Auxin is an endogenous hormone containing unsaturated aromatic ring and acetic acid side chain. IAA for short, IAA is used internationally. 4- chloro-indoleacetic acid, 5- hydroxy-indoleacetic acid, NAA and indolebutyric acid are auxins. In 1872, the Polish horticulturist Xie studied the root tip which controls the growth of root elongation area. Later, Darwin and his son studied the phototropism of grass coleoptiles. 1928 winter confirmed that the tip of the germ did produce something that could control the growth of the germ. 1934, Kaigl and others isolated this substance from some plants and named it indoleacetic acid, so indoleacetic acid is often used as a synonym for auxin.

Auxin is synthesized in swollen young leaves and apical meristem, and accumulated to the base from top to bottom through long-distance transportation of phloem. Roots can also produce auxin, which is transported from bottom to top. Plant auxin is formed by tryptophan through a series of intermediate products. Its main route is through indole acetaldehyde. Indole acetaldehyde can be formed by oxidative deamination of tryptophan into indole pyruvic acid and decarboxylation, or by oxidative deamination of tryptophan after decarboxylation into amine. Then indole acetaldehyde is oxidized to indole acetic acid. Another possible synthetic route is to convert tryptophan into indoleacetic acid by indoleacetic acid.

In plants, indoleacetic acid can be combined with other substances and lose its activity, such as indoleacetic acid combined with aspartic acid, inositol combined with inositol, glycoside combined with glucose, and indoleacetic acid-protein complex combined with protein. Binding indoleacetic acid often accounts for 50~90% of plant indoleacetic acid, which may be a storage form of auxin in plant tissues, and they can be hydrolyzed to produce free indoleacetic acid.

Indole acetic acid oxidase widely exists in plant tissues and can oxidize and decompose indole acetic acid.

Auxin has many physiological functions, which are related to its concentration. Low concentration can promote growth, while high concentration can inhibit growth and even kill plants. This inhibition is related to whether ethylene can be induced. The physiological function of auxin is manifested in two levels.

At the cellular level, auxin can stimulate the division of cambium cells; Stimulate branch cell elongation and inhibit root cell growth; Promote the differentiation of xylem and phloem cells, promote the rooting of cuttings, and regulate the morphogenesis of callus.

At the level of organs and the whole plant, auxin plays a role in the process from seedling to fruit ripening. Reversible red light inhibition of auxin controlling hypocotyl elongation of seedlings: when indoleacetic acid is transferred to the lower side of branches, it produces geotropism of branches; When indoleacetic acid is transferred to the backlight side of the branch, it produces phototropism of the branch; Indoleacetic acid leads to apical dominance; Delaying leaf senescence; Spraying auxin on the leaves inhibits shedding, and spraying auxin on the near-axis end of abscission layer promotes shedding; Auxin promotes flowering, induces the development of parthenocarpy and delays fruit ripening.

In recent years, the concept of hormone receptor has been put forward. Hormone receptor is a kind of macromolecular cell component, which can specifically bind with corresponding hormones and then launch a series of reactions. The complex of indoleacetic acid and its receptor has two functions: first, it acts on membrane proteins, affecting medium acidification, ion pump transport and tension change, which is a fast reaction (< 10 min >); Second, it acts on nucleic acid, causing cell wall changes and protein synthesis, which is a slow reaction (10 minute). Acidification of medium is an important condition for cell growth. Indoleacetic acid can activate ATP (adenosine triphosphate) enzyme on plasma membrane, stimulate hydrogen ions to flow out of cells, and reduce the pH value of culture medium, so related enzymes are activated, hydrolyze polysaccharides in cell walls, soften cell walls, and expand cells.

The application of indoleacetic acid led to the appearance of specific messenger ribonucleic acid (mRNA) sequences, which changed the synthesis of protein. Indoleacetic acid treatment also changed the elasticity of cell wall and made cells grow.

Auxin promotes cell growth, especially cell elongation, but has no effect on cell division. The part of plants that feel light stimulation is at the shoot tip, but the bending part is at the lower part of the shoot tip. This is because the cells under the tip are growing and elongating, which is the most sensitive period to auxin, so auxin has the greatest influence on its growth. Tissue auxin, which tends to age, has no effect. The reason why auxin can promote the development of fruits and the rooting of cuttings is that auxin can change the distribution of nutrients in plants, get more nutrients in the parts where auxin is rich, and form distribution centers. Auxin can induce the formation of seedless tomato because the ovary of tomato bud becomes the distribution center of nutrients after auxin treatment, and the nutrients produced by leaf photosynthesis are continuously transported to the ovary, and the ovary develops.

