevent flower and fruit fall, but also thin flowers and fruits. This phenomenon is related to the concentration of auxin and the type of plant organs, among other things.

In general, low concentrations of auxin can promote plant growth, while high concentrations of auxin inhibit plant growth. Different organs of the same plant respond differently to different concentrations of auxin, for example, for roots, the optimal concentration of auxin is about 10-10mol l; For buds, the optimal concentration is about 10-8mol l; For stems, the optimal concentration is about 10-4mol l (as shown in the figure).

The apical advantage of plants - the phenomenon that the terminal buds of plants grow preferentially and the side buds are inhibited, is because the auxin produced by the terminal buds is transported downward and accumulates in large quantities in the side buds, so that the growth of the side buds is inhibited. If the apical bud is removed, the auxin concentration in the side bud decreases, the inhibition effect on the side bud is lifted, and the side bud can soon develop into a branch (as shown in the figure).

In 1880, Darwin and his son, in their last book, The Ability to Move Plants, stated that the germ sheath of the Canary grass of the grass family lost its phototropic response when the tip was cut off.

His explanation is that when the seedling receives light from the side, the effect of the tip is transmitted downward, causing the growth rate of the light to be different from that of the backlight, which causes the bending to the light receiving side, so that the top does not show a phototropic response after cutting off.

In 1928, Winter experimentally proved that there is a growth-promoting substance at the tip of the germ sheath, called auxin. It can spread into small agar squares, and placing the resulting small squares back on the side of the cut of the coleoplasty with the tip cut off can cause the coleoplasts to bend to the other side.

And the curvature is roughly proportional to the amount of growth-promoting substance it contains. This experiment not only proved the existence of growth-promoting substances, but also created the famous "oat test" for the determination of auxin.

1933 fIndoleacetic acid was isolated from human urine and yeast, and after it could cause the coleoplasty to bend in the oat test, it was proved that indoleacetic acid is auxin, which is ubiquitous in various plant tissues.

Auxin is one of the earliest and most studied plant hormones that is most commonly found in plants. As early as 1880, when Darwin and his son conducted phototropism experiments, they found for the first time that the germ sheath at the tip of a plant seedling bends and grows towards the light under the light in one direction, but if the tip is cut off or the light is covered with a black cover, the seedling will not bend to the light even if the light is illuminated in one direction. They speculated at the time because of this

When the coleoplasty is irradiated with light on one side, a substance may be produced at the tip that is transmitted to the lower part, causing phototropic bending of the seedling. Later, inspired by Darwin's experiments, many scholars have successively carried out studies in this area and confirmed the existence of this substance. The most successful of these was the Dutchman Winter, who succeeded for the first time in collecting auxin in agar cubes in 1928, demonstrating that this substance was associated with the phototropic curving growth of plants.

The auxin withering tolerance biological identification method he established, which was defeated in the first banquet test of oats, is still used today. It was not until 1946 that the first active substance related to growth was isolated from higher plants, and it was identified as an organic compound with a relatively simple structure, indoleacetic acid.