The duality of physiological function of auxin;

Low concentration promotes growth and high concentration inhibits growth. Different organs of plants have different requirements for the optimal concentration of auxin. The optimum concentration of roots is about 10e- 10 mol/L, buds are about 10e-8 mol/L, and stems are about10.3e-5mol/L. In production, auxin analogues (such as NAA, 2,4-) ) is often used to regulate the growth of plants. For example, when bean sprouts are produced, the bean sprouts are treated with a concentration suitable for stem growth, and as a result, the roots and buds are inhibited, and stems developed from hypocotyls. The apical advantage of plant stem growth is determined by two factors: the transport characteristics of auxin and the duality of physiological function of auxin. The terminal bud of plant stem is the most active part of auxin production, but the concentration of auxin produced by the terminal bud is continuously transported to the stem through active transportation, so the concentration of auxin in the terminal bud itself is not high, but it is higher in the young stem which is most suitable for stem growth, but it has inhibitory effect on the bud. The closer to the terminal bud, the higher the auxin concentration, and the stronger the inhibitory effect on the lateral bud, which is also the reason why many tall plants form pagodas. However, not all plants have a strong top advantage. After a period of development, some shrubs began to degenerate or even shrink, losing their original top advantage, so the tree shape of shrubs was not pagoda-shaped. Because high concentration of auxin can inhibit plant growth, its analogues can also be used as herbicides in production, especially for dicotyledonous weeds.

Auxin analogues: 2,4-D. Because auxin rarely exists in plants, auxin analogues have been found in order to regulate plant growth, which are similar to auxin and can be produced in large quantities, and have been widely used in agricultural production.

Effect of gravity on auxin distribution;

The underground growth of stems and roots is caused by the gravity of the earth, because the gravity of the earth leads to uneven distribution of auxin, with more distribution near the stems and less distribution on the underground side. Because the optimum concentration of auxin in the stem is very high, there is more auxin on the near-ground side of the stem to promote it, so the near-ground side grows faster than the back side, keeping the stem growing upward; For roots, because the optimum concentration of auxin is very low, the more auxin near the ground, the more it can inhibit the growth of root cells, so the growth near the ground is slower than that on the back ground, and the roots keep geotropism. If there is no gravity, the roots will not necessarily grow down.

Effects of weightlessness on plant growth;

The underground growth of roots and stems is induced by the gravity of the earth, which is caused by the uneven distribution of auxin. In the state of weightlessness in space, due to the loss of gravity, the growth of stems lost their backs, and the roots also lost their characteristics of growing to the ground. But the apical advantage of stem growth still exists, and the polar transport of auxin is not affected by gravity.

Discovery of auxin:

Auxin is the earliest discovered plant hormone.

1880

When Darwin studied the phototaxis of plants with canary grass, he found that one-way illumination of coleoptile would cause phototaxis bending of coleoptile. Cutting off the top of the coleoptile or covering it with opaque tin foil will not cause phototropism when irradiated with unilateral light. Therefore, Darwin believed that the coleoptile produced downward moving substances under unilateral illumination, which led to the coleoptile growing at different speeds from the backlight surface to the smooth surface, and made the coleoptile bend to the smooth surface.

1928

In derwent, the Netherlands, the top of the cut oat coleoptile is straight on the agar block. After a period of time, the top of the coleoptile is removed and these agar blocks are placed on one side of the germinated coleoptile. As a result, the side with agar grows faster and bends in the opposite direction. This experiment confirmed that a substance produced at the top of the coleoptile can be transferred to the lower part of the coleoptile and promote the growth of the lower part when it is diffused into agar and placed on the coleoptile. Later, got isolated the growth-related substance produced by sheath tip for the first time and named it auxin.

1934

Kaergel and others in the Netherlands isolated a compound from human urine and added it to agar, which can also induce coleoptile bending. This compound proved to be indoleacetic acid. Subsequently, Kaergel and others also found indoleacetic acid (IAA) in plant tissues.

Summary: The discovery of auxin embodies the basic ideas of scientific research: a. Ask questions, make assumptions, design experiments and draw conclusions; B. The experiment embodies the univariate principle of design experiment; The univariate of Darwin's experiment is whether there is a tip, and the univariate of Winter's experiment is whether the agar touches the tip of the coleoptile